Summary notes (Scholar) Unit 1 Chemical Changes and Structure

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1 Summary notes (Scholar) Unit 1 Chemical Changes and Structure 1A Rates of Reaction Rates of chemical reactions can be controlled by chemists. If reaction rates are too low a manufacturing process will not be economically viable. If reaction rates are too high there is a risk of thermal explosion. The rates of reactions are affected by changes in concentration, particle size and temperature and the collision theory can be used to explain these effects. The relative rate of a reaction can be calculated using the formula Rate =1/t. Graphs which use the same axes and place the results for different experiments in which the concentration or temperature are varied from one experiment to the next are common and show up how that variable affects the reaction progress. Temperature is a measure of the average kinetic energy of the particles of a substance. Activation energy is the minimum kinetic energy required by colliding particles before reaction can occur. Energy distribution diagrams (Maxwell-Boltzmann) can be used to explain how an increase in temperature or, in some chemical reactions the energy from light, increases the number of particles with energy greater than the activation energy (Ea/EA). Reactions increase their rate at higher temperatures because a higher proportion of the molecules involved have energy in excess of the activation energy and more successful collisions can occur. It is observed that a 10 C rise is responsible for an approximate doubling of rate in many reactions. The effect of temperature on reaction rate can be explained in terms of an increase in the number of particles with energy greater than the activation energy. In general, the lower the activation energy the faster the reaction. Enzymes catalyse the chemical reactions which take place in the living cells of plants and animals. They are also widely used in industrial processes. The activated complex is an unstable arrangement of atoms formed during a reaction at the maximum of a potential energy barrier. A potential energy diagram can be used to show the energy pathway for a reaction. The enthalpy change, which can be calculated from the potential energy diagram, is the energy difference between products and reactants. The enthalpy change has a negative value for exothermic reactions, which cause heat energy to be released to the surroundings. The enthalpy change has a positive value for endothermic reactions, which cause absorption of heat energy from the surroundings. The activation energy is energy required by colliding molecules to form the activated complex. The activation energy can be calculated from potential energy diagrams. Catalysts are substances which speed up chemical reactions without being used up in the process. They are widely used in industrial processes. Catalysts allow chemical reactions to occur more quickly at lower temperatures and so reduce energy costs. Catalysts speed up reactions by providing an alternative pathway which has a lower activation energy. A potential energy diagram can be used to show the effect of a catalyst on activation energy.

2 Summary notes (Scholar) Unit 1 Chemical Changes and Structure 1B Periodicity Elements are arranged in the Periodic Table in order of increasing atomic number. The Periodic Table allows chemists to make accurate predictions of physical properties and chemical behaviour for any element based on its position. There are periodic variations in the densities, melting points and boiling points of the elements across a Period and down a Group. Metallic bonding is the electrostatic force of attraction between positively charged ions and delocalised outer electrons. A metallic structure consists of a giant lattice of positively charged ions and delocalised outer electrons. Atoms in a covalent bond are held together by electrostatic forces of attraction between positively charged nuclei and negatively charged shared electrons. A covalent molecular structure consists of discrete molecules held together by intermolecular forces. A covalent network structure consists of a giant lattice of covalently bonded atoms. Ionic bonding is the electrostatic force of attraction between positively and negatively charged ions. An ionic structure consists of a giant lattice of oppositely charged ions. Elements can be categorised into four classes according to their bonding and structure. Metallic Covalent molecular Covalent network Monatomic The covalent radius decreases across a Period because the increase in nuclear charge attracts the electrons more strongly. The covalent radius increases on going down a group as the number of occupied electron shells increases. The first ionisation energy is the energy required to remove one mole of electrons from one mole of gaseous atoms. The second and subsequent ionisation energies refer to the energies required to remove further moles of electrons. First ionisation energies increase across a Period and decrease down a group. This can be explained in terms of atomic size (covalent radius), nuclear charge and the screening effect due to inner shell electrons. Electronegativity is a measure of the attraction an atom in a bond has for the electrons of the bond. Electronegativity values increase across a Period and decrease down a group. This can be explained in terms of atomic size (covalent radius), nuclear charge and the screening effect due to inner shell electrons. Electronegativity is a measure of the attraction an atom in a bond has for the electrons of the bond. Electronegativity values increase across a Period and decrease down a group.

3 Summary notes (Scholar) Unit 1 Chemical Changes and Structure 1C Structure and Bonding Polar covalent bonds occur when the atoms of the bond attract the bonding electrons unequally causing the atoms to have partial positive and negative charges. The polarity of a covalent bond depends on the difference in electronegativity between the bonded atoms, the most electronegative becoming more negative. Between pure covalent and pure ionic bonds there are polar covalent bonds. The type of bonding between two atoms depends mainly on the difference in electronegativity between the atoms. Permanent dipole-permanent dipole interactions act in addition to London dispersion electrostatic attractions between polar molecules and are stronger than these attractions for molecules of equivalent size. Not all covalent molecules with polar bonds result in polar molecules. Molecules which are highly symmetrical tend to be non-polar. An electrostatically charged rod can be used to detect the presence of polar molecules in a liquid. Polar molecules are attracted to both a negative and positive rod. There is a complete range of bond types leading to a bonding spectrum mainly based on electronegativity. All molecular elements and compounds and monatomic elements condense and freeze at sufficiently low temperatures. For this to occur, some attractive forces must exist between the molecules or discrete atoms. Intermolecular forces acting between molecules are known as van der Waals forces. There are several different types of van der Waals forces such as London dispersion forces and permanent dipole: permanent dipole interactions, which include hydrogen bonding. London dispersion forces are forces of attraction that can operate between all atoms and molecules. These forces are much weaker than all other types of bonding. They are formed as a result of electrostatic attraction between temporary dipoles and induced dipoles caused by movement of electrons in atoms and molecules. The strength of London dispersion forces is related to the number of electrons within an atom or molecule. Bonds consisting of a hydrogen atom bonded to an atom of a strongly electronegative element such fluorine, oxygen or nitrogen are highly polar. Hydrogen bonds are electrostatic forces of attraction between molecules which contain these highly polar bonds. A hydrogen bond is stronger than other forms of permanent dipole-permanent dipole interaction but weaker than a covalent bond. Melting points, boiling points and viscosity can all be rationalised in terms of the nature and strength of the intermolecular forces which exist between molecules. By considering the polarity and number of electrons present in molecules, it is possible to make qualitative predictions of the strength of the intermolecular forces. The melting and boiling points of polar substances are higher than the melting and boiling points of non-polar substances with similar numbers of electrons. The anomalous boiling points of ammonia, water and hydrogen fluoride are a result of hydrogen bonding. Boiling points, melting points, viscosity and solubility/miscibility in water are properties of substances which are affected by hydrogen bonding. Hydrogen bonding between molecules in ice results in an expanded structure which causes the density of ice to be less than that of water at low temperatures. Ionic compounds and polar molecular compounds tend to be soluble in polar solvents such as water and insoluble in non-polar solvents. Non-polar molecular substances tend to be soluble in non-polar solvents and insoluble in polar solvents.

4 Summary notes (Scholar) Unit 1 Chemical Changes and Structure Glossary Activated complex the activated complex is a very unstable arrangement of atoms formed at the maximum of the potential energy barrier, during a chemical reaction Activation energy is the minimum kinetic energy required by colliding particles before reaction will occur, since a high energy activated complex must be formed Allotropes one of two or more existing forms of an element. For example, graphite and diamond are allotropes of carbon Bonding electrons are shared pairs of electrons from both atoms forming the covalent bond Chemical bonding is the term used to describe the mechanism by which atoms are held together Chemical structure describes the way in which atoms, ions or molecules are arranged Collision theory of reactions suggests that, for a chemical reaction to occur, particles must collide Covalent bond a covalent bond is formed when two atoms share electrons in their outer shell to complete the filling of that shell Covalent radius half the distance between the nuclei of two bonded atoms of an element Delocalised delocalised electrons, in metallic bonding, are free from attachment to any one metal ion and are shared amongst the entire structure Diatomic molecules with only two atoms are described as diatomic (e.g. oxygen, O2, and carbon monoxide, CO.) Dipole an atom or molecule in which a concentration of positive charges is separated from a concentration of negative charge Electronegativity a measure of the attraction that an atom involved in a bond has for the electrons of the bond Enthalpy change for a reaction is defined as the change in heat energy when 1 mole of reactant is converted to product(s) at constant pressure, and has the symbol ΔH and units of kj mol -1 Fullerenes are molecules of pure carbon constructed from 5- and 6-membered rings combined into hollow structures. The most stable contains 60 carbon atoms in a shape resembling a football Hydrogen bonds are electrostatic forces of attraction between molecules containing a hydrogen atom bonded to an atom of a strongly electronegative element such as fluorine, oxygen or nitrogen, and a highly electronegative atom on a neighbouring molecule Intermolecular forces are those which attract molecules together. They are weaker than chemical bonds Intramolecular forces are forces of attraction which exist within a molecule Ionisation energy the energy required to remove one mole of electrons from one mole of atoms in the gaseous state Isoelectronic means having the same arrangement of electrons. For example, the noble gas neon, a sodium ion (Na + ) and a magnesium ion (Mg 2+ ) are isoelectronic Lattice a lattice is a regular 3D arrangement of particles in space. The term is applied to metal ions in a solid, and to positive and negative ions in an ionic solid London dispersion forces the forces of attraction which result from the electrostatic attraction between temporary dipoles and induced dipoles caused by movement of electrons in atoms and molecules Lone pairs are pairs of electrons in the outer shell of an atom which take no part in bonding Miscible fluids are fluids which mix with or dissolve in each other in all proportions Periodicity is the regular recurrence of similar properties when the elements are arranged in order of increasing atomic number Polar covalent bond a covalent bond between atoms of different electronegativity, which results in an uneven distribution of electrons and a partial charge along the bond Potential energy diagram shows the enthalpy of reactants and products, and the enthalpy change during a chemical reaction Properties of a substance are their physical and chemical characteristics. These are often a reflection of the chemical bonding and structure of the material. Viscosity is the resistance to flow that is exhibited by all liquids

5 Summary notes (Scholar) Unit 2 Nature s Chemistry 2A Esters, Fats and Oils An ester can be identified from the ester group and by the name containingthe - yl-oate endings. An ester can be named given the names of the parent carboxylic acid and alcohol or from structural formulae. Structural formulae for esters can be drawn given the names of the parent alcohol and carboxylic acid or the names of esters. Esters have characteristic smells and are used as flavourings and fragrances. Esters are also used as industrial solvents. Esters are formed by the condensation reaction between carboxylic acid and an alcohol. The ester link is formed by the reaction of a hydroxyl group and the carboxyl group. In condensation reactions, the molecules join together with the elimination of a small molecule, in this case water. Esters can be hydrolysed to produce a carboxylic acid and alcohol. Given the name of an ester or its structural formula, the hydrolysis products can be named and their structural formulae drawn. The parent carboxylic acid and the parent alcohol can be obtained by hydrolysis of an ester. In a hydrolysis reaction, a molecule reacts with water breaking down into smaller molecules. Fats and oils are a concentrated source of energy. Fats and oils can be classified as animal, vegetable or marine. Fats and oils are important in a balanced diet and supply the body with energy in a more concentrated form than carbohydrates. There is evidence of a link between a high intake of saturated fat in the diet and heart disease. Fats and oils are essential for the transport and storage of fat-soluble vitamins in the body. The lower melting points of oils compared to those of fats is related to the higher degree of unsaturation of oil molecules. The low melting points of oils are a result of the effect that the shapes of the molecules have on close packing, hence on the strength of van der Waals forces of attraction. Fats and oils are esters formed from the condensation of glycerol (propane- 1,2,3-triol) and three carboxylic acid molecules. The carboxylic acids are known as fatty acids and are saturated or unsaturated straight-chain carboxylic acids, usually with long chains of carbon atoms. Bromine solution can be used to test fats and oils for the degree of unsaturation. The higher the unsaturation levels the lower the melting point. The hydrolysis of triglycerides produces one molecule of glycerol (a trihydric alcohol) and three molecules of fatty acids which can be identical to or different from each other. The fatty acids produced can be saturated or unsaturated and always contain even numbers of carbon atoms C4 to C24, primarily C16 and C18. The conversion of oils into hardened fats involves the partial removal of unsaturation by the addition of hydrogen. 2B Proteins Nitrogen is essential for protein formation by plants and animals. Proteins are the major structural materials of animal tissue. Proteins are also involved in the maintenance and regulation of life processes. Enzymes are proteins. The structure of a section of protein is based on the constituent amino acids. Amino acids, the building blocks from which proteins are formed, are relatively small molecules which all contain an amino group (NH2), and a carboxyl group (COOH). The body cannot make all the amino acids required for body proteins and is dependent on dietary protein for supply of certain amino acids known as essential amino acids. Proteins are made of many amino acid molecules linked together by condensation reactions. Condensation polymers are made from monomers with two functional groups per molecule. A small molecule is also produced as condensation occurs.

6 Summary notes (Scholar) Unit 2 Nature s Chemistry In these condensation reactions, the amino group on one amino acid and the carboxyl group on a neighbouring amino acid join together, with the elimination of water. The link which forms between the two amino acids can be recognised as an amide link (CONH) also known as the peptide link when in living things. Proteins which fulfil different roles in the body are formed by linking differing sequences of amino acids together. During digestion, enzyme hydrolysis of dietary proteins can produce amino acids. The structural formulae of amino acids obtained from the hydrolysis of proteins can be identified from the structure of a section of the protein. Chromatography can separate and identify these amino acids by comparison with a bank of known amino acids. 2C Chemistry of Cooking Food flavours mainly excite the senses of taste and smell; Molecular size and functional groups present affect the volatility of food molecules; Flavour molecules can be water- or oil-soluble, consequently cooking methods can affect the quality of the food; cooking methods might enhance or destroy the food's flavour; cooking changes (denatures) proteins, in particular it can make tough collagen palatable; different cooking methods would be appropriate for different foods. within proteins, the long-chain molecules may be twisted to form spirals, folded into sheets, or wound around to form other complex shapes; the chains are held in these forms by intermolecular bonding between the side chains of the constituent amino acids; when proteins are heated, during cooking, these intermolecular bonds are broken allowing the proteins to change shape (denature). these changes alter the texture of foods. 2D Oxidation When applied to carbon compounds, oxidation reactions result in an increase in the oxygen to hydrogen ratio. Primary and secondary alcohols can be oxidised by a number of oxidising agents, including copper(ii) oxide and acidified potassium dichromate. Primary alcohols are oxidised first to aldehydes and then to carboxylic acids. Secondary alcohols are oxidised to ketones. Tertiary alcohols are resistant to oxidation. Alkanals and alkanones are homologous series of aldehydes and ketones respectively, identified by the presence of the carbonyl functional group. They are named in a similar way to alkanols. Aldehydes and ketones can be identified from the -al and -one name endings respectively. Straight-chain and branched-chain aldehydes and ketones, with no more than eight carbon atoms in their longest chain, can be named from structural formulae. Given the names of straight-chain or branched-chain aldehydes and ketones, structural formulae can be drawn and molecular formulae written. Aldehydes, but not ketones, can be oxidised to carboxylic acids. Fehling s solution, Tollens reagent and acidified dichromate solution can be used to differentiate between an aldehyde and a ketone. Oxygen reacts with edible oils giving the food a rancid flavour. Antioxidants are molecules which will prevent these oxidation reactions taking place. Ion-electron equations can be written for the oxidation of many antioxidants.

7 Summary notes (Scholar) Unit 2 Nature s Chemistry 2E Soaps, Detergents and Emulsions production of soaps by the alkaline hydrolysis of fats and oils to form water soluble ionic salts called soaps; soap ions have a long covalent tail, readily soluble in covalent compounds (hydrophobic), and an ionic carboxylate head which is negatively charged and water soluble (hydrophilic); during cleaning using soaps and detergents, the hydrophobic tails dissolve in a droplet of oil or grease, whilst the hydrophilic heads face out into the surrounding water; agitation of the mixture results in ball-like structure forming with the hydrophobic tails on the inside and the negative hydrophilic head on the outside; repulsion between these negative charges results in an emulsion being formed and the dirt released; detergents are particularly useful in hard water areas; an emulsion contains small droplets of one liquid dispersed in another liquid. Emulsions in food are mixtures of oil and water; to prevent oil and water components separating into layers, a soap-like molecule known as an emulsifier is added; emulsifiers for use in food are commonly made by reacting edible oils with glycerol to form molecules in which either one or two fatty acid groups are linked to a glycerol backbone rather than the three normally found in edible oils; the one or two hydroxyl groups present in these molecules are hydrophilic whilst the fatty acid chains are hydrophobic; when applied to carbon compounds, reduction reactions result in a decrease in the oxygen to hydrogen ratio. 2F Fragrances essential oils are concentrated extracts of the volatile, non-water soluble aroma compounds from plants; essential oils can be extracted from suitable plant sources by steam distillation or solvent extraction; essential oils are widely used in perfumes, cosmetic products, cleaning products and as flavourings in foods; essential oils are mixtures of organic compounds; terpenes are key components in most essential oils; terpenes are unsaturated compounds formed by joining together isoprene (2-methylbuta-1,3- diene) units; terpenes are components in a wide variety of fruit and floral flavours and aromas; terpenes can be oxidised within plants to produce some of the compounds responsible for the distinctive aroma of spices. 2G Skin care ultraviolet radiation (UV) is a high-energy form of light, present in sunlight; exposure to UV light can result in molecules gaining sufficient energy for bonds to be broken; this is the process responsible for sunburn and also contributes to aging of the skin; sun-block products prevent UV light reaching the skin; when UV light breaks bonds, free radicals are formed; free radicals have unpaired electrons and, as a result, are highly reactive; free radical chain reactions include the following steps: initiation, propagation and termination; many cosmetic products contain free radical scavengers; free radical scavengers are also added to food products and to plastics.

8 Summary notes (Scholar) Unit 2 Nature s Chemistry Glossary Aldehyde an organic compound with a carbonyl functional group (C=O) at the end of the molecule Alkanals a homologous series of aldehydes based on the corresponding alkanes by changing one of the terminal carbon atoms into a carbonyl group Alkanones a homologous series of ketones based on the corresponding alkanes by changing one of the middle chain carbon atoms into a carbonyl group Amide links a group of atoms formed by condensation polymerisation of amino acids during the formation of proteins. The amide link can be identified as -CO-NH- and occurs where each pair of amino acids has joined together Condensation a condensation reaction is one in which two molecules combine to form a larger molecule at the same time eliminating a small molecule such as water Denaturing denaturing of a protein involves physical alteration of the molecular shape as a result of temperature or ph changes Electronegative electronegativity is a measure of the ability of an atom to attract a bonded pair of electrons - the more electronegative, the stronger the attraction Enzymes protein molecules which act as catalysts in biological processes Essential (in the sense of an amino acid) is a necessary material required by living organisms for normal growth which cannot be made in the body Free radicals atoms or molecule containing unpaired electrons Free radical scavengers, molecules which can react with free radicals to form stable molecules and prevent chain reactions Heterolytic fission both of the shared electrons go to only one of the two atoms producing ions Homolytic fission the two shared electrons separate equally, one going to each atom Hydrogenation the addition of hydrogen to a carbon to carbon multiple bond Hydrolysis the breakdown of a molecule by reaction with water Ketone an organic compound with a carbonyl functional group (C=O) within the carbon chain (ie. not on one of the end carbons) Oxidation when applied to carbon compounds, oxidation reactions result in an increase in the oxygen to hydrogen ratio Peptide links an amide link which is found in a living organism Polyunsaturated a polyunsaturated molecule has more than one carbon to carbon unsaturated bond Proteins biological polymers of small molecules called amino acids Redox reaction a reaction in which one reactant gains electrons and another reactant loses electrons Reduction when applied to carbon compounds, reduction reactions result in a decrease in the oxygen to hydrogen ratio Saponification the process by which soaps are made from fats and oils in a hydrolysis reaction Triglycerides molecules formed through the condensation of one glycerol molecule with three fatty acid molecules Unsaturated an unsaturated molecule has at least one carbon to carbon double bond. An unsaturated hydrocarbon does not contain the maximum number of hydrogen atoms for a given carbon atom framework Volatile a volatile substance evaporates very easily to form a gas Volatility a measure of how easily a molecule will evaporate

9 Summary notes (Scholar) Unit 3 Chemistry in Society 3A Getting the Most from Reactants Industrial processes are designed to maximise profit and minimise the impact on the environment. Factors influencing process design include: availability, sustainability and cost of feedstock(s); opportunities for recycling; energy requirements; marketability of by-products; product yield. Environmental considerations include: minimising waste; avoiding the use or production of toxic substances; designing products which will biodegrade if appropriate. Balanced equations show the mole ratio(s) of reactants and products. Using a balanced equation & gram formula masses (GFM), mass to mass calculations can be performed. The quantity of a reactant or product can also be expressed in terms of moles. The concentration of a solution can be expressed in mol l -1. Balanced equations can be used in conjunction with concentrations and volumes of solutions and/or masses of solutes to determine quantities of reactants and/or products. The molar volume (in units of l mol -1 ) is the same for all gases at the same temperature and pressure. The volume of a gas can be calculated from the number of moles and vice versa. The molar volume is the same for all gases at the same temperature and pressure. (approx. 24 l mol -1 ). The volumes of reactant and product gases can be calculated from a balanced equation using the number of moles of each reactant and product. The efficiency with which reactants are converted into the desired product is measured in terms of the percentage yield and atom economy. Percentage yields can be calculated from mass of reactant(s) and product(s) using a balanced equation. Given costs for the reactants, a percentage yield can be used to calculate the feedstock s cost for producing a given mass of product. The atom economy measures the proportion of the total mass of all starting materials successfully converted into the desired product. It can be calculated using the formula shown below in which the masses of products and reactants are those appearing in the balanced equation for the reaction. Atom Economy = (mass of desired product(s) / total mass of reactants) x 100%. Reactions which have a high percentage yield may have a low atom economy value if large quantities of unwanted by-products are formed. In order to ensure that costly reactant(s) are converted into product, an excess of less expensive reactant(s) can be used. By considering a balanced equation, the limiting reactant and the reactant(s) in excess can be identified. Whilst the use of excess reactants may help to increase percentage yields, this will be at the expense of the atom economy so an economic/environmental balance must be struck. 3B Equilibria Many reactions are reversible, so products may be in equilibrium with reactants. At equilibrium, the concentrations of reactants and products remain constant, but are rarely equal. This may result in costly reactants failing to be completely converted into products. The same equilibrium position is reached irrespective of whether starting from reactants or products. In a closed system, reversible reactions attain a state of dynamic equilibrium when the rates of forward and reverse reactions are equal. Le Chatelier's principle states that If the conditions of a chemical system at equilibrium are changed, the system responds by minimising the effect of the changes. Le Chatelier's principle can be used to predict changes in the position of equilibrium caused by a change in temperature in a reaction. Le Chatelier's principle can be used to predict changes in the position of equilibrium in a reaction involving gas molecules due to pressure changes. Le Chatelier's principle can be used to predict changes in the position of equilibrium in a reaction when concentration of reactants or products are altered. Changes in concentration, pressure and temperature can alter the position of equilibrium. Le Chatelier's principle can be used to predict changes in the position of equilibrium in a reaction. A catalyst does not alter this position, merely changes the rate at which it is attained.

10 Summary notes (Scholar) Unit 3 Chemistry in Society To maximise profits, chemists employ strategies to move the position of equilibrium in favour of products. Pressure, temperature and catalyst can be controlled in the Haber process in order to maximise a safe, cost-effective yield of ammonia. The effects of pressure and temperature, use of catalyst, removal of product and recycling of unreacted gases can be considered in relation to the conditions actually applied in the Haber process. 3C Chemical Energy For industrial processes, it is essential that chemists can predict the quantity of heat energy taken in or given out. Exothermic changes cause heat to be released to the surroundings. Endothermic changes cause absorption of heat from the surroundings. With endothermic reactions, costs will be incurred in supplying heat energy in order to maintain the reaction rate. With exothermic reactions, the heat produced may need to be removed to stop the temperature rising. Chemical energy is also known as enthalpy. The change in chemical energy associated with chemical reactions can be measured. The enthalpy change is the energy difference between products and reactants. The enthalpy change can be calculated from a potential energy diagram. The enthalpy change is negative for exothermic reactions and positive for endothermic reactions. The specific heat capacity, mass and temperature can be used to calculate the enthalpy change. The enthalpy of combustion of a substance is the enthalpy change when one mole of the substance burns completely in oxygen. These values can often be directly measured using a calorimeter and values for common compounds are available from data books and online databases for use in Hess s law calculations. Enthalpy of solution is the energy change when one mole of a substance dissolves in water. The units are kj mol -1 The enthalpy of neutralisation of an acid is defined as the energy change when it is neutralised to form 1 mole of water. The units are kj mol -1. Hess's law is used to calculate enthalpy change values that would be difficult to measure by experiment or cannot be measured directly because the reaction does not occur under normal conditions. Hess s law states that the enthalpy change for a chemical reaction is independent of the route taken, providing the starting point and finishing point is the same for both routes. Enthalpy changes can be calculated by applying Hess s law. Calculations can be carried out using enthalpy cycle diagrams and/or by using algebraic calculations from balanced equations. Algebraic calculations from balanced equations can be used to calculate enthalpy changes for reactions which are difficult to measure. If an equation is reversed, then the sign of ΔH also changes. If an equation is multiplied the value of ΔH should be multiplied by the same number. If an equation is divided the value of ΔH should be divided by the same number. e.g. Knowing the value for (1), we can write equations like (2) and (3). equation (1) C(s) + O2(g) CO2(g) ΔH = -394 kj equation (2) 2C(s) + 2O2(g) 2CO2(g) ΔH = -798 kj equation (3) ½C(s) + ½O2(g) ½CO2(g) ΔH = -197 kj For a diatomic molecule, XY, the molar bond enthalpy is the energy required to break one mole of XY bonds. Mean molar bond enthalpies are average values which are quoted for bonds which occur in different molecular environments. Bond enthalpies can be used to estimate the enthalpy change occurring for a gas phase reaction by calculating the energy required to break bonds in the reactants and the energy released when new bonds are formed in the products.

11 Summary notes (Scholar) Unit 3 Chemistry in Society 3D Oxidising and Reducing Agents A redox reaction is a reaction in which reduction and oxidation occur together, reduction being the gain of electrons by a reactant and oxidation being the loss of electrons by a reactant in a reaction. An oxidising agent is a substance which accepts electrons. A reducing agent is a substance which donates electrons. Oxidising and reducing agents can be identified in redox reactions. Elements with low electronegativities (metals) tend to form ions by losing electrons (oxidation) and so can act as reducing agents. Elements with high electronegativities (non-metals) tend to form ions by gaining electrons (reduction) and so can act as oxidising agents. The strongest reducing agents are found in Group 1. The strongest oxidising agents come from Group 7. The electrochemical series indicates the effectiveness of oxidising and reducing agents. Compounds can also act as oxidising or reducing agents. Electrochemical series contain a number of ions and molecules. The dichromate and permanganate ions are strong oxidising agents in acidic solutions whilst hydrogen peroxide is an example of a molecule which is a strong oxidising agent. Carbon monoxide is an example of a gas that can be used as a reducing agent. Oxidising and reducing agents can be selected using an electrochemical series from a data booklet or can be identified in the equation showing a redox reaction. Oxidising agents are widely employed because of the effectiveness with which they can kill fungi and bacteria, and can inactivate viruses. The oxidation process is also an effective means of breaking down coloured compounds making oxidising agents ideal for use as bleach for clothes and hair. Oxidation and reduction reactions can be represented by ion-electron equations. When molecules or group ions are involved, if the reactant and product species are known, a balanced ion-electron equation can be written by adding appropriate numbers of water molecules, hydrogen ions and electrons. Ion-electron equations can be combined to produce redox equations. Displacement reactions are example of redox reactions and oxidising and reducing agents can be identified in these and other redox reactions. The technique of titration can be applied to redox reactions, allowing the concentration of a reactant to be calculated from results of volumetric titrations. 3E Analysis and Researching Chemistry in chromatography, differences in the polarity / size of molecules are exploited to separate the components present within a mixture; depending on the type of chromatography in use, the identity of a component can be indicated either by the distance it has travelled or by the time it has taken to travel through the apparatus (retention time); the results of a chromatography experiment can sometimes be presented graphically showing an indication of the quantity of substance present on the y-axis and retention time on the x-axis. Note: You are not required to know the details of any specific chromatographic method or experiment. volumetric analysis involves using a solution of accurately known concentration in a quantitative reaction to determine the concentration of another substance a solution of accurately known concentration is known as a standard solution the volume of reactant solution required to complete the reaction is determined by titration calculations from balanced equations can then be carried out to calculate the concentration of the unknown solution redox titrations are based on redox reactions substances such as potassium permanganate(vii), which can act as their own indicators, are very useful reactants in redox titrations the concentration of a substance can be calculated from experimental results by use of a balanced equation quality control of chemical processes requires analysis to ensure that the process requirements are met.

12 Summary notes (Scholar) Unit 3 Chemistry in Society Glossary Bond enthalpies bond enthalpy is the amount of energy needed to break one mole of a bond in a gaseous molecule Chromatography an analytical method where mixtures are separated into their components by partitioning between a stationary and mobile phase. The stationary/mobile phases are solid/liquid in paper and thin layer chromatography, and liquid/gas in gas-liquid chromatography. Dynamic equilibrium a dynamic equilibrium is achieved when the rates of two opposing processes become equal, so that no net change results Endothermic reactions absorb heat energy from the surroundings End-point the point at which the reaction is just complete Enthalpy change for a reaction is defined as the change in heat energy when 1 mole of reactant is converted to product(s) at constant pressure, and has the symbol ΔH and units of kj mol -1 Enthalpy of combustion is the enthalpy change that occurs when 1 mole of a substance is burned completely in oxygen Enthalpy of neutralisation is the energy change (in kj) when an acid is neutralised to form 1 mole of water Enthalpy of solution is the energy change (in kj) when 1 mole of the substance dissolves in water Equilibrium chemical equilibrium is the state reached by a reaction mixture when the rates of forward and reverse reactions have become equal Exothermic reactions release heat energy, which is given up to the surroundings Feedstocks a feedstock is a reactant from which other chemicals can be extracted or synthesised. Feedstocks are themselves derived from raw materials either by physical separation or by chemical reaction Formula unit the term 'formula unit' is a general term. A formula unit may be an atom (for all elements which do not exist as diatomic molecules), a molecule (for all covalent molecular substances) or the simplest ratio of atoms or ions (for network or lattice substances). Hess's law the enthalpy change for a chemical reaction is independent of the route taken, providing the starting point and finishing point is the same for both routes Ion-electron equation a half-equation, either an oxidation or a reduction, which in combination of the opposite type, can be part of a complete redox equation Molar volume the molar volume is the volume occupied by one mole of a substance. For gases, the units used are l mol -1 Oxidation an oxidation is a loss of electrons by a reactant in any reaction Oxidising agent an oxidising agent is a substance which accepts electrons Potential energy diagram shows the enthalpy of reactants and products, and the enthalpy change during a chemical reaction Reducing agent a reducing agent is a substance which donates electrons Reduction a reduction is a gain of electrons by a reactant in any reaction Retention time the time taken for an individual peak to traverse the gas-liquid chromatographic column after the injection time Specific heat capacity relates the energy change in a liquid to the change in temperature. For water it has a value of 4.18 kj kg -1 C -1. In other words, when 1 kg of water absorbs 4.18 kj of heat its temperature will rise by 1 C Standard solution a solution of accurately known concentration Theoretical yield the theoretical yield is the maximum possible amount of product in a reaction, i.e. all of the reactant(s) have been converted into product Titration determines the volume of reactant solution required to react completely with the test solution Volumetric analysis involves analysis using a solution of accurately known concentration in a quantitative reaction to determine the concentration of another substance