# CI abcde: Kinetics: rates & orders of reactions; Arrhenius equation Flashcards

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1
Q

Why does increasing the concentration of a reactant not necessarily mean that the rate of reaction will increase?

A

The reactant may not be involved in the rate-determining step.

2
Q

What influences how fast a rate-determining step is?

A

Its activation enthalpy.

3
Q

2H2O2 → 2H2O + O2

0.01 moles of oxygen are formed per second. What is the rate in terms of hydrogen peroxide?

A

- 0.02 mol s-1

4
Q

Rate = k[H2O2][catalase] at temperature T.

What is the unit of the rate constant?

A

k = rate / product of concentrations

mol dm-3 s-1

(mol dm-3)(mol dm-3)

So unit of k = dm3 mol-1 s-1

Write positive indices first by convention

5
Q

Experimental evidence showed the rate of a reaction is given by the following equation:

rate = k[S2O82-][I-]

What is the order of the reaction:

• With respect to each of the reactants?
• Overall?
A
• First order with respect to S2O82-
• First order with respect to I-
• Second order overall (add the powers)
6
Q

What is the unit of the rate constant for zero order reactions?

A

mol dm-3 s-1

7
Q

What is the unit of the rate constant for first order reactions?

A

s-1

8
Q

What is the unit of the rate constant for second order reactions?

A

dm3 mol-1 s-1

9
Q

This is the Arhenius equation:

k = Ae-Ea/RT

What is represented by each symbol?

A

k, rate constant

A, frequency factor

e, the constant

Ea, activation enthalpy

R, gas constant

T, temperature in Kelvin

10
Q

k = Ae-Ea/RT

Prove that the Arrhenius equation can be used to plot a straight line graph. State its gradient and y-intercept.

A

Take natural log of both sides (on data sheet)

lnk = -Ea/RT + lnA

lnk = -Ea/R x 1/T + lnA

Since k and T are the only variables, this is equivalent to y = mx + c

Plot lnk against 1/T

Gradient = -Ea/R and y-intercept = lnA

11
Q

What is the unit of A in the Arrhenius equation?

A

The same as that of k.

12
Q

What is the magnitude of the change in rate of a reaction with activation energy 50 kJ mol-1 when the temperature is increased from 20oC to 30oC?

A

In both cases, k = [X]m[Y]n so k is only factor affecting rate; k ∝ rate

Convert Ea into J (unit of gas constant) and T into K.

k1 = A e-50,000 / 8.314 x 293 = 1.2 x 109 A

k2 = A e-50,000 / 8.314 x 303 = 2.4 x 10-9 A

A is a constant, so number of collisions + therefore rate is multiplied by approx. 2

13
Q

Which statement about the Arhenius equation is correct?

1. A plot of lnk against T gives a straight line
2. When T is very large, ln k almost equals A
3. Ea is the gradient of a plot of ln k against 1/T
4. A plot of k against 1/T gives a straight line
A

2

1. Should be 1/T
2. lnk = -Ea/R x 1/T + lnA = -Ea/RT + lnA so, as T → ∞, -Ea/RT → 0
3. Should be -Ea/R
4. Should be lnk
14
Q

Suggest 5 methods of analysis which could be used to measure the rate of a reaction.

A
• Titration
• pH
• Colorimetry
• Volume of gas evolved
• Mass change
15
Q

How does quenching work?

A
• Sample isolated from reaction mixture
• Chemical added which stops reaction (e.g. neutralises a catalyst)
• Sample titrated to find conc. of a reactant/product
16
Q

To work out the rate equation for a reaction, you need to vary the concentration of the reactants one at a time.

In what 2 ways could you ensure that you are investigating the effect of a single reactant?

A
• Run one experiment with a given concentration of one reactant. Have other in excess so that its concentration does not change significantly
• Run several experiments with different concentrations of one reactant. Keep concentration of other reactant constant
17
Q

Explain how the initial rate method can be used to determine the order of a reaction with respect to one reactant.

A
• Run several experiments with different concentrations of one reactant. Keep concentration of other reactant(s) constant
• Measure conc of first reactant throughout
• For each, plot concentration against time, then take gradient of tangent at origin: initial rate
• Plot initial rate against concentration:
• Flat line = 0 order wrt this reactant
• Straight line with gradient > 0: 1st order wrt this reactant
• If initial rate against conc2 is straight line with gradient > 0: 2nd order wrt this reactant
18
Q

Explain how reaction time can be used to determine the order of a reaction with respect to one reactant.

A
• Run several experiments with different concentrations of one reactant. Keep concentration of other reactant(s) constant
• For each conc, measure time taken to form a small, fixed amount of a product
• Rate = amount of product / time. For given amount, initial rate ∝ 1/t
• Plot 1/t against concentration:
• Flat line = 0 order wrt this reactant
• Straight line with gradient > 0: 1st order wrt this reactant
• If 1/t against conc2 is straight line with gradient > 0: 2nd order wrt this reactant
19
Q

Explain how half-lives can be used to determine whether a reaction is first order with respect to one reactant.

A
• Run one experiment with a given concentration of one reactant. Have other(s) in excess
• Measure concentration of reactant of interest throughout reaction
• Plot concentration against time
• Find 3 half-lives: time for concentration to halve
• Constant half-life indicates reaction is 1st order with respect to reactant of interest
20
Q
A

B

A: gradient is constant, so rate with respect to A is constant; [A] doesn’t affect rate so 0, not 1st, order

C: insufficient information to know

D: they are

21
Q

Which of these graphs is a straight line for the first order reaction Z → products?

1. [Z] against time
2. rate against [Z]
3. ln k against 1/T
• A 1, 2 and 3
• B only 1 and 2
• C only 2 and 3
• D only 1
A

C

Not 1 as there is constant deceleration, not constant decrease.

22
Q

A reaction is first order with respect to a catalyst, Y. What is true about a graph of [Y] against time?

1. It will be a curve going downwards with constant half life
2. It will be a straight line going downwards
3. It will be a straight line parallel with the x-axis
4. It will be a curve rising with decreasing rate
A

3

(Catalyst, so not used up during reaction)

23
Q

H2O2 + 2H+ + 2I- → 2H2O + I2

Rate = k [H2O2] [I-]

Discuss the two mechanisms in terms of the orders of reaction and the individual steps. (4)

Mechanism 1

1. H2O2 + I- → H2O + IO- (slow)
2. IO- + 2H+ + I-→ I2 + H2O (fast)

Mechanism 2

1. H2O2 + I- → H2O + IO- (fast)
2. H+ + IO- →HIO (slow)
3. H+ + HIO → H2IO+ (fast)
4. H2IO+ + I- → I2 + H2O (fast)
A
• Mechanism 1 has correct slow step not involving H+
• The slow step would be first order with respect to H2O2 and I-
• Mechanism 2 has all two-body (AW) collisions
• One of:
• Slow step in 2 would give 1st order with respect to all 3 reagents
• Fast step of 1 very unlikely as 4 particles collide