Reaction Rates (OCR AS Chemistry A): Exam Questions

Which statement(s) explain(s) why reaction rates increase as temperature increases?

1. The activation energy is less.

2. Collisions between molecules are more frequent.

3. A greater proportion of molecules have energy greater than the activation energy.

The Maxwell-Boltzmann distribution of molecular energies in a sample of a gas at a given temperature is shown.

If the temperature is increased, what will happen to the position of point X?

The dotted-line curve on each graph below represents the corresponding Boltzmann distribution for a gas at 200 K.

Which solid-line curve most accurately represents the Boltzmann distribution of molecular energies in the same gas at 400 K?

The Boltzmann distribution of the kinetic energies of molecules in a slowly reacting gaseous mixture, without a catalyst, at a given temperature, is shown.

The activation energy for the reaction, E_a, is marked for the uncatalysed reaction.

What would the position of E_a be if the reaction took place with an effective catalyst?

The diagram shows a Boltzmann distribution of molecular energies for a gaseous mixture. The distribution has a peak, labelled Q on the diagram.

What happens when an effective catalyst is added to the mixture?

A student performs two reactions and measures the rate of product formation.

• Reaction 1: 1.5 g of solid calcium carbonate is added to 100 cm³ of 0.5 mol dm^-3 hydrochloric acid.

• Reaction 2: 100 cm³ of distilled water is added to 100 cm³ of 0.5 mol dm^-3 hydrochloric acid and then 1.5 g of solid calcium carbonate is added.

The rate of reaction 1 was faster than the rate of reaction 2.

Which of the following 3 hypotheses correctly describes the difference in the rate as a result of adding water?

The Boltzmann distribution of molecular energies in sample 1 of a gas is shown.

A change was made to the reaction conditions, as shown by the dotted curve, labelled sample 2.

What factor was changed in the reaction conditions to produce the dotted curve, labelled sample 2?

The Boltzmann distribution shows the number of molecules that have particular kinetic energy at a constant temperature.

If the temperature is decreased by 10 °C, what happens to the size of the areas labelled H, J and K?

The addition of a catalyst to a reaction provides an alternate mechanism with

Consider the following reaction:

NaOH (aq) + HCl (aq) → H₂O (l) + NaCl (aq)

Which species could a student monitor the concentration of, in order to determine the rate of reaction?

For the reaction of sulfuric acid and sodium hydroxide, the standard enthalpy change of neutralisation is -62 kJ mol^-1 and the energy from bond-making is -157 kJ mol^-1.

Which of the following statements is/are correct?

1. The value for energy from bond breaking is less than the value of energy from bond making.

2. The standard enthalpy change of neutralisation is H₂SO₄ (aq) + 2NaOH (aq) → Na₂SO₄ (aq) + 2H₂O (l)

3. The activation energy is 219 kJ mol^-1.

A nucleophilic substitution reaction takes place between 2-bromopropane and sodium hydroxide in a total volume of 300 cm³. An organic compound and 0.32 g of an inorganic salt are formed in 30 seconds.

Which row is correct?

Iron is an important catalyst in many reactions including the Haber process. The reaction between peroxodisulfate(VI) ions (S₂O₈^2-) and iodide ions uses a Fe²⁺ catalyst.

Which of the following statements is/are correct?

1. In the peroxodisulfate(VI) ion and iodide reaction, iron is oxidised from Fe²⁺ to Fe³⁺ with the electron configurations 1s²2s²2p⁶3s²3p⁶3d⁶ and 1s²2s²2p⁶3s²3p⁶3d⁵ respectively.

2. Iron is a heterogeneous catalyst in the Haber process and will shift the equilibrium to the right.

3. Adding a catalyst increases the proportion of molecules with energy greater than or equal to the activation energy, and the peak of the Boltzmann distribution curve decreases in height and moves to the right, increasing the number of effective collisions.

In an investigation of the reaction between hydrochloric acid and magnesium, the following graphs were obtained.

Which of the following statements is/are correct?

1. As the reaction progresses the concentration of the reactants decreases so the rate of reaction also decreases.

2. As the concentration increases the amount of heat given out also increases.

3. As the concentration of HCl doubles the rate of reaction will double.

Which statement about the molecules in a gas is correct?