CHAPTER 9 PAGE 111

COMPLETE CHEMISTRY FOR CAMBRIDGE IGCSE: ANSWERS

EDUCATALYST 1

CHAPTER 9 ENERGY CHANGES & REVERSIBLE REACTIONS

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1 a. exothermic (but needs energy to start it off) b. exothermic c. endothermic 2 kilojoule (kJ) 3 It is exothermic. 4 See the diagrams on pages 110 and 111.


EDUCATALYST 2

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1 Bonds must be broken, then new bonds must form. 2 The energy that has to be put in to break bonds is greater than the energy given out when new bonds form. 3 2H–H + O = O → 2H–O–H 4 – 486 kJ / mole

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1 a. i. A good fuel gives out plenty of energy. So, on the energy level diagram the products should be at a much lower energy level than the reactants.

ii. A very poor fuel will give out very little energy, so the energy gap should be very small.

b. Think about: pollution, reliability of supply, ease and safety of storage and use, cost. 2 Hydrogen gives out most heat per gram, and produces only water. This makes it an attractive fuel. (But it is highly flammable, so safety is an issue.) 3 2C4H10 (g) + 13O2 (g) → 8CO2 (g)+ 10H2O (l)


EDUCATALYST 3

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1 No. Electrons must be given up during the reaction, to supply a current. So it must be a redox reaction. 2 the more reactive one 3 H+ ions (from water) accept electrons at the copper strip, to form hydrogen gas. 4 No, the two strips must be of different metals. The more reactive metal will give up electrons, to form a current. 5 a. hydrogen b. At the negative pole, hydrogen and OH– ions (from the electrolyte) react together, releasing electrons. These flow through the wires to the positive pole where they take part in a reaction between oxygen and water, to form more OH– ions. c. The ions in it carry the current, to complete the circuit.


EDUCATALYST 4

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1 one that can go backwards as well as forwards 2 anhydrous copper(II) sulfate + water → hydrated copper(II) sulfate 3 heat it (to expel the water of crystallisation) 4 It will turn blue (as water is driven off). 5 Dynamic means the forward and back reactions are both going on. But they are going on at exactly the same rate, so the quantities of reactants and products do not change. A balance or equilibrium hasbeen reached. 6 a. N2 + 3H2 → 2NH3, and 2NH3 → N2 + 3H2. b. No. Equilibrium is not established until the forward and back reactions are taking place at the same rate.


EDUCATALYST 5

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1 Because it is not possible to convert all the nitrogen and hydrogen to ammonia. So special steps must be taken to increase the yield. 2 If a reversible reaction has reached a state of equilibrium and you change a condition, it will act to oppose the change, and establish a new equilibrium. 3 a. The system acts to oppose the change by reducing the number of molecules present – so more ammonia forms. b. When ammonia is removed, the system acts to restore equilibrium by forming more ammonia. 4 a. 2SO2 + O2 ⇌ 2SO3. b. i. The yield increases (because an increase in pressure favours the side of the equation with fewer molecules). ii. The yield decreases (because a rise in temperature favours the back reaction, which is endothermic).


EDUCATALYST 6

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Core

1 a. neutralisation

b. It gives out energy.

c. It will rise.

d. The diagram should be like the one on page 110, but with NaOH (aq) and HCl (aq) as reactants, and NaCl (aq) and H2O (l) as products.

2 a. beaker, spatula, thermometer

b. a fall of 4°C for ammonium nitrate, and a rise of 20°C for calcium chloride

c. i. calcium chloride

ii. because of the temperature rise

iii. The energy level of the ions in solution is lower than their energy level in the solid.

d. The answers are:


EDUCATALYST 7

3 a. to condense water vapour b. The blue solid turns white. c. It can go backwards as well as forwards. d. Add water to the white solid left in the test-tube after heating. e. CuSO4.5H2O (s)⇌CuSO4 (s) + 5H2O (l)

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Extended 4 a. The energy given out when new bonds form is greater than the energy needed to break bonds. b. i.


EDUCATALYST 8

ii. Some energy must be put in, to start the bonds breaking. A spark or flame can provide this energy. c. i. Energy is given out, overall. ii. exothermic d. 55.6 kJ 5 a. to remove impurities (for example the magnesium is likely to be coated with a fine layer of magnesium oxide) b. sulfuric acid c. because electrons were flowing through it d. magnesium e. Chemical energy is being changed into electrical energy. f. a cell g. The acid could spill out – and it is corrosive. The set-up would be difficult to carry around. 6 a.


EDUCATALYST 9

Bonds broken: 1 N–N, 4 N–H, 1 O=O b. New bonds formed: 1 N≡N, 4 O–H. c. i. 2220 kJ/mol ii. 2801 kJ/mol d. – 581 kJ/mol e. exothermic f. from the chemicals to the surroundings. g. Since energy is released during combustion, it could be used as a fuel. (It is used in some rocket fuels.) But it is corrosive, unstable, and highly toxic, which means it is dangerous. 7 a. i and iii b. The number of molecules is the same on both sides of the equation, so equilibrium will not shift in response to a change in pressure. c. Increasing the pressure will speed up both the forward and back reactions, so equilibrium is reached faster. (The molecules are pushed closer together so more collisions occur.)


EDUCATALYST 10

8 a. N2 (g) + 3H2 (g)⇌2NH3 (g) b. exothermic; the energy change has a negative sign c. i. In this reaction, 4 molecules of reactants give 2 molecules of product. When pressure is increased, molecules are pushed into a smaller space. So the system acts to oppose this change by forming fewer molecules (that is, more product). ii. Raising the temperature favours the endothermic reaction – in this case the back reaction. So the yield falls. d. i. The rate at which ammonia is made increases. (Increasing the pressure increase the rate of both reactions. This is because molecules are pushed closer together, so more collisions occur.)

ii. The rate at which ammonia is made increases. (Increasing the temperature increases the rate of both reactions, because collisions have more energy, so more are successful.) e. to give an acceptable rate 9 a. The yellow solution will turn orange. b. Add alkali, to remove the H+ ions in a neutralisation reaction. The colour change will reverse

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CHAPTER 9 PAGE 111