How Fast? The Rate of Chemical Change (HL IB Chemistry)

1 dm³ of gas X and 1 dm³ of gas Y are kept at the same temperature and pressure.

The speed of the molecules is plotted against the fraction of molecules with that speed. From the graph what can be said about the two gases?

A beaker containing 100 cm³ of 2M sulfuric acid, H₂SO₄ (aq), was placed on a balance and then an excess of powdered zinc was added. The change in mass was recorded against time and the results plotted on a graph as shown in line I below.

What change in conditions could result in line II shown on the graph?

Bleach solution contains sodium chlorate (I) which naturally decomposes according to the equation below:

2NaClO (aq)  → 2NaCl (aq)  +  O₂ (g)

The bleach decomposes faster as the temperature increases. A 100 cm³ sample of 10% bleach was heated to 40 C and the amount of oxygen evolved was measured against time.

The experiment was repeated under the same conditions, but this time using a 100 cm³ sample of 20% bleach. If the first experiment produces the dotted line, which line is produced by the second experiment?

How does the average kinetic energy of a gas vary with temperature?

Three batches of sulfuric acid containing different volumes and concentrations were reacted with an excess of zinc powder. The hydrogen evolved was measured against time and the results were plotted on a graph. Which lines on the graph could correspond to the specified volumes and concentrations of the sulfuric acid shown in the table?

 

	25 cm3 of 1.0 M H2SO4	50 cm3 of 0.5 M H2SO4	25 cm3 of 0.5 M H2SO4
A	I	III	IV
B	I	IV	III
C	I	II	III
D	II	I	III

When 1 cm³ of 0.1 mol dm^–3 HCl is added to 10 cm³ of 0.02 mol dm^–3 Na₂S₂O₃ solution, a pale yellow precipitate forms slowly.

When the experiment is repeated with 1 cm³ of 0.1 mol dm^–3 HCl and 10 cm³ of 0.05 mol dm^–3 Na₂S₂O₃ the precipitate forms more quickly.

Why is this?

On the graph, curve 1 was produced from measuring the decomposition of 100 cm³ of 1.0 mol dm^–3 hydrogen peroxide in the presence of a manganese (IV) oxide catalyst.

Which alteration to the original experimental conditions would produce curve 2?

The stoichiometry of a catalysed reaction is shown by the equation below.

R (g) + S (g)  ⇌ T (g) + U (g)

Two experiments were carried out in which the amount of product T was measured against time. The results are shown in the diagram below.

Which changes in the conditions from experiment 1 to experiment 2 might explain the results shown?

1. Product U was continuously removed from the equilibrium mixture.
2. A different catalyst was used.
3. Less of reactant R was used

Boltzmann distributions are shown in the two diagrams below.

In diagram 1, one line, P or Q, corresponds to the activation energy in the presence of a catalyst and the other line corresponds to the activation energy of the same reaction in the absence of a catalyst.

In diagram 2, one curve, X or Y, corresponds to a temperature higher than that of the other curve.

Which combination gives the correct curve and line?

	presence of catalyst	higher temperature
A B C D	P P Q Q	X Y X Y

The potential energy level profile for the hydrolysis of a tertiary halogenoalkane is shown.

Which of the following conclusions can be deduced from the potential energy level profile?

1. The reaction proceeds via an S_N1 mechanism 
2. The reactants are more stable than the products
3. The rate-determining step is the first step of the reaction mechanism

The rate information below was obtained for the following fourth order reaction at a constant temperature: 

5Br^– (aq) + BrO₃^– (aq) + 6H⁺ (aq) → 3Br₂ (aq) + 3H₂O (l)

Which rate expression is consistent with the data?

The rate information below was obtained for the following reaction at a constant temperature: 

C₂H₅Br (aq) + OH^– (aq) → C₂H₅OH (aq) + Br^– (aq)

[C2H5Br] / mol dm-3	[OH-] / mol dm-3	Rate / mol dm-3 s-1
2.0 x 10-3	1.0 x 10-2	4.0 x 10-4
4.0 x 10-3	1.0 x 10-2	8.0 x 10-4
8.0 x 10-3	2.0 x 10-2	3.2 x 10-3

What is the correct equation to calculate the value of the rate constant, k?

The mechanism for the following reaction between nitrogen(II) oxide and carbon monoxide is shown.

NO₂ (g) + CO (g) → NO (g) + CO₂ (g)

 

      Step 1:   NO₂ + NO₂ ⇌ N₂O₄   fast step

      Step 2:   N₂O₄ + 2CO → 2NO + 2CO₂   slow step

Which rate expression is consistent with the mechanism?

The rate information below was obtained for the following reaction of aqueous solutions of F and G, in the presence of a homogeneous catalyst, H, 

2F + 3G → J + 2K

When the concentrations of F and G are doubled, the rate of reaction increases by a factor of four.

When the concentrations of F and H are doubled, the rate of reaction increases by a factor of eight.

Which rate expression is consistent with the data?

Which of the following statements about the constant A in the Arrhenius equation are correct?

1. It is a steric factor for the fraction of collisions where the particles have the correct mutual orientation
2. It takes into account the energy of the colliding particles
3. It takes into account the number of collisions in a chemical reaction

The following information was obtained for the rate constant, k, for a reaction at 25 ^oC.

 

k	Ea	R
3.46 × 10–8 s–1	96.2 kJ mol–1	8.31 J K–1 mol–1

 

What is the value of the constant, A?

 

Which is the correct value for the activation energy? (R = 8.31J K^–1 mol^–1)

The rate constant data for a reaction at two different temperatures is shown.

Temperature / oC	Rate constant / mol-1 dm3 s-1
5	6.81 x 10-6
35	6.11 x 10-5

 Using the following equation, what is the correct value for the activation energy of the reaction, in J mol^-1? 

When the temperature increases from 25 ^oC to 55 ^oC, the rate constant for the reaction increases by a factor of 1.45.

Using the equation above, what activation energy of the reaction in kJ? (R = 8.31J K^–1 mol^–1)

The diagram shows a reaction pathway for an endothermic reaction.

Which arrow represents the activation energy for the forward reaction?

The dotted-line curve on each graph below represents the corresponding distribution for a gas at 300 K.

Which solid-line curve most accurately represents the distribution of molecular energies in the same gas at 500 K?

The energies of the reactants, the products and the transition state of a reaction are shown in the reaction pathway diagram below.

Which expression correctly represents how to calculate the activation energy of the forward reaction?

The diagram below represents, for a given temperature, the Boltzmann distribution of the kinetic energy of the molecules in a mixture of two gases that react slowly together without a catalyst.

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?

Four possible energy profiles are shown below, for reactions carried out at the same temperature.

Which energy profile shows a reaction that is likely to proceed most rapidly and with a good yield?

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: 1.5g of solid calcium carbonate is added to 100 cm³ of 0.5 M hydrochloric acid

Reaction 2: 100 cm³ of distilled water is then added to 100 cm³ of 0.5 M hydrochloric acid then 1.5g 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?

I	Adding water reduces the frequency of collisions between reactant molecules.
II	Adding water reduces the proportion of effective collisions between reactant molecules.
III	Adding water reduces the proportion of reactant molecules possessing the activation energy.

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?

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

Zinc reacts with copper sulfate according to the following equation:

Zn (s) + CuSO₄ (aq) → Cu (s) + ZnSO₄ (aq)

Rates of reaction can be found by measuring how certain properties change during the course of the reaction. Which of these properties could be used?

1. Change in volume.
2. Change in temperature.
3. Change in colour.

Which of the following statements about the rate constant, k, are correct? 

1.   High values of k are associated with fast reactions
2.   The rate constant is affected by temperature
3.   The units of k are independent of the orders of reaction

A student experimentally determined the rate expression for the reaction between iodine and propanone to be: 

Rate = k [H⁺] [C₃H₆O] 

Which graph is consistent with this information?

The rate information below was obtained for the following reaction at a constant temperature: 

C₂H₅Br (aq) + OH^- (aq) → C₂H₅OH (aq) + Br ^- (aq)

[C2H5Br] / mol dm-3	[OH-] / mol dm-3	Rate / mol dm-3 s-1
3.0 x 10-3	2.0 x 10-2	4.0 x 10-4
6.0 x 10-3	2.0 x 10-2	8.0 x 10-4
6.0 x 10-3	4.0 x 10-2	1.6 x 10-3

What are the orders of reaction with respect to C₂H₅Br and OH^-?

The mechanism for the following reaction between iodine and propanone is shown. 

CH₃COCH₃ (aq) + I₂ (aq) → CH₃COCH₂I (aq) + HI (aq)

Step 1: CH₃COCH₃ + H⁺ → CH₃COH⁺CH₃        

Step 2: CH₃COH⁺CH₃ → CH₃COHCH₂ + H⁺ 

Step 3: CH₃COHCH₂ + I₂ → CH₃COHCH₂I + I^-

Step 4: CH₃COHCH₂I → CH₃COCH₂I + H⁺ 

Which classifications of CH₃COCH₃, H⁺ and CH₃COHCH₂ are correct?

	CH3COCH3	H+	CH3COHCH2
A	Intermediate	Intermediate	Catalyst
B	Reactant	Intermediate	Product
C	Reactant	Catalyst	Intermediate
D	Reactant	Product	Intermediate

The proposed mechanism for the following reaction where ethanal dimerises in dilute alkaline solution to form 3-hydroxybutanal is shown. 

2CH₃CHO → CH₃CH(OH)CH₂CHO

Step 1: CH₃CHO + :OH^- → :CH₂CHO + H₂O                                       slow step

Step 2: CH₃CHO + :CH₂CHO → CH₃CH(O:^-)CH₂CHO                        fast step

Step 3: CH₃CH(O:^-)CH₂CHO + H₂O → CH₃CH(OH)CH₂CHO + :OH^-  fast step

Which of the following statements is not correct?

Which of the following statements about the Arrhenius plot are correct? 

1.     The gradient has a value of E_a / R.
2.     The intercept on the y-axis is ln A.
3.     The Arrhenius plot will give a value for activation energy in J mol^-1.

The following information was obtained for the rate constant, k, for a reaction at 25 ^OC

A	Ea	R
2.57 × 109 s–1	96.2 kJ mol–1	8.31 J K–1 mol–1

Which expression correctly represents how to calculate the rate constant, k?

The following experimental data was collected. 

Activation energy, Ea	111 kJ mol–1
Rate constant, k	1.30 x 10-4  mol-1 dm3 s-1
Arrhenius constant, A	4.55 × 1013 mol-1 dm3 s-1

 

What is the temperature of the reaction?

Which statements are correct?

1. Many reactions show a doubling of the rate with a temperature increase of 10 K.
2. The units of k for a second order reaction are mol^-1 dm³ s^-1.
3. In the Arrhenius equation, A relates to the energy requirements of the collisions.

Which graph shows the correct relationship between the rate constant, k, and temperature?