1. 04/24/2016 Topic 5 – Chemical Reactions

2. 04/24/201604/24/16 Endothermic and exothermic reactions Step 1: Energy must be SUPPLIED to break bonds: Step 2: Energy is RELEASED when new bonds are made: A reaction is EXOTHERMIC if more energy is RELEASED then SUPPLIED. If more energy is SUPPLIED then is RELEASED then the reaction is ENDOTHERMIC Energy

3. 04/24/2016 Examples you need to know Combustion Photosynthesis ExothermicEndothermic Ammonium Nitrate and water (ice packs)

4. 4 Heat is given off during a reaction.This shows the reaction is(1) A a decomposition reaction B an endothermic reaction C an exothermic reaction D a precipitation reaction 04/24/2016

5. 5 (d) When ammonium nitrate crystals are dissolved in water, heat energy is taken in. A student puts some water in a beaker. He puts a thermometer in the water. He adds some ammonium nitrate crystals and stirs the mixture. Describe what the student would see. (2)

6. 6 04/24/2016

7. 7 Exam example: When zinc reacts with copper sulfate solution, zinc sulfate solution and copper are formed. (i) An experiment was carried out to measure the temperature change when zinc powder reacts with copper sulfate solution. initial temperature of copper sulfate solution = 20 °C final temperature of mixture after the reaction = 46 °C Explain what the temperature readings show about the type of heat change that occurs during this reaction.(2)

8. 8 04/24/2016

9. 9 Rates of reactions How to increase the rate of a reaction The rate of a reaction increases if: The temperature is increased The concentration of a dissolved reactant is increased The pressure of a reacting gas is increased Solid reactants are broken into smaller pieces A catalyst is used Different reactions can happen at different rates. Reactions that happen slowly have a low rate of reaction. Reactions that happen quickly have a high rate of reaction. Collisions: http://www.bbc.co.uk/schools/gcsebitesize/science/add_edexcel/chemical_reactions/ratesr ev3.shtml http://www.bbc.co.uk/schools/gcsebitesize/science/add_edexcel/chemical_reactions/ratesr ev3.shtml

10. 10 04/24/2016

11. Catalysts Summary Words – surface area, speed up, used up, cheaper, monoxide Effect of catalysts A catalyst is a substance that can increase the rate of a reaction. The catalyst itself remains unchanged at the end of the reaction it catalyses. Catalytic converters Modern cars have a catalytic converter to help reduce the production of toxic gases. Catalytic converters use a platinum and rhodium catalyst with a high surface area. This increases the rate of reaction of carbon monoxide and unburnt fuel from exhaust gases with oxygen from the air. Carbon monoxide + oxygen carbon dioxide

12. 12 (i) For a reaction to take place the reacting particles must (1) A dissolve B boil C collide D evaporate (ii) The reaction can be slowed down by (1) A using a bigger volume of the same acid B cooling the hydrochloric acid C increasing the concentration of the hydrochloric acid D adding a catalyst (iii) When zinc powder is used, instead of larger pieces of zinc, the reaction is faster. Explain, using ideas about particles, why the reaction is faster when zinc powder is used. (2) 04/24/2016 12

13. 13 04/24/2016

14. (b) A length of magnesium ribbon was added to excess hydrochloric acid. The time for all of the magnesium to react was recorded. The experiment was repeated with the same lengths of magnesium ribbon but different concentrations of the acid. The graph shows the time taken for the magnesium to react with different concentrations of this acid. Use the graph to explain how the rate of this reaction changes as the concentration of hydrochloric acid increases. (2) 04/24/2016 14

15. 04/24/2016

16. Model 6 Marks Question Describe experiments to investigate what effect using smaller marble chips has on the rate of this reaction. (6) 5-6 MARKS BRIEF

17. 04/24/2016 Measuring the Rate of Reaction Two common methods:

18. 04/24/2016

19. Topic 6 – Quantitative Chemistry

20. 04/24/201604/24/16 Mass and atomic number revision ParticleRelative MassRelative Charge Proton1+1 Neutron10 ElectronVery small He 2 4 MASS NUMBER = number of protons + number of neutrons SYMBOL PROTON NUMBER = number of protons (obviously)

21. 04/24/2016 Relative formula mass, M r The relative formula mass of a compound is the relative atomic masses of all the elements in the compound added together. E.g. water H 2 O: Therefore M r for water = 16 + (2x1) = 18 Work out M r for the following compounds: 1)HCl 2)NaOH 3)MgCl 2 4)H 2 SO 4 5)K 2 CO 3 H=1, Cl=35 so M r = 36 Na=23, O=16, H=1 so M r = 40 Mg=24, Cl=35 so M r = 24+(2x35) = 94 H=1, S=32, O=16 so M r = (2x1)+32+(4x16) = 98 K=39, C=12, O=16 so M r = (2x39)+12+(3x16) = 138 Relative atomic mass of O = 16Relative atomic mass of H = 1

22. 22 Calculate the relative formula mass of iron chloride, FeCl 3. (Relative formula masses: Cl = 35.5, Fe = 56) (2) 04/24/2016 Exam example

23. Empirical formula: all you need to learn is the steps Empirical formulae is simply a way of showing how many atoms are in a molecule.

24. 04/24/2016 Example questions 1)Find the empirical formula of magnesium oxide which contains 48g of magnesium and 32g of oxygen. 2)Find the empirical formula of a compound that contains 42g of nitrogen and 9g of hydrogen. 2)Find the empirical formula of a compound containing 20g of calcium, 6g of carbon and 24g of oxygen. MgO NH 3 CaCO 3

25. 25 04/24/2016 You may also be asked to simplify molecular formulas (This is also the empirical formula) C 2 H 6 C 5 H 10 C 4 H 16 CH 3 CH 2 CH 4

26. 04/24/2016 Calculating percentage mass Calculate the percentage mass of magnesium in magnesium oxide, MgO: Mass of magnesium = 24 Mass of oxygen = 16 Mass of magnesium oxide = 24 + 16 = 40 Therefore percentage mass = 24/40 x 100% = 60% Percentage mass (%) = Mass of element A r Relative formula mass M r x100%

27. 04/24/201604/24/16 Recap questions Work out the percentage mass of: 1)Carbon in carbon dioxide CO 2 2)Calcium in calcium oxide CaO 3)Hydrogen in methane CH 4 27% 71% 25% Percentage mass (%) = Mass of element A r Relative formula mass M r x100%

28. 28 Oxygen from the air reacted with the hot copper to form copper oxide. These are Rosie's results mass of copper used = 3.2 g mass of copper oxide formed = 3.6 g Calculate the percentage of oxygen in copper oxide, CuO. (Relative atomic masses: Cu = 64, O = 16) 04/24/2016 Exam example 28 Percentage mass (%) = Mass of element A r Relative formula mass M r x100%

29. 04/24/2016 Percentage Yield Percentage yield = actual yield (in g) x 100 theoretical yield Theoretical yield = the amount of product that should be made as calculated from the masses of atoms Actual yield = what was actually produced in a reaction

30. 30 04/24/2016 Percentage yield = actual yield (in g) x 100 theoretical yield Example question: 65g of zinc reacts with 73g of hydrochloric acid and produces 102g of zinc chloride. What is the percentage yield? Theoretical Yield = 65 + 73 = 138g Actual Yield = 102g Percentage yield = (102/138) x 100 = 74%

31. 04/24/2016 Percentage yield Percentage yield = Actual yield Predicted yield X 100% 70% 13% 150g 1)The predicted yield of an experiment to make salt was 10g. If 7g was made what is the percentage yield? 2)Dave is trying to make water. If he predicts to make 15g but only makes 2g what is the percentage yield? 3)Sarah performs an experiment and has a percentage yield of 33%. If she made 50g what was she predicted to make?

32. 32 (c) If calcium carbonate is heated strongly, it decomposes. CaCO3 → CaO + CO2 If 100 g of calcium carbonate is heated a calculation shows that 44 g carbon dioxide should be formed. (i) In an experiment 100 g of calcium carbonate was heated and only 40 g carbon dioxide was formed. Calculate the percentage yield of carbon dioxide in this reaction. (2) Percentage yield of carbon dioxide =........................................................... % 04/24/2016 40/44 (1) (= 0.909) (any fraction) x 100 (1) (= 90.9 / 90.91 / 91(%))