1. 1
   A student adds solid sodium hydroxide pellets to a beaker of water and stirs with a thermometer. The temperature of the solution rises from 19°C to 31°C over one minute. Later that day, the same student dissolves a different salt, ammonium nitrate, in water using an identical method, and records the temperature falling from 20°C to 13°C.

   (a)

   Which term correctly describes this change?

   [1 mark]

   • AEndothermic, because heat energy is taken in from the surroundings
   • BExothermic, because heat energy is given out to the surroundings
   • CEndothermic, because heat energy is given out to the surroundings
   • DNeutral, because no heat energy is transferred

   (b)

   Which statement correctly classifies the dissolving of ammonium nitrate described above, and gives the correct reason?

   [1 mark]

   • AExothermic, as heat energy is given out to the solution
   • BEndothermic, as heat energy is given out to the surroundings
   • CEndothermic, as heat energy is taken in from the surroundings
   • DExothermic, as the solution becomes colder

   (c)

   Explain, in terms of bond breaking and bond making, why dissolving sodium hydroxide releases heat energy while dissolving ammonium nitrate absorbs heat energy.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A technician places 50 cm³ of copper sulfate solution into a polystyrene cup with a lid and records a starting temperature of 21°C. She adds an excess of zinc powder, stirs continuously with a thermometer, and records the highest temperature reached as 68°C. The blue colour of the solution fades as a red-brown solid forms.

   (a)

   Which term correctly describes this displacement reaction?

   [1 mark]

   • AEndothermic, because the temperature rises
   • BNeutral, because a colour change occurs rather than a temperature change
   • CEndothermic, because zinc displaces copper
   • DExothermic, because heat energy is released to the solution and surroundings

   (b)

   Which statement correctly explains, in terms of bond energy, why this displacement reaction is exothermic overall?

   [1 mark]

   • AThe energy released when new bonds/attractions form in the products is greater than the energy required to break bonds/attractions in the reactants
   • BThe energy required to break bonds/attractions in the reactants is greater than the energy released when new bonds/attractions form in the products
   • CNo bonds are broken or formed during a displacement reaction
   • DThe reaction is exothermic only because a solid catalyst is present

   (c)

   Explain why using a polystyrene cup with a lid, rather than an open glass beaker, gives a more accurate measurement of the heat energy change in this reaction.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Hydrogen gas reacts with chlorine gas to form hydrogen chloride gas: H2 + Cl2 -> 2HCl. The bond energies are: H-H = 436 kJ per mole, Cl-Cl = 242 kJ per mole, H-Cl = 431 kJ per mole.

   (a)

   Using the idea of activation energy, describe what happens to the reacting particles as this reaction proceeds, and state whether the overall reaction profile would show the products at a higher or lower energy level than the reactants.

   [3 marks]

   (b)

   Using the bond energies given, calculate the overall energy change for the reaction H2 + Cl2 -> 2HCl. Show your working and state whether the reaction is exothermic or endothermic.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A group of students plans an investigation to compare the heat energy changes of two neutralisation reactions: dilute hydrochloric acid mixed with sodium hydroxide solution, and dilute ethanoic acid mixed with sodium hydroxide solution. Their method is to place 25 cm³ of acid into an open glass beaker, record the starting temperature, add 25 cm³ of the sodium hydroxide solution in one go, stir briefly with a thermometer, and record the highest temperature reached. They plan to carry out each reaction once.

   (a)

   Explain, in terms of bond breaking and bond making and using the idea of activation energy, why neutralisation reactions between an acid and an alkali are exothermic overall.

   [6 marks]

   (b)

   Evaluate the students' planned method for comparing the heat energy changes of the two neutralisation reactions, and suggest improvements that would make their comparison more valid and reliable.

   [6 marks]

   Total for question 4: 12 marks

End of questions