Year 11 Double Award Chemistry. Bangor Revision Guide

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1 Year 11 Double Award Chemistry Bangor Revision Guide

Materials and Bonding Bonding and Structure When a chemical reaction occurs new bonds are formed. The can form by the transfer of electrons or by the sharing of electrons.

2 Materials and Bonding Bonding and Structure When a chemical reaction occurs new bonds are formed. The can form by the transfer of electrons or by the sharing of electrons. Ionic Bonding Charged particles called ions are formed when electrons are transferred between atoms during chemical bonding. e.g. When sodium chloride (NaCl) forms, one electron is transferred to chlorine. This will form a full stable outer shell (like noble gasses) for the two particles ( atom ). Before bonding Electron Transfer Na Cl After bonding One electron lost One electron gained Proton(+) = 11 Electron(-) = 10 Charge = +1 POSITIVE ION Proton(+) = 17 Electron(-) = 18 Charge = -1 NEGATIVE ION Strong Electrostatic attraction between 2,8 the two ions 2,8,8 Covalent Bond When hydrogen gas(h 2 ) forms electrons are shared between two atoms to form a molecule. There is no charge on molecules. Electrons share to form a full outer shell 10 GCSE Science: Chemistry 2

3 Materials and Bonding Ionic Bonding Here is the information needed to draw ionic bonding diagrams. Check the ionic charges by using the ions table at the back of the examination paper. Sodium Chloride (NaCl) Group 1 with Group 7 Before Bonding Electron Transfer After bonding Strong electrostatic charge between the ions cryf rhwng y ddau ion Magnesium oxide (MgO) Group 2 with Group 6 Before Bonding Electron Transfer After bonding Strong electrostatic charge between the ions cryf rhwng y ddau ion MgO has a higher melting point than NaCl as the charges are greater there is more attraction between ions GCSE Science: Chemistry 2 11

4 Materials and bonding Lithium Oxide (Li 2 O) Group 1 with Group 6 Before Bonding Electronic Transfer After bonding Strong electrostatic charge between the ions cryf rhwng y ddau ion Magnesium Chloride (MgCl 2 ) Group 2 with Group 7 Before Bonding Electronic Transfer After bonding Strong electrostatic charge between the ions cryf rhwng y ddau ion 12 GCSE Science: Chemistry 2

5 Materials and Bonding Covalent Bonds Hydrogen (H 2 ) Electrons share to form a full outer shell Chlorine (Cl 2 ) Electrons share to form a full outer shell Water (H 2 O) Electrons share to form a full outer shell Hydrogen Chloride (HCl) Electrons share to form a full outer shell GCSE Science: Chemistry 2 13

6 Materials and Bonding Covalent Bonding Ammonia (NH 3 ) Methane (CH 4 ) Covalent examples with double bonds (Higher Tier) Oxygen (O 2 ) Carbon Dioxide (CO 2 ) Ethene (C 2 H 4 ) 14 GCSE Science: Chemistry 2

sodium chloride, magnesium oxide), High Melting and Boiling points Solubility = Dissolved in water

carbon dioxide, water) Low Melting and Boiling points Solubility = Dissolved in water CO 2 H 2 O

7 Materials and Bonding Simple and Giant structures Giant ionic structure(e.g. sodium chloride, magnesium oxide), High Melting and Boiling points Solubility = Dissolved in water Simple molecular structure (e.g. carbon dioxide, water) Low Melting and Boiling points Solubility = Dissolved in water CO 2 H 2 O (water) CH 4 (methane) As the forces between molecules are weak the melting and boiling points are low Giant covalent structure (e.g. diamond, graphite), Very High Melting and Boiling points Solubility = Does not dissolve in water Diamond Graphite GCSE Science: Chemistry 2 15

8 Materials and Bonding Diamond and Graphite Carbon atoms linked covalently to four other carbon atoms Carbon atoms Carbon atoms linked covalently to three other carbon atoms Diamond Graphite Properties related to uses Graphite Sea of free moving electrons because the fourth electron is unbonded. Appearance Grey/black shiny solid Hardness very soft Conductivity Conducts electricity Melting point Very high over 3600ºC Uses Pensil lubricants Diamond Uses Appearance Transparent/crystalline Gemstones Hardness very hard Glass cutting, Drill bits Conductivity Electrical insulator Melting point Very high over 3500ºC Carbon nanotubes They are roles of carbon hexagons similar to graphite. They conduct electricity / used in semi-conductors The have a very small diameter which is about 10,000 times less than a human hair. They are extremely strong. Very low density They are proposed to be used in small electronic circuits 16 GCSE Science: Chemistry 2

Materials and Bonding Materials The uses of everyday materials depends on their

Aluminium Ceramic Diamond Low density Hardness, strength Hardness, lusture Aluminium

The properties of all materials, are determined by:- the types of atoms present, the

giant structures with free electrons Metallic bonds are strong, so metals can maintain a

Outer shell electrons of metals are free to move.

9 Materials and Bonding Materials The uses of everyday materials depends on their properties. Aluminium Ceramic Diamond Low density Hardness, strength Hardness, lusture Aluminium Oxide Teflon Glass Low friction and wear Low friction high Mpt Hardness transmits light The properties of all materials, are determined by:- the types of atoms present, the types of bonding between the atoms, and the way the atoms are packed together Metals are giant structures with free electrons Metallic bonds are strong, so metals can maintain a regular structure and usually have high melting and boiling points. Outer shell electrons of metals are free to move. The strength of bond in a metal is the force of attraction between the metal ions and free moving electrons. More free electrons and more protons in the ions increase the strength of a metal. GCSE Science: Chemistry 2 17

Metals are also malleable (hit into shape) and ductile (drawn into wires) because the free electrons allow the metal atoms to slide over each other.

10 Materials and Bonding Metallic bonding Free electrons allow electricity to be carried as well as heat energy. Outer electron free to move Sodium metal These electrons allow metals to conduct electricity and conduct heat. Metals are also malleable (hit into shape) and ductile (drawn into wires) because the free electrons allow the metal atoms to slide over each other. Having more free electrons in the outer shell e.g. Aluminium compared to sodium above and more protons in each nucleus the forces of attraction for the free electrons is greater. This makes the metal stronger. More outer electron free to move. Aluminium metal More protons in each ion attracting more electrons 18 GCSE Science: Chemistry 2

The materials properties change with a change in the surroundings, such as changes in temperature, light, ph.

11 Smart Materials Smart Materials The term smart material has been given to a range of modern materials. A variety of smart materials exist which can change shape and colour, retain shape after bending and can expand greatly with different liquids. This means that their properties change. The materials properties change with a change in the surroundings, such as changes in temperature, light, ph. Thermochromic Paint This smart material has the ability to change colour with a change in temperature. The boat seen in this t shirt appears because thermochromic paint has been used. Under cold conditions the pigments are white, but when heated in warm weather, or if the person becomes warmer the pigments change colour to reviles a picture of a boat. Photochromic Paint This material has the ability to change colour with a change in the light strength. The sunglass lenses become darker when exposed to strong light and become lighter in weak light GCSE Science: Chemistry 2 19

Smart Materials Shape memory alloy This smart material is a mixture of metals (alloy) that retains its original shape when heated A mixture of nickel and titanium make up the alloy called NiTi or

Stents are metal structures that can be inserted in veins to prevent them from sticking together.

12 Smart Materials Shape memory alloy This smart material is a mixture of metals (alloy) that retains its original shape when heated A mixture of nickel and titanium make up the alloy called NiTi or nitinol. This metal can be bent into any shape at low temperature, but when heated it can remember its original shape so it bends back very quickly. It can be used as a coffeepot thermostat. Stents are metal structures that can be inserted in veins to prevent them from sticking together. The stents are cooled to below 37C so they change shape and become thinner, when inserted into the vein it warms up to body temperature and changes shape to open the vein. This alloy can also be used in super elastic spectacle frames. These retain their original shape after bending them. Shape memory polymer This smart material is a form of plastic that can retain its original shape when heated. These could be used for:- Surgical stutures are threads of smart polymer that can tighten to the right tension automatically when heated. Car bumpers could be made from this material. If the car body such as bumpers were dented, on heating they would regain their original shape. 20 GCSE Science: Chemistry 2

13 Smart Materials Polymer gels This smart material is a form of plastic with cross linkage (see diagram below) that can swell or shrink with different liquids. No cross linkage across polymer Cross linkage across polymer These gels can swell to 1000 times their volume depending on the temperature or ph. Artificial snow this smart material expands greatly by adding water. It can also shrink by heating. Nappies this smart material is similar to artificial snow and expands greatly when it becomes wet. Contact Lens within these lenses there is a smart material which prevents them from drying up. They can then be used for weeks instead of days. Artificial muscles gels can be used to swell and shrink creating an artificial muscle. Robot actuators gels can be used to swell and shrink creating movement. Toxic Chemical absorber gels can be used to block dangerous chemicals in the body. GCSE Science: Chemistry 2 21

Titrations You can find out the number of moles that react together in solutions using a technique called titration. In titration experiments, you use a burette to add one solution to another.

Example: A student was asked to carry out a titration to check the concentration of the sodium hydroxide using the apparatus below Burette 0.

21 Titrations You can find out the number of moles that react together in solutions using a technique called titration. In titration experiments, you use a burette to add one solution to another. You need a way to decide when the reaction is complete - The end point of reaction. Indicators are used to determine end points in acid/alkali titrations. Example: A student was asked to carry out a titration to check the concentration of the sodium hydroxide using the apparatus below Burette 0.2 mol dm -3 Hydrochloric acid (HCl) Methyl orange in alkali Conical flask 25 cm 3 of sodium hydroxide (NaOH) solution and indicator End point - neutral Some practical points: When solution are made up they are made up using a volumetric flask A volumetric pipette is used to accurately measure the solutions that go into the conical flask When you fill the burette make sure that you remove the funnel afterwards it could affect the level of solution in the burette and your results. A white tile is placed under the flask so that the colour change is easier to see. Volumetric flask Volumetric pipette 23

22 In the example the acid is added slowly by means of a burette, the volume of acid needed to change the indicator colour is recorded. It is easy to overshoot the end point the first time (turning the mixture acidic rather than neutral); the first titration is normally a practice run. Results: Volume of Hydrochloric acid added (cm 3 ) Titration Average * Calculating the average= Titration = = cm * Titration 1 not used in average as it is not reliable (practice run) Brain teasers: 1. A student reacted 25 cm 3 of 1 mol dm -3 HCl with 25 cm 3 of 1 mol dm -3 NaOH, is the solution alkali, acidic or neutral? 2. A student reacted 25 cm 3 of acid (HCl) with 20 cm 3 alkali (NaOH) to make a neutral solution, which had the highest concentration, the acid or the alkali? 3. A student reacted 10 cm 3 of acid (HCl) with 40 cm 3 alkali (NaOH) to make a neutral solution, which had the weakest concentration, the acid or the alkali? Answers; 1 neutral, 2 alkali, 3 alkali 24

23 Using titration to prepare pure solutions Once you have established the end point of a titration you can repeat the experiment without an indicator to obtain a pure sample of the compound. NaOH + HCl NaCl + H 2 O In the reaction on page 23, using an indicator it was found that exactly cm 3 of the hydrochloric acid was required to neutralise cm 3 of the sodium hydroxide alkaline solution. The experiment could be repeated without an indicator to make a pure solution which could be further evaporated to give pure sodium chloride salt Step 1: Neutralisation Step 2: Evaporation Evaporating dish NaCl and H 2 O 20 cm 3 of Hydrochloric acid (HCl) Evaporated until 1/3 solution remains 25 cm 3 of sodium hydroxide (NaOH) solution without indicator Step 3: Evaporate slowly to dryness Left to evaporate at room temp/near window to obtain pure crystals of sodium chloride (NaCl) 26

24 Energy in reactions Changes in temperature happen often during chemical reactions. Exothermic reactions Endothermic Reaction A reaction where temperature rises. e.g. magnesium and acid A reaction where temperature falls. e.g. ammonium nitrate and acid Here is a reaction for making ammonia Hydrogen + Nitrogen Ammonia 3 H 2 (g) + N 2 (g) 2 NH 3 (g) To create the product ammonia its is necessary to break bonds between hydrogen and nitrogen (the reactants). To break bonds energy in required. 436kJ Creating bonds releases energy. 388 kj 388 kj 436kJ 388 kj 436kJ 944kJ 388 kj 388 kj 388 kj Bonds Broken = 2252kJ Bonds Made 388 x 6 = 2328kJ In an endothermic reaction the number would be positive. Overall Energy Change = Bonds broken Bonds made = -76kJ Exothermic Reaction - Because energy is released (negative number) 32 GCSE Science: Chemistry 2

25 Chemical Calculations Every atom has different mass. This is determined by the number of protons and neutrons in the nucleus. A lithium atom has a mass of 7. 3 protons and 4 neutrons Relative atomic mass (A r ) is a way of saying how heavy different atoms are compared to each other. The A r of Lithium is 7 and that of Carbon is 12. We use the top number to determine this; this is called the mass number Relative formula mass or relative molecular mass (M r ) is the mass for a compound (e.g. MgCl 2 ) so the masses for each element are Mass numbers Mg Cl Cl = 94 What is the molecular mass of ammonium sulphate (NH 4 ) 2SO 4? (N=14, S=32, O=16, H=1) Calculate (NH 4 ) 2 first = = 18 x 2 = 36 S= 32 4 oxygen atoms 16 x 4 = 64 Mr= 132 Calculating % composition After calculating M r it is possible to calculate % composition, this shows how much of a specific element is in a compound in percentage form e.g. % Mg in MgCl 2 = total M r of Mg in MgCl 2 x 100 M r MgCl 2 24 x 100 = 25.5 % 94 GCSE Science: Chemistry 2 33

26 Chemical Calculations Calculating Reacting Masses By using relative atomic masses and (Ar) and relative molecular masses (Mr) it is possible to calculate how much of a product is produced or how much reactants are needed. e.g. (product calculation) What is the mass of magnesium oxide is produced when 60g of magnesium is burned in air? Symbol Equation 2Mg + O 2 2MgO Mr = (24+16) Therefore 48g (or tonnes) will produce 80g 1g = 1.67g 60g will produce = 100.2g e.g. (reactant calculation) What is the mass of magnesium needed to produce 90g of magnesium oxide? 2Mg + O 2 2MgO Mr = (24+16) Therefore 48g (or tonnes) will produce 80g Or 80g of MgO will be produced with 48g of Mg 1g 90g 48 80=0.6g will produce = 54g 34 GCSE Science: Chemistry 2

27 Calculations Determining the formula of a compound from experimental data When 4 g of copper oxide is reduced in a steam of hydrogen, 3.2 g of copper remains. 1. Work out how much oxygen was contained in the compound = 0.8 g Cu O Divide with Ar Divide with smallest Whole number 1 1 Example 2 1 Cu 1 O Formula = CuO Find the formula of iron oxide produced when 44.8g of iron react with 19.2g of oxygen. (Ar Fe = 56 and O = 16) Fe O Mass Divide with Ar Divide with the smallest value A formula must have whole numbers therefore 2 3 Formula = Fe 2 O 3 GCSE Science: Chemistry 2 35

28 Chemical Calculations Calculating reactants or product masses Reactants Products NaOH + HCl NaCl + H 2 O Units g / tones g / tones Calculating the percentage yield When we want to create a chemical, the aim is to work carefully and to produce the maximum amount possible. The amount formed or yield is calculated in percentage. It is very unlikely that 100% yield will be achieved e.g. some might be stuck in filter paper, evaporating dish, the product might react with the air. Example Magnesium metal dissolves in hydrochloric acid to form magnesium chloride. Mg (s) + 2HCl (aq) MgCl 2(aq) + H 2(g) 24g 95g ( ) (a) What is the maximum theoretical mass of magnesium chloride which can be made from 12g of magnesium? 12g 95/2 = 47.5g (b) If only 47.0g of purified magnesium chloride was obtained after crystallising the salt from the solution, what is the % yield of the salt preparation? % yield = actual amount obtained x 100 maximum possible 47.0 x % yield = = 98.9% (to 1 decimal place) 36 GCSE Science: Chemistry 2

for sodium ion (Na + )

for sodium ion (Na + ) 3.4 Unit 2 Chemistry 2 Throughout this unit candidates will be expected to write word equations for reactions specified. Higher tier candidates will also be expected to write and balance symbol equations