Rates

Question 1

Give a basic definition of rate of reaction.

Answer 1

Rate = change in concentration / time

Question 2

What are the units of rate of reaction?

Answer 2

mol dm-3 s-1

Question 3

How might we measure rates practically? (x5)

Answer 3

• Change in mass
• pH measurement
• Colorimetry
• Chemical analysis: titration with quenching to stop the reaction’s progression
• Production of a solid precipitate

Question 4

What factors affect rate of reaction? (x4)

Answer 4

• Concentration
• Temperature
• Presence of a catalyst
• Surface area of solid reactants

Question 5

How can we explain the effect of concentration on the rate of reaction, using collision theory?

Answer 5

• More particles per unit volume
• Effective collisions occur more frequently
• Increase in rate

Question 6

What are the two axes of a Maxwell-Boltzmann distribution graph?

Answer 6

X-axis: energy, E
Y-axis: number of particles

Question 7

What does the area beneath a Maxwell-Boltzmann distribution represent?

Answer 7

The total number of particles in a given system.

Question 7

Describe the basic features of a Maxwell-Boltzmann distribution.

Answer 7

• Asymmetrical bell curve
• The graph starts at the origin; no particles have E=0
• The graph never reaches the x-axis beyond this, as there is no maximum energy carried by a given particle
• The peak of the curve represents the most probably energy of particles E(mp)

Question 8

How would a higher temperature affect the Boltzmann distribution?

Answer 8

E(mp) will shift to the right, and the height of the distribution curve will decrease to maintain a constant area. The area lying beyond threshold E(a) would increase so more particles collide successfully.

Question 9

How would the presence of a catalyst affect the Boltzmann distribution?

Answer 9

The curve itself wouldn’t change, but the area lying beyond the threshold E(a) would be greater and hence more particles would collide successfully.

Question 10

How do we use a Maxwell-Boltzmann distribution to explain rate of reaction?

Answer 10

We consider how changing an external factor affects the total area of the graph lying beyond the threshold E(a).

Question 11

Define catalyst.

Answer 11

A substance providing an alternative reaction pathway with a lower activation energy.

Question 12

Explain the difference between heterogeneous and homogeneous catalysts.

Answer 12

Heterogeneous - DIFFERENT phase as the reactants.
Homogeneous - SAME phase as the reactants.

Question 13

How do heterogeneous catalysts work?

Answer 13

• Reactant molecules absorb onto the surface of the catalyst.
• Bonds in the reactants weaken and break, and new ones are formed.
• As bonds are weakened, the products now easily resorb away.

Question 13

What is the most important aspect of catalytic function?

Answer 13

The catalyst is fully re-formed at the end of the reaction.

Question 14

Why might we use a catalyst for a reaction involving multiple negative-charged reactants?

Answer 14

The activation energy of this process will be very high as strong e.static forces of attraction need to be overcome. Catalysts work to reduce this activation energy.

Question 15

Explain the significance behind the order of a reactant in a rate equation.

Answer 15

The order represent how changing the concentration of a reactant changes the rate of reaction.

Question 16

If a reactant doesn’t feature in the rate equation, how does it affect rate of reaction?

Answer 16

It doesn’t - the reaction is zero order wrt. the concentration of this reactant.

Question 17

How do we determine the overall order of a reaction?

Answer 17

Sum of all individual reactant orders.

Question 18

What is the rate constant, k?

Answer 18

This is a proportionality constant, converting orders and concentrations to rates.

Question 19

How do we determine the units of k?

Answer 19

Re-arrange the rate equation to make k the subject.

Question 20

Describe how the shape of a RATE-CONCENTRATION graph relates to order.

Answer 20

• Straight horizontal line: zero order
• Straight diagonal line: first order
• Upwards quadratic: second order

Question 21

Describe how the shape of a CONCENTRATION-TIME graph relates to order.

Answer 21

• Straight line: zero order, concentration is independent of rate as gradient is constant
• Quadratic curve: first order, concentration is proportional to rate as half-life is constant
• Steeper quadratic curve: second order

Question 21

How can we determine k graphically?

Answer 21

With the conc.-time graph for a first-order reactant, divide the natural log of 2 by the half-life t(1/2).

Question 22

Why do reactions typically occur over multiple steps?

Answer 22

It's highly unlikely that >2 reactants will collide at once.

Question 23

What is the rate-determining step of a general reaction mechanism?

Answer 23

The slowest step; the overall rate is dependent on the rate of this step, as the others steps are fast and hence have negligible imppact

Question 24

What species will appear in the rate equation? (x2)

Answer 24

- Catalysts (rate eqtn. only) - Species involve in steps up to and including the RDS

Question 25

How can be determine the nature of the reaction mechanism practically?

Answer 25

- We can determine the rate equation experimentally, this shows the RDS. - This step forms an intermediate for use in the later fast step. - This step uses up remaining reactants from the overall eqtn.

Question 26

How might reactants in a mechanism form an intermediate?

Answer 26

- Adding reactants - Splitting reactants to form a product and an intermediate - Combining reactants to form a product and an intermediate

Question 27

How is the Arrhenius equation written?

Answer 27

k=Ae^(-E(a)/RT) ln k = ln A - E(a)/RT

Question 28

What is the role of the Arrhenius equation?

Answer 28

It links the rate constant k with temperature and the activation energy of a reaction.

Question 29

How does the rate equation describe the rate-determining step?

Answer 29

- Only the species in the rate equation feature in the RDS - The orders of reactants in the rate eqtn. match the number of species in the RDS

Question 30

How can we re-arrange the Arrhenius equation to apply it graphically?

Answer 30

- We know that ln k = ln A - E(a)/RT - This re-arranges as ln k = -E(a)/R * (1/T) - This resembles y=mx+c - The modulus * R=8.314 of a graph of ln k against 1/t is equal to E(a)

Question 30

How can we use the Arrhenius equation to predict changes in rate of reaction?

Answer 30

- We compare values of k under different conditions - We know that k is a factor of proportionality in the rate equation, so if k doubles so will rate.