Chemistry WebTeachers 2010

Transcription

1 Chemistry WebTeachers 2010

2 Particle Theory & Properties All matter exists in one of 3 states - solid, liquid or gas, dependent on the temperature of the substance. A substances is: Solid at temperatures below its melting point Liquid at temperatures between its melting point and its boiling point Gas at temperatures above its boiling point All substances have their own melting point, boiling point and density which define them. Melting Point - The temperature at which a solid becomes hot enough to melt and become a liquid. Boiling Point - The temperature at which a liquid is hot enough to boil and become a gas. Density - This is a measure of how much mass of a substance is contained in a given unit volume (density = mass/volume). Put simply, if mass is a measure of how much ʻstuffʼ there is in an object, density is a measure of how tightly that ʻstuffʼ is packed together. Expansion, Gas Pressure & Diffusion Expansion Substances expand when they are heated up and contract when they are cooled down. This property can be useful e.g. thermometer but can also cause problems e.g. bridges and railway lines need to have special joints with gaps for the materials to expand into in hot weather. When substances expand, their particles stay the same size but the space between the particles changes as the particles move about more - in solids the particles vibrate more, in liquids the particles move around each other more and in gases the particles move around more quickly in all directions. In each case, this increase in movement takes up more room and hence the substance expands. Gas Pressure When gas particles hit the walls of their container they cause pressure. The more particles that hit the walls, the higher the pressure. If you heat a gas its particles move more quickly and collide Solids Particles in a fixed, regular pattern, forces between particles are strong, particles are very closely packed & vibrate, definite shape & volume, not easily compressed, do not flow. Liquids Particles in an irregular patter, forces between particles are strong, particles close together but move freely, definite volume, take shape of container, not easily compressed but flow easily Gases Particle pattern irregular, forces between particles are very weak, particles are far apart & move fast, particles fill the container they are in, are easily compressed and flow easily. CLASSIFYING MATERIALS Solids, Liquids and Gases with the container walls more often and harder, so the pressure increases. The pressure also increases if you make the container smaller, as the particles will collide more often with the walls. Diffusion Diffusion is the spreading out of particles in a gas or liquid. An example of this is if you release a smelly gas e.g. turn on a gas tap, at one end of a room, you can soon smell it at the other end of the room without having to mix the air yourself. This is because the gas particles move very quickly in amongst the air particles in all directions and so spread out rapidly across the room. Diffusion also happens in liquids but slower, because there isnʼt the same space and the particles do not travel as fast. Before After Heating causes the particles in a solid to vibrate more & eventually they have enough energy to break their bonds (melt) to become a liquid. Further heating will cause more movement of particles and eventually the liquid will boil as the particles ʻjumpʼ free of the liquid state into the gas state. Changes of State Solid Liquid Gas Atoms and Molecules Atoms Everything is made from atoms, which are tiny particles that are far too small to see, even with a microscope. An atom has a nucleus at the centre (containing protons & neutrons) and electrons moving about in the space surrounding the nucleus, orbiting it. Molecules When two or more atoms are chemically joined together they form a molecule. Molecules can be a combination of the same type of atom e.g. oxygen (O2), or they can be a combination of atoms of different kinds e.g. water (H2O). When we talk about the particles of a substance we are actually referring to the molecules, as a single molecule of a substance can exist on its own, i.e. a single molecule of water is still water, even on its own. A molecule of oxygen (O2) Cooling causes the particles in a gas to sow down & eventually they donʼt have enough energy and condense to become a liquid. Further cooling will cause less movement of particles and eventually the liquid particles will bond (freeze) to form the solid state, where the particles just vibrate. O O

3 Elements & The Periodic Table A substance whose molecules are made up of just one kind of atom is called an element. Elements are pure substances that cannot be split into simpler substances. They are listed on the Periodic Table and each element is represented by a symbol e.g. O for oxygen, C for Carbon etc. When different elements chemically combine during a chemical reaction they form compounds. Here are some examples of elements: Element Magnesium Symbol Mg Metals and non-metals The elements in the Periodic Table can be classified as metals or non-metals due to their properties. Metals have many common properties whilst non-metals have much more varied properties. Examples of metallic elements: iron, copper, aluminium, zinc Examples of non-metallic elements: oxygen, carbon (carbon can take different forms), sulphur, nitrogen. Properties of Elements Metals Good electrical conductors Good thermal conductors Usually strong Shiny Flexible Some are magnetic Non-metals Poor electrical conductors Poor thermal conductors Usually weak Dull Brittle None are magnetic Copper Calcium Hydrogen Cu Ca Formation of Compounds Compounds are substances whose molecules are made up of the atoms of two or more different elements, that have joined together during a chemical reaction. Compounds have chemical properties that are different to the properties of the elements from which they are made. They are chemical combinations of elements in fixed proportions and can be represented by a chemical formula e.g. H2O for water, showing the proportions of atoms involved. The Rules for Naming Compounds There are three key rules to remember the right way to name compounds: 1) When two elements combine the compound ends in ʻideʼ. e.g. sodium chloride 2) When two or more elements combine with oxygen the compound ends in ʻateʼ. e.g. copper sulphate 3) When two identical elements combine the name of the substance doesnʼt change. e.g. hydrogen H CLASSIFYING MATERIALS Elements, Compounds & Mixtures Formulae and Word Equations A word equation gives a simple summary of a chemical reaction between two or more substances. e.g. Magnesium + Oxygen Magnesium Oxide A chemical formula uses symbols to represent the substances in a chemical reaction and in the compound formed, and shows how many atoms of each substance are involved e.g. 2Mg + O2 2MgO Compound Sodium Chloride (salt) Carbon Dioxide Glucose Chemical Formula (one molecule) NaCl CO2 C6H12O6 Most are solids at room temperature Usually have a high melting point Solid, liquid or gas at room temperature Usually have a low melting point Mixtures & Their Separation A mixture is a collection of substances that are not chemically combined. The substances that make up mixtures can be easily separated using a wide range of separation techniques (e.g. filtration, distillation, chromatography), as there are no chemical bonds to break. Examples of mixtures: the air, salt water, sand and salt, ink in water Element 1 Element 2 Compound 1 Compound 2 Mixture of elements & compounds

4 What are Physical Changes? Physical changes are changes in the condition of a substance, but not changes in its chemical properties. Physical changes do not produce new substances and they are usually easy to reverse. Different substances have different physical properties, such as their appearance and their melting and boiling points. So, physical changes in a substance involve changes such as melting, boiling and change in colour. The substance does not change its chemical properties. For example, water is still H2O whether it is in the form of ice, water or steam - its chemical properties are the same, it just looks different due to its state, depending on the temperature. The changes of state shown earlier are typical examples of physical changes. Energy Transfers during State Change When heat energy is supplied to a substance, its temperature increases but if you view a graph of this (a heating curve) you will see that the curve isnʼt smooth all the way up. At the points where the substance changes from solid to liquid and from liquid to gas, there are plateaus (flat bits) on the graph. These flat sections show where the heat energy supplied is being used to break the bonds, rather than raise the temperature. Cooling curves show similar flat bits where bonds are forming as heat is removed. Evaporation Boiling happens all through a liquid but evaporation happens at its surface. Some particles in a liquid may have enough energy to break their bonds and escape from the surface as gas. Evaporation happens faster when the liquid is hotter and is maximum at boiling point. Conservation of Mass The mass of the substance stays the same during a physical change. For example, if you freeze 100g of water, you get 100g of ice. And if you boil 100g of water, you get 100g of steam. This is called the conservation of mass. e.g. ICE 100g e.g. WATER 100g CHANGING MATERIALS Physical Changes Subliming e.g. STEAM 100g Sublimation is the change of state from solid to gas or from gas to solid without going through the liquid stage. It is quite rare, only happening in certain substances e.g. carbon dioxide, iodine. The State of a Substance As different substances have different melting and boiling points, You can easily work out the state of a substance if you know its current temperature, its melting point and its boiling point: If the temperature is below the melting point, the substance will be a solid. If the temperature is between the melting point and the boiling point, the substance will be a liquid. If the temperature is above the boiling point, the substance will be a gas. Dissolving and Solubility Some substances will dissolve - they mix completely with a liquid e.g. water, to form a solution in which their molecules have spread out completely amongst the liquid molecules (the substance hasnʼt disappeared, but it may look like it has!) Other substances do not dissolve and are said to be insoluble. Here are some important terms to do with dissolving: Soluble - a substance that can dissolve. Solution - the mixture formed when a substance dissolves. Solute - the substance that dissolves Solvent - the liquid in which the solute dissolves Saturated - if a lot of solute is added to the solvent, eventually no more will dissolve. This is then called a saturated solution. Sediment - at the bottom of a saturated solution this is a layer of undissolved solute. Solubility - the ability of a substance to dissolve in another substance. Solubility usually increases with temperature meaning that more of a solute will dissolve if the temperature of the solvent is greater. The mass of a solution is equal to the combined mass of the solute plus the solvent g Solvent 2g Solute 102g Solution

5 The Water Cycle About Water Water is a common chemical substance that is essential for the survival of all known forms of life. In typical usage, water refers only to its liquid form or state, but the substance also has a solid state, ice, and a gaseous state, water vapor. Water covers 71% of the Earth's surface, mostly in oceans and other large water bodies. Clean, fresh water is essential to human and other life. Water comprises 50 to 70 per cent of an adult's total body weight, and without regular top-ups, our body's survival time is limited to a matter of hours or days. It is vital for a number of biological processes. Water has a huge number of uses making it a very important substance. Water can act as a solvent (particularly useful for cleaning), but not everything dissolves in it. It is used in agriculture, for drinking and wastewater services, as a heat transfer fluid, for extinguishing fires, for chemical uses, for recreation, for power generation, in food processing etc. Pure water has a melting point of 0 º C, a boiling point of 100 º C and a density of 1g/cm 3. The Water Cycle shows the three states of matter in everyday situations, with all three states possibly on display at the same time. It is a continuous exchange of water between the atmosphere, soil water, surface water, groundwater, and plants. Heat CHANGING MATERIALS Physical Changes - Water Testing for Water If water is pure it will boil at 100 º C whilst if it contains impurities the boiling and melting points will change (this is why salt is spread on roads in freezing conditions - it changes the melting point so that the temperature has to drop much further before the water on the roads turns to ice). To test for the presence of water you can use anhydrous copper sulphate, which is a white powder that turns blue in the presence of water. To find out if a clear liquid produced during a chemical reaction is water, we can test it with blue cobalt chloride paper, which turns pink if there is water present. Comparing Types of Water As water can be found in so many different places and situations on the Earth, it is useful to know in what ways the contents of water may differ, depending on where samples are taken from. Some situations may be obvious due to the content of water being clearly visible e.g. muddy water. But, if you took relatively clear samples of sea water, river water, rain water and pure water (distilled), how could the differences be examined and what would you expect? The most obvious comparison test would be to heat the same mass of each sample and view what contents remain once all of the water has evaporated. The mass of the residue left behind could be weighed to see which on the samples contained the greatest amount of impurities. Clearly, out of the four samples, sea water should contain the most mass of impurity due to its salt content. River water is likely to have the next highest impurity content, as the water making up rivers has flowed over land and through rocks etc. Rain water should produce very little mass of impurities and pure water should not have any. If the rain water does not produce any mass after evaporation, you can always tell it apart from pure water due to its ph (which will show rain to be slightly acidic due to what it has absorbed from the atmosphere), or by checking its boiling point.

6 What are Chemical Reactions? A chemical reaction is a change that produces one or more new substances. A chemical reaction always starts with initial substances, called the reactants, which react together to form new substances, called the products. Chemical reactions usually produce permanent (irreversible) changes. Examples of chemical reactions are respiration, photosynthesis, iron rusting, baking a cake, burning fuels etc. In each case, new substances are produced and it is very difficult (if not impossible) to get the products back into the original state of the reactants. e.g. magnesium + oxygen The Bunsen Burner The Bunsen Burner is used to provide the heat for experiments in the laboratory. It is a device which causes an exothermic reaction between natural gas (or bottled gas), the fuel, and oxygen from the air. When lit, the gas/air mixture burns to produce a hot flame which can be controlled using the collar at the lower part of the barrel. The collar acts like a valve to control the amount of air that mixes with the gas and can be moved round to open or close the air hole. magnesium oxide With the air hole open, a very hot, blue, clean and roaring flame is produced (shown above). A closed air hole produces a yellow, smokey, wavy, quiet flame, which isnʼt as hot. Conservation of Mass When a chemical reaction takes place, the overall mass of the reactants remains the same when they react to form the products. In other words the overall mass stays the same throughout the reaction. There is no gain or loss of atoms in a chemical reaction, they just join together in different ways. For example, if you burn carbon in air it produces carbon dioxide gas. This is because the carbon combines with the oxygen in the air. The overall mass of the carbon plus the oxygen is equal to the mass of the carbon dioxide that is produced. e.g. carbon + oxygen carbon dioxide 3g 7g 10g CHANGING MATERIALS Chemical Reactions Everyday Chemical Reactions There are many chemical reactions that happen regularly in everyday life, some of which are useful whilst others cause problems. Examples of useful chemical reactions include smelting (extracting metals from ores), fermentation (used to make alcohol, bread and medicines), neutralisation of acid soils and combustion (used for heating and producing electricity). Examples of less useful chemical reactions are rusting, food going ʻoffʼ and pollution due to combustion (which causes global warming and acid rain). Recognising Chemical Change When a chemical reaction occurs, usually one or more of the following takes place: a change in colour a change in temperature a gas is given off (fizzing or bubbling) a solid may be formed when two solutions are mixed together These visible changes should allow you to recognise that a chemical reaction has taken place. Types of Chemical Reactions These are the main types of chemical reaction you need to know about: combustion - a substance burns oxidation - the addition of oxygen reduction - the removal of oxygen decomposition - breakdown of a compound exothermic reactions - heat energy given out endothermic reactions - heat energy taken in neutralisation - acid reacts with base or alkali displacement - a metal displaces a less reactive metal from its compound. Details of each of these can be found in the ʻKey Knowledgeʼ section Burning Fossil Fuels Fuels are substances that release energy when they burn. Fossil fuels are substances such as coal, oil and natural gas which formed millions of years ago from the remains of living things. Fossil fuels are non-renewable energy resources - once they have all been used up they cannot be replaced. When a fossil fuel is burned, the contents of the fuel react with the oxygen in the air to form oxides. Many fuels such as natural gas, oil etc. are hydrocarbons; when they burn they produce the oxides of hydrogen (H2O) and carbon (CO2). This can be seen in the laboratory if you burn a candle (wax is a hydrocarbon) and test the products of the reaction for water and carbon dioxide. See the ʻKey Knowledgeʼ section for more details of this experiment. Of course, burning fossil fuels also produces pollution due to the CO2 released.

7 The Reactivity Series How strongly metals react with other substances depends on the type of metal involved in the reaction. Some metals are highly reactive whilst others are not reactive at all. The Reactivity Series lists metals in order of their reactivity towards other substances: Metals will react with other substances more or less vigorously depending on their position in the reactivity series. The least reactive metals show very little, if any reaction with other substances, whilst the metals at the top of the series may react violently. Reactions of Metals Reaction of Metals with Water When a metal reacts with water, a metal hydroxide and hydrogen are formed. The general word equation for the reaction is: metal + water metal hydroxide + hydrogen Reaction of Metals with Air When metals react with oxygen in the air, compounds called metal oxides are produced, e.g. magnesium reacts with oxygen to produce magnesium oxide. The general word equation for this is: Some metals do not easily react with water, even though they are more reactive than hydrogen, e.g. Aluminium and zinc. These will react with steam though, to produce their oxide + hydrogen. Iron and steel objects rust if in contact with both water and oxygen. metal + oxygen metal oxide Reaction of Metals with Acids When a metal reacts with an acid, a salt and hydrogen are formed. The general word equation for the reaction is: Non-metals but used for comparison Extraction of Metals The most reactive metals are difficult to extract from their ores and the ones above carbon in the reactivity series require electrical energy to separate them from the ore (electrolysis). The less reactive metals such as zinc, iron and lead can be extracted using a reduction reaction with carbon e.g. extracting iron using the blast furnace. The unreactive metals such as silver and gold can be found lying in the ground and in rivers, not requiring a reaction for extraction. Uses of Metals Metals are used in all sorts of ways but tend to be chosen for a particular function due to the properties they display. For example, copper is used for wiring due to its electrical conductivity, iron is used in construction (iron girders etc.) due to its strength, whilst gold and silver are used for jewelry due to their appearance and lack of reactivity. PATTERNS OF BEHAVIOUR Metals When metals react with compounds of less reactive metals a displacement reaction occurs. When this happens the more reactive metal displaces the less reactive metal from its compound. e.g. iron + copper sulphate Displacement iron sulphate + copper In general, the further apart the two metals are in the reactivity series, the bigger the reaction. There will be no reaction if the more reactive metal is already in the compound. When displacement reactions occur you may observe changes in colour, heat being generated (displacement is exothermic), the less reactive metal coating the more reactive metal etc. metal + acid salt + hydrogen The salt formed depends on the metal and the acid used. Hydrochloric acid forms chloride salts, sulphuric acid forms sulphates and nitric acid forms nitrates. e.g. zinc + nitric acid zinc nitrate + hydrogen In each of the three types of reactions of metals, silver, gold and platinum do not react at all. In the metal reactions that produce hydrogen, you can test for the gas produced using the usual gas test (see ʻKey Knowledgeʼ section). Blue copper sulphate fades as iron sulphate forms in its place.

8 Acids and Bases Acids are substances that have a ph of less than 7 and can be corrosive. Examples of acids include hydrochloric acid, sulphuric acid, citric acid (lemon juice), acetic acid (vinegar) and many others. Acids can be neutralised by alkalis or bases to form a salt and water. Acids can be formed when non-metal oxides (e.g. carbon dioxide, sulphur dioxide) dissolve in water. Bases are usually metal oxides, metal hydroxides or metal carbonates (e.g. copper oxide, sodium hydroxide, calcium carbonate) which can neutralise an acid to form a salt and water (and sometimes CO2). Alkalis are formed when bases dissolve in water and have a ph greater than 7. Alkalis can be corrosive. Indicators & The ph Scale Indicators are substances that change colour when they are added to acidic solutions or alkaline solutions. You can make homemade indicators using red cabbage or beetroot juice. Litmus and universal indicator are two indicators that are commonly used in the laboratory. Red litmus paper turns blue in alkaline solutions and blue litmus paper turns red in acidic solutions. Universal Indicator can show us how strong an acidic or alkaline solution is, due to the colour change when compared to the ph sclae: Neutralisation & Salt Formation If you mix together an acid and a base, a chemical reaction called neutralisation takes place. A neutral solution is made if you add just the right amount of acid and base together. The general word equation for this type of reaction is: acid + base salt + water The salt produced depends on the acid and base used. Hydrochloric acid forms chloride salts, sulphuric acid forms sulphates and nitric acid forms nitrates. e.g. hydrochloric acid + sodium hydroxide sodium chloride +water Using carbonates & hydrogen carbonates If the base is a metal carbonate or metal hydrogen carbonate, we still get salt and water when we neutralise it with acid, but CO2 is also produced: acid + metal carbonate Acids in the Environment salt + water + carbon dioxide Rain is naturally slightly acidic due to carbon dioxide in the air. Carbon dioxide dissolves in the rainwater to produce carbonic acid. Pollutants such as sulphur dioxide and oxides of nitrogen are acidic and when they mix with water in the atmosphere they produce acid rain. Reactive metals corrode when they react with the acid rain which weakens the metal producing a salt plus hydrogen. Acid rain also weathers rocks like limestone, chalk and marble which can cause damage to buildings, statues and cliffs (see below). Of course it also damages trees and plants and affects rivers, ponds and streams, killing the living organisms in them. PATTERNS OF BEHAVIOUR Acids and Bases Everyday Applications of Neutralisation There are some useful everyday applications for neutralisation reactions: Your stomach contains hydrochloric acid, and too much of this causes indigestion - antacid tablets can be used to settle your stomach because they contain bases such as magnesium hydroxide and magnesium carbonate to neutralise the extra acid. Bee stings are acidic but can be neutralised using baking powder, which contains sodium hydrogen carbonate. Farmers use lime (calcium oxide) to neutralise acid soils, which then allows plants to grow properly in those soils. Chemical Weathering Minerals in rocks may react with the rainwater, causing a neutralisation reaction, which weathers the rocks. Some types of rock are easily weathered by chemicals. For example, limestone and chalk are made of the mineral calcium carbonate (a base). When acidic rainwater falls on limestone or chalk, a salt, water and CO2 are formed. The salt and water are washed away and the rock is broken down. Chemical weathering can make caves form and cliffs fall away.

9 Separation Techniques Filtration This is used for separating an insoluble solid from a liquid (an insoluble substance does not dissolve in water). For example, sand can be separated from a mixture of sand and water using filtration. The solid residue (sand) remains in the filter paper while the water (filtrate) passes through. Evaporation This is used to separate a soluble solid from the solvent it is dissolved in e.g. separating salt from water - heating the salt water solution causes the water to evaporate leaving the salt crystals behind in the basin. Chromatography This is used for separating dissolved substances that have different colours, such as inks and plant dyes. It works because some of the coloured substances dissolve in the liquid better than others, so they travel further up the paper. Distillation This is used for separating two or more liquids from each other. For example, ethanol (alcohol) can be separated from a mixture of ethanol and water by distillation. This method works because the two liquids have different boiling points.the Liebigʼs Condenser is used to cool the hot gas which condenses back to a liquid.

10 Preparation of Gases and Gas Tests Preparation of hydrogen gas Test for hydrogen gas Example: zinc + hydrochloric acid ===> zinc chloride + hydrogen Preparation of carbon dioxide gas Test for carbon dioxide gas Example: calcium carbonate + hydrochloric acid ===> calcium chloride + water + CO2 Preparation of oxygen gas Test for oxygen gas