Kinetics I

Question 1

Factors that effect rates of reaction:

Answer 1

Concentration (or pressure when using gases).
Temperature.
Surface area of solids.
Catalysts.

Question 2

How can we measure rates of reaction?

Answer 2

We can measure rates of reaction by measuring how fast reactants are being used up or by how quickly the product is formed.

Question 3

What can rate of reaction be defined as in terms of concentration?

Answer 3

The change in concentration of reactant (or product) in a given time.

Question 4

What is the equation for rate of reaction and what are the units?

Answer 4

Rate = change in concentration/time

Where:
Rate is in moldm^-3s^-1.
Change in concentration is in moldm^-3.
And time is in s.

Question 5

Describe a typical rate of reaction curve with concentration plotted against time.

Answer 5

Rate of reaction is fastest at the start of the reaction (reactants have the highest concentration).
Rate of reaction slows down as the reaction proceeds.
Rate of reaction is zero when all the reactants have been used up.

Question 6

How do we find the rate of reaction at any given point on a graph?

Answer 6

We draw a tangent to the curve at that point and calculate the gradient by doing Δy/Δx.

Question 7

Under what conditions will a successful collision occur?

Answer 7

The particles must have enough energy to overcome the activation barrier of the reaction.
The particles must collide in the correct orientation.

Question 8

For a reaction to take place what must two particles initially do?

Answer 8

Collide.

Question 9

If concentration or pressure are increased why does the rate of reaction increase?

Answer 9

There is an increased frequency of collisions meaning more change of successful collisions. This is due to more particles in a given volume.

Question 10

If a gas is the product of a reaction what two methods can be used to measure the rate of reaction?

Answer 10

You can measure the volume of gas produced with a gas syringe.
You can measure the loss of mass by conducting the experiment on a top pan balance and bunging the mixture with cotton wool.

Question 11

If I use a method for measuring rate of reaction and I can take readings every 10/20/30 seconds etc. what is that known as in terms of monitoring method?

Answer 11

It is a continuous monitoring method.

Question 12

How can we plot data gained by continuous monitoring methods?

Answer 12

We can plot it on a concentration-time graph.

Question 13

When should we stop when using a continuous monitoring method?

Answer 13

When you get a repeated result one after the other.

Question 14

Why does a Maxwell-Boltzmann curve never start at (0,0)?

Answer 14

No particles have 0 energy.

Question 15

Why does a Maxwell-Boltzmann never reach the x-axis?

Answer 15

There is no maximum energy that a particle can have.

Question 16

What are the axis labels for a Maxwell-Boltzmann curve?

Answer 16

Number/fraction of particles with a given energy on the y-axis.
Energy on the x-axis.

Question 17

What does a catalyst do to the Ea on a Maxwell-Boltzmann curve?

Answer 17

It moves the line to the left so a greater number/proportion of particles have the required activation energy for the reaction.

Question 18

What happens to a Maxwell-Boltzmann curve if you increase the temperature?

Answer 18

The peak moves to the right and decreases in height but there is the same area under the curve as before.

Question 19

How can catalysts provide an alternative pathway for a reaction?

Answer 19

It may provide an intermediate state or a surface for the reaction to occur on.

Question 20

What does a catalyst do?

Answer 20

Lowers the activation energy for a successful reaction.

Question 21

If a catalyst is present for a reversible reaction what happens to the rate of reaction?

Answer 21

The rate of both the forward and backward reactions increase.

Question 22

What state is a homogeneous catalyst in?

Answer 22

The same state as the reactants.

Question 23

How do homogeneous catalysts work?

Answer 23

They react with the reactants forming an intermediate, this then breaks down to give the products and the regenerated catalyst.

Question 24

Examples of homogeneous catalysts:

Answer 24

Sulphuric acid in making esters.

Cl. in ozone depletion.

Question 25

What state is a heterogeneous catalyst in?

Answer 25

A different state to the reaction mixture (usually solid).

Question 26

How does a heterogeneous catalyst work?

Answer 26

They work by binding with a reactant and holding it in an orientation until the other reactant comes and reacts with it. This holding it in a certain orientation increases the frequency of successful collisions thus increasing the rate of reaction.