Chapter 3 Practice Paper: Chemical Energetics — question paper

Section A: Multiple-choice questions[10 marks]

Choose the one correct answer for each question.

1. 1

   Which observation always accompanies an exothermic reaction carried out in a beaker?

   1. Athe temperature of the mixture rises
   2. Bthe mixture changes colour
   3. Ca gas is released
   4. Dthe mixture becomes more dilute
   ( )
2. 2

   Which reaction has a positive ΔH?

   1. Athe burning of hydrogen in oxygen
   2. Bthe neutralisation of nitric acid by potassium hydroxide
   3. Cthe decomposition of silver bromide by light in photography
   4. Drespiration in yeast
   ( )
3. 3

   A catalyst speeds up a reaction because it

   1. Aincreases the enthalpy change
   2. Bprovides a pathway of lower activation energy
   3. Craises the temperature of the reactants
   4. Dincreases the concentration of the reactants
   ( )
4. 4

   When bonds in the reactants are broken, the process is

   1. Aexothermic and releases energy
   2. Bendothermic and absorbs energy
   3. Cneutral in energy terms
   4. Donly possible with a catalyst
   ( )
5. 5

   Hydrogen scores over petrol as a fuel chiefly because its combustion

   1. Areleases more heat per gram with only water formed
   2. Bneeds no ignition source
   3. Cproduces carbon dioxide for greenhouses
   4. Dcan be stored loose at room pressure
   ( )

Section B: Structured questions[20 marks]

Answer all questions in the spaces provided. Include state symbols where the question asks, and balance every equation.

1. 1

   A reaction has an energy profile in which the reactants sit on a shelf at 80 kJ and the products at 155 kJ, while the curve between them climbs to a maximum of 220 kJ before falling again.

   1. (a)

      Calculate ΔH for the reaction and state whether it is exothermic or endothermic.

      [2]
   2. (b)

      Calculate the activation energy of the uncatalysed reaction.

      [1]
   3. (c)

      A catalyst lowers the peak to 180 kJ. State the new activation energy and the new ΔH.

      [2]
   4. (d)

      Sketch-in-words how the catalysed curve compares with the uncatalysed one on the same axes.

      [1]
2. 2

   50 cm³ of 2.0 mol/dm³ hydrochloric acid is mixed with 50 cm³ of 2.0 mol/dm³ potassium hydroxide. The temperature rises by 13.6 °C. (c = 4.2 J/(g °C); density of mixture 1.0 g/cm³; the neutralisation is 1 : 1.)

   1. (a)

      Calculate the heat released, in kJ.

      [2]
   2. (b)

      Calculate the moles of water formed.

      [2]
   3. (c)

      Calculate ΔH for the neutralisation in kJ/mol.

      [2]
3. 3

   Bond energies: H–H = 436 kJ/mol, O=O = 498 kJ/mol, O–H = 464 kJ/mol. The reaction is 2H₂ + O₂ → 2H₂O.

   1. (a)

      Find the energy absorbed when every bond inside the reactant molecules is pulled apart.

      [2]
   2. (b)

      Find the energy given out as every bond inside the product molecules is assembled.

      [2]
   3. (c)

      Calculate ΔH and classify the reaction.

      [1]
4. 4

   A camping stove burns propane, C₃H₈. A student closes the air vents to simmer the flame.

   1. (a)

      Propane (C₃H₈) burns fully in a well-adjusted gas ring. Construct the balanced equation for this reaction.

      [1]
   2. (b)

      Name the toxic gas that forms when the vents are closed, and explain why it is dangerous.

      [2]