Topic 13

Question 1

What do you understand by the term ‘first order reaction?’ (1 mark) [DSE Practice Paper]

Answer 1

The rate of reaction is proportional to the concentration of the reactant.

Question 2

What is a catalyst?

Answer 2

A chemical that can change the rate of reaction that remains chemically unchanged at the end of the reaction.

Question 3

What is the effect of a catalyst?

Answer 3

It can increase the rate of reaction (1) by providing an alternative pathway that requires lower activation energy. (1)

Question 4

Why does making the catalyst finely divided/in a porous form increase the efficiency of the catalyst?

Answer 4

It provides a larger surface area that makes the catalyst more effective.

Question 5

How can N₂ be obtained in industry? (1 mark) [2016 DSE]

Answer 5

fractional distillation of liquid air

Question 6

What effect does the concentration of the reactant have on the rate for a 0th order reaction?

Answer 6

The rate is constant and DOES NOT change with the concentration of the reactant.

Question 7

What order is a reaction if its concentration decreases linearly with time?

Answer 7

0th order

Question 8

What order is a reaction if its rate increases linearly with concentration of reactant?

Answer 8

1st order

Question 9

How to prove a reaction (A → B) is 2nd order?

Answer 9

Rate = k[A]²

Plot a graph of rate against [A]². A straight line with positive slope passing through origin should be obtained.

Question 10

For A + B + C → D + E + F, how to keep [B] and [C] constant so as to find order of reaction with respect to A?

Answer 10

[B]_initial and [C]_initial&raquo_space;> [A]_initial

Question 11

Explain why ‘initial rate’ is commonly used in the study of the kinetics of a reaction. (1 mark) [2012 DSE]

Answer 11

Initial rate is used because the initial concentrations of reactants are known.

(Note: extra expt would be needed to find [A] mid-experiment if initial rate method is not used)

Question 12

Determine graphically the order of reaction with respect to (reactant) and the rate constant from (some data).

How to structure the answer? (1 sentence + calculation)

Answer 12

Easier method:
- A plot of log(initial rate) against log[reactant]_initial gives a straight line with a slope of (order).
- The reaction is x order with respect to reactant and rate = k[reactant]^x

More annoying method:
- A plot of initial rate against initial [reactant] gives a straight line passing through the origin. The order with respect to reactant is (answer).
(Plot rate against [reactant]² if order = 2)

• Rate equation: rate = k[reactant]
  (corresponds with y = mx)
• Find rate constant by calculating slope
  (Don’t use origin (0, 0) for calculating slope in exam! Preferably 用最遠兩個point)

Question 13

N₂O₅ (in CCl₄) → 4NO₂ (in CCl₄) + O₂ (g)
Suggest a method that can be used to follow the progress of the decomposition and state the principle of the method.

Answer 13

Measure the pressure (or volume) of the system at regular time intervals (because both N₂O₅ and NO₂ are soluble in CCl₄ while O₂ is insoluble). The pressure (or volume) of the system will increase during the course of the reaction.

Question 14

In an experiment using colorimetry, describe briefly how the initial rate of formation of the product is determined. (3 steps)

Answer 14

1. Measure the absorbance of the product in the reaction mixture during the course of the reaction by using a colorimeter.
2. By using a calibration curve (a graph of absorbance against [product]), find the concentration of the product at different times. Plot a curve of [product] against time.
3. Find the slope of the tangent to the curve at time = 0. This corresponds to the initial rate of formation of the product.

Question 15

In an experiment A + B → C to find the order of reaction with respect to A,
1. Why is it necessary to keep [B] much higher than [A]? (2 marks)
2. Suggest how the order of reaction with respect to B can be determined. (2 marks)

Answer 15

1. Rate = k[A]^x[B]^y
   When [B]&raquo_space; [A], the rate equation becomes rate = k’[A]^x (1)
   [A]^x is the only factor which affects the reaction rate. (1)
2. Repeat the experiment using the same reagents, keep [A] used constant, and vary [B] used. (1)
   Measure the initial rate in each case. Compare the initial rates to obtain the order with respect to B. (1)

Question 16

For this clock reaction:
Na₂S₂O₃ (aq) + H₂SO₄ (aq) → Na₂SO₄ (aq) + SO₂ (g) + H₂O (l) + S (s)

1. What is the purpose of adding different volume of H₂O (l) in each experiment?
2. Why is a large excess of H₂SO₄ (aq) added to all the experiments?

Answer 16

1. To make the total volume of the reaction mixture same for all the cases, so that the volume of Na₂S₂O₃ (aq) used is directly proportional to its concentration in the reaction mixture (after mixing).
   (Plot rate (1/t) against volume or volume² to show order)
2. Rate = k[H₂SO₄ (aq)]^x[Na₂S₂O₃ (aq)]^y
   When [H₂SO₄ (aq)]&raquo_space; [Na₂S₂O₃ (aq)], the rate equation becomes rate = k’[Na₂S₂O₃ (aq)]^y
   [Na₂S₂O₃ (aq)]^y is the only factor which affects the reaction rate.

Question 17

VERY IMPORTANT

For this clock reaction:
S₂O₈^2- (aq) + 2I^- (aq) → 2SO₄^2- (aq) + I₂ (aq)

Describe and explain how time (t) can be found experimentally. (2+2 marks)

Answer 17

Describe:
1. Add a known volume of standard S₂O₃^2- (aq) and some starch indicator to the I^- (aq) before adding S₂O₈^2- (aq).
2. Add S₂O₈^2- to the mixture of I^- (aq), S₂O₃^2- (aq) and starch indicator and record the time for the first appearance of a blue color.

Explain:
1. S₂O₈^2- (aq) + 2I^- (aq) → 2SO₄^2- (aq) + I₂ (aq)
The I₂ (aq) formed will react with the S₂O₃^2- (aq) present:
I₂ (aq) + 2S₂O₃^2- (aq) → 2I^- (aq) + S₄O₆^2- (aq)
2. When all S₂O₃^2- (aq) ions are consumed, the excess I₂ (aq) will react with the starch indicator to give a blue color.

Question 18

VERY IMPORTANT

For this clock reaction:
S₂O₈^2- (aq) + 2I^- (aq) → 2SO₄^2- (aq) + I₂ (aq)

Why should we add a small, fixed amount of S₂O₃^2- to the reaction mixture?

Answer 18

I₂ (aq) + 2S₂O₃^2- (aq) → 2I^- (aq) + S₄O₆^2- (aq)
To delay the finishing point of the reaction so that a longer time is needed to form I₂ (aq) and a smaller percentage error can be obtained in measurement.

(Note: think 5s ± 0.5s vs 60s ± 0.5s)

Question 19

VERY IMPORTANT

For this clock reaction:
S₂O₈^2- (aq) + 2I^- (aq) → 2SO₄^2- (aq) + I₂ (aq)

The reaction itself has a color change, so why should we add starch to signal the finishing point of the experiment?

Answer 19

To ensure the color change at the finishing point of the experiment can be observed clearly.

No starch: colorless → yellow → orange → brown
With starch: colorless → dark blue

Question 20

What is the definition of order of reaction?

Answer 20

The effect of change in [reactant] on the reaction rate

Question 21

If the question gives you volume instead of concentration of reactants, what does that mean?

Answer 21

Total volume should be equal for each experiment, hence volume of reactants is proportional to [reactants] after mixing
→ don’t use calculation to find order + ignore water (only used to keep volume equal)

Question 22

If you see terms like ‘acid catalysis’ or ‘catalyzed by H⁺’ in the question, what MUST you remember to do??

Answer 22

Take [H⁺] into account for rate equation!!

E.g. for A + B → C (catalyzed by H⁺)
rate = k[A]^x[B]^y[H⁺]^z

Question 23

Which 2 conditions are necessary for a chemical reaction to occur? (Topic 13)

Answer 23

1. have sufficient amount of energy
2. collide in the right orientation

Question 24

What is the activation energy (E_a)?

Answer 24

The minimum energy that reactants must have before they can change to products

Question 25

In an energy profile diagram, what reactions have a higher energy value in reactants but a lower one in products? What reactions are the opposite?

Answer 25

Exothermic reactions have a higher energy value in reactants than products.
Endothermic reactions have a lower energy value in reactants than products.

Question 26

Why is there a distribution of kinetic energy in Maxwell–Boltzmann distributions (energy distribution curves)? (3 marks)

Answer 26

• The motion of molecules is random, so collisions between molecules occur randomly.
• At each collision, kinetic energy is transferred between the molecules. Collisions between two molecules usually result in a gain of kinetic energy for one molecule and a loss of kinetic energy for the other.
• If a molecule undergoes a series of collisions such that each collision results in a gain of kinetic energy, it will end up with a kinetic energy higher than average.
  Conversely, if a molecule undergoes a series of collisions such that each collision results in a loss of kinetic energy, it will end up with a kinetic energy lower than average.

Question 27

Why does the shape of a Maxwell–Boltzmann distribution curve change with temperature but the area under the curve always remains constant? (from 2016, short ans)

Answer 27

The area under the curve is directly proportional to the total number of gas molecules in the system.

Question 28

How does a Maxwell–Boltzmann distribution curve change with temperature? (2 changes)

Answer 28

1. The peak of the curve shifts to the right at higher temperature.
2. The curve flattens at higher temperature.

Question 29

What are the major and minor reasons most rates of chemical reactions increase with increasing temperature?

Answer 29

Major reason:
Number of molecules with >= E_a

Minor reason:
KE increase, molecular speed increase, collision frequency increase

In the end, for both:
effective collision frequency increase,
reaction rate increase

Question 30

Why does reaction rate increase with temperature? (Topic 13) (3 marks)

Answer 30

• Increase in temperature increases average KE
• 1) collision frequency increase, 2) no. of molecules >= E_a increase
• effective collision frequency increases and rate increases

Question 31

Discuss, in terms of the Arhenius equation (logk = logA - E_a / 2.3RT), the effect of temperature on the rate of reaction.

Answer 31

temperature increase, E_a / 2.3RT decrease, logo increase, k increase, rate increase

vice versa for temperature decrease

Question 32

What is the Arrhenius equation?

Answer 32

logk = logA - E_a / 2.3RT

Question 33

What is the version of the Arrhenius equation used for 2 different temperatures?

Answer 33

log(k₁ / k₂) = -E_a / 2.3R (1/T₁ - 1/T₂)
or
log(rate₁ / rate₂) = -E_a / 2.3R (1/T₁ - 1/T₂)

Question 34

If no information about the reaction mechanism is given, how many steps does the reaction have?

Answer 34

One. Assume it is a single-step reaction.

Question 35

For an endothermic reaction, predict, with a reason, which reaction, forward or backward, will increase more in rate as the temperature increases. (Topic 13)

Answer 35

Since the E_a of forward reaction is higher than E_a of backward reaction (think energy profile diagram), forward reaction rate increases more.

Question 36

What is Green Chemistry?

Answer 36

The utilization of a set of principles to design and apply chemical processes that maximize the production efficiency, but at the same time, reduce or eliminate the use and generation of hazardous substances.

Question 37

What is sustainable development?

Answer 37

The development which meets the needs of the present without compromising the ability of future generations to meet their own needs.