reaction rate

Question 1

Give 2 ways of improving an experiment that uses a gas syringe

Answer 1

1. Let the reaction carry on until the gas syringe has stopped moving.
2. Wait for the gas to cool to room temperature before reading.

Question 2

What do catalysts do?

Answer 2

Provide an alternate reaction pathway with a lower activation energy without being used up.

If the activation energy is lower, more particles will have energy > EA, so there will be a higher frequency of effective collisions. The reaction will be faster.

Question 3

Give 2 reasons for why catalysts are useful

Answer 3

1. Allow reactions to occur at lower temperatures and pressure ∴ less (thermal) energy demand.
2. Allow for reactions with better atom economies ∴ less waste.

Question 4

What is a homogeneous catalyst and its benefits and drawbacks

Answer 4

● A catalyst in the same phase/state as the reactants.
+ It means more molecules can react.
- It makes separating molecules at the end harder.

Question 5

What is a heterogeneous catalyst and its benefits and drawbacks?

Answer 5

● A catalyst in a different phase/state to the reactants.
+ Can be easily separated.
• Reactions only take place on the catalyst’s surface.

Question 6

Describe what occurs on the surface of the nickel catalyst in the hydrogena;on of ethene

Answer 6

1. The reactants (H2 and C2H4) are ADSORBED onto the catalyst surface.
2. The bonds weaken (1) and they react to form C2H6.
3. The product (C2H6) is DESORBED from the surface.

Question 7

Explain effect of increasing concentra;on and pressure

Answer 7

At higher concentrations(and pressures) there are more particles per unit volume and so the particles collide with a greater frequency and there will be a higher frequency of effective collisions.

Note: If a question mentions a doubling of concentration/rate then make sure you mention double the number of particles per unit volume and double the frequency of effective collisions.

Question 8

How do you measure reac;on rates?

Answer 8

The rate of reaction is defined as the change in concentration of a substance in unit time Its usual unit is mol dm-3s-1
When a graph of concentration of reactant is plotted vs time, the gradient of the curve is the rate of reaction.
The initial rate is the rate at the start of the reaction where it is fastest.
Reaction rates can be calculated from graphs of concentration of reactants or products, by drawing a tangent to the curve (at different times) and calculating the gradient of the tangent.

Question 9

What is the key idea behind the ‘Boltzmann Distribution

Answer 9

Particles gain or lose energy in collisions with others and thus travel at different speeds.

Question 10

Give the key features of the ‘Boltzmann Distribution

Answer 10

● It starts at the origin because no particles have no energy.
● The peak (Emp) is the most probable energy (which is different
from the mean).
● It never meets the x-axis as there is no maximum energy for
particles .

Question 11

Explain effect of increasing temperature

Answer 11

A t higher temperatures the energy of the particles increases. They collide more frequently and more oBen with energy greater than the activation energy. More collisions result in a reaction
As the temperature increases, the graph shows that a significantly bigger proportion of particles have energy greater than the activation energy, so the frequency of successful collisions increases.

Question 12

Explain effect of increasing surface area

Answer 12

Increasing surface area will cause collisions to occur more frequently between the reactant particles and this increases the rate of the reaction.