OCR AS Chemistry

Topic Questions

ASChemistryOCRTopic Questions3. Periodic Table & Energy3.5 Reaction Rates

3.5 Reaction Rates

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11 mark

Which of the following factors can affect the value of the activation energy of a reaction?

 

The presence of a catalyst

Changes in temperature

Changes in the concentration of the reactants

A

✓

⨯

⨯

B

✓

✓

⨯

C

⨯

⨯

✓

D

✓

⨯

✓

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21 mark

Why does a mixture of hydrogen gas and chlorine gas react faster at a temperature of 600 K than at 500 K?

 

A higher proportion of successful collisions occurs at 600 K

Hydrogen molecules and chlorine molecules collide more frequently at 600 K

The activation energy of the reaction is lower at 600 K

A

✓

⨯

⨯

B

✓

✓

⨯

C

⨯

⨯

✓

D

✓

✓

✓

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31 mark

This energy profile diagram represents the reaction pathway for the following reaction:

Y (g) + Z (g) → W (g) + X (g)10-3

Which statement is true about the reverse reaction, W(g) + X(g) → Y(g) + Z(g)?

  • It will have a negative ΔH

  • It will have a positive ΔH

  • It will have a smaller activation energy

  • The temperature of the surroundings increase

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41 mark

This question concerns the Maxwell-Boltzmann energy distribution shown below.

l6MxV2dH_6

How does the curve change when the temperature is increased?

  • Stays the same

  • Peak height decreases and moves to the left

  • Peak height increases and moves to the right

  • Peak height decreases and moves to the right

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51 mark

The Maxwell–Boltzmann distribution for a sample of a gas, Z, at two different temperatures, T1 and T2, is shown.

eVlqSWD9_8

Which letter represents the mean energy of molecules at a higher temperature?

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61 mark

The Maxwell-Boltzman distribution of molecular energies in a sample of gas is shown.

h9t9An5R_2

What does the area underneath the curve represent?

  • The number of particles which can react with each other

  • The total number of particles

  • The total energy of the particles

  • The total number of particles that have activation energy

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71 mark

The Maxwell-Boltzman distribution of molecular energies in a sample of gas is shown.

YUhzs3Q~_3

What effect would adding a catalyst have on the activation energy, Ea?

  • Move to the left and the peak would be unchanged

  • Move to the left and the peak would move to the right

  • Move to the left and the peak would move to the left

  • The activation energy would be unchanged and the peak move to the right

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81 mark

Which of these statements is / are true for the Haber process?

N2 + 3H2 rightwards harpoon over leftwards harpoon 2NH3    ΔH = −92.4 kJ mol–1 

  1. Increasing the temperature will increase the rate of reaction
  2. Increasing the temperature will increase the yield of ammonia
  3. An iron catalyst will increase the yield of ammonia

  • 1, 2 and 3

  • Only 1 and 2

  • Only 2 and 3

  • Only 1

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91 mark

Which of the following does not increase the rate of reaction by affecting the frequency or nature of collisions?

  • Addition of a catalyst

  • Increasing the concentration of a reactant

  • Increasing the pressure 

  • Increasing the temperature

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101 mark

Which of these statements is / are true for the Haber process?

N2 + 3H2 rightwards harpoon over leftwards harpoon 2NH3    ΔH = −92.4 kJ mol–1 

  1. A catalyst will shift the equilibrium to the right and increase the rate.
  2. An increase in pressure will shift the equilibrium to the right and increase the rate.

  3. An increase in temperature will shift the equilibrium to the left and increase the rate.

  • 1, 2 and 3

  • Only 1 and 2

  • Only 2 and 3

  • Only 1

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11 mark

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.

  • 1, 2 and 3

  • Only 1 and 2

  • Only 2 and 3

  • Only 1

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21 mark

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?

 

Organic product formed

Rate of reaction (mol dm-3 min-1)

A

Propan-1-ol

2.07 x 10-2

B

Propan-2-ol

3.46 x 10-4

C

Propan-1-ol

3.46 x 10-4

D

Propan-2-ol

2.07 x 10-2

 

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31 mark

Iron is an important catalyst in many reactions including the Haber process. The reaction between peroxodisulfate(VI) ions (S2O82-) and iodide ions uses a Fe2+ catalyst.

  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 the E  Ea and the peak of the Boltzmann distribution curve decreases in height and moves to the right increasing the number of effective collisions.

  • 1, 2 and 3

  • Only 1 and 2

  • Only 2 and 3

  • Only 1

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41 mark

In an investigation of the reaction between hydrochloric acid and magnesium, the following graphs were obtained.
ocr-as-3-5h-mcq-q4-rate-graphs

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.

  • 1, 2 and 3

  • Only 1 and 2

  • Only 2 and 3

  • Only 1

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51 mark

Which statement about the molecules in a gas is correct?

  • Most molecules have the mean energy

  • At a certain temperature, the average kinetic energy of the molecules is constant

  • As temperature increases, more molecules have the most probable energy

  • As temperature decreases, fewer molecules have the mean energy

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11 mark

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.

  • 1, 2 and 3

  • Only 1 and 2

  • Only 2 and 3

  • Only 1

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21 mark

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

3-5-reaction-rates-q2

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

  • Fewer molecules possess the most probable energy value so X will shift higher and to the left

  • Fewer molecules possess the most probable energy value so X will shift lower and to the right

  • More molecules possess the most probable energy value so X will shift to the left

  • The position of X will stay the same but the area under the distribution curve increases

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31 mark

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

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

3-5-reaction-rates-q3-variants

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41 mark

The Maxwell-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, Ea, is marked for the uncatalysed reaction.

3-5-reaction-rates-q4

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

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51 mark

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

3-5-reaction-rates-q5

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

  • The height of the peak remains the same and the activation energy moves to the right

  • The height of the peak decreases and the activation energy moves to the left

  • The height of the peak remains the same and the activation energy moves to the left

  • The height of the peak decreases and the activation energy moves to the right

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61 mark

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?

  Frequency of collisions between reactant molecules is reduced Proportion of effective collisions between reactant molecules is reduced Proportion of reactant molecules possessing the activation energy is reduced
A X X ✓
B ✓ ✓ X
C ✓ X X
D ✓ ✓ ✓

 

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71 mark

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

3-5-reaction-rates-q7

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?

  Temperature increased Catalyst added  Volume reduced
A X X ✓
B X X X
C ✓ X X
D X ✓ X

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81 mark

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

3-5-reaction-rates-q8

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

  H J  K
A decreases increases decreases
B decreases decreases decreases
C increases decreases increases
D increases decreases decreases

 

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91 mark

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

  • Lower activation energy and lower reaction rate

  • Lower activation energy and higher reaction rate

  • Higher activation energy and lower reaction rate

  • Higher activation energy and higher reaction rate

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101 mark

Consider the following reaction:

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

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

  • H+

  • Cl-

  • Na+

  • H2O

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