Module 3.2.2 - Reaction rates

Question 1

What is rate of reaction?

Answer 1

The rate of a chemical reaction is the change in concentration of a reactant/product per unit time

Question 2

What are the factors that affect the rate of reaction (5)?

Answer 2

-Concentration of reactant (liquid)
-Temperature
- Use of a catalyst
- Surface area (solids only)
- Pressure (gases only)

Question 3

What is the simple collision theory?

Answer 3

For a chemical reaction to occur; molecules must collide with each other, with energy greater than/equal to the activation energy, in the correct orientation (successful collision)
If molecules dont have collide with enough energy/wrong orientation = ineffective collision

Question 4

How does concentration affect the rate of reaction?

Answer 4

As concentration of reactant molecules increase, rate of reaction increases.
At higher concentrations there are more molecules in a given volume, meaning more frequent successful collisions

Question 5

How does pressure affect the rate of reaction?

Answer 5

When the pressure of a gas increases, the gas molecules are pushed closer together; the number of gas molecules in a given volume increases and more frequent successful collisions occur.

SAME EFFECT AS INCREASING CONCENTRATION OF GASES

Question 6

How does temperature affect the rate of reaction?

Answer 6

As temperature of reaction mixture increases, rate of reaction increases.
At a higher temperature, the average (kinetic) energy of the molecules increases, a greater proportion of molecules have energy greater than/equal to the activation energy meaning more frequent successful collisions occur

Question 7

What is a catalyst?

Answer 7

Increases the rate of a reaction and is not used up by the overall reaction

Question 8

What is a heterogeneous catalysis?

Answer 8

The catalyst has a different physical state from the reactants

Question 9

What is a homogeneous catalysis?

Answer 9

The catalyst of a reaction has the same physical state as the reactants

Question 10

How does a catalyst function?

Answer 10

A catalyst increases the rate of a reaction by providing a different reaction pathway/mechanism for the reaction, which has a lower activation energy

Question 11

Why use a catalyst?

Answer 11

They have economic importance and are beneficial for sustainability

Question 12

What does lower temperature needed by using a catalyst lead to?

Answer 12

Reduced energy demand from combustion of fossil fuels -> Less CO2 emissions from combustion of fossil fuels-> Less cost due to lower energy demand

Question 13

What are examples of heterogenous catalysts used in industry?

Answer 13

Haber process (Manufacture of ammonia) uses Fe(s) catalyst
Manufacture of poly(ethene) uses Ziegler-Natta catalyst
Catalytic converter to remove toxic gases from car exhaust, Rh/Pd/Pt alloy catalyst

Question 14

What does the Boltzmann distribution show?

Answer 14

The distribution of energies of molecules at a particular temperature

Question 15

How do you draw the Boltzmann distribution?

Answer 15

• x axis; energy, y axis; number of molecules
• starts at origin (0,0)
• asymptotes with x-axis at high energies
• has asymmetrical shape towards the left

Question 16

How does the Boltzmann distribution show its information?

Answer 16

• Peak is the mode energy as the greatest number of molecules have that energy
• Total area under the curve is equal to the total number of molecules in the mixture
• Shaded area under curve represents the proportion of molecules that have the energy greater/equal to activation energy
• asymmetrical shape as more molecules have lower energies

Question 17

How does the Boltzmann distribution show the effect of temperature?

Answer 17

At a higher temperature, more molecules have higher energies as the average energy of molecules increases.
Boltzmann flattens and shifts to the right and the shaded area under the curve is greater
A greater proportion of molecules have energy greater than/equal to the activation energy so more frequent successful collisions occur

Question 18

How does the Boltzmann distribution show the effect of a catalyst?

Answer 18

A catalyst increases the rate of reaction by lowering the activation energy.
A greater proportion of molecules have energy greater than/equal to activation energy so more frequent successful collisions occur.

Question 19

What are some methods for identifying the rate of reaction? (5)

Answer 19

• measuring the mass of a reactant/product at regular time intervals
• using titration at regular time intervals on a sample
• using a colorimeter/UV (visible spectrophotometer) at regular time intervals
• measuring the gas evolved at regular time intervals
• measure the change in pH / electrical conductivity at regular time intervals

Question 20

How do you find rate of reaction at time (t) from a concentration-time graph?

Answer 20

Draw a tangent to the curve at time, t
Find the gradient of the tangent to the curve

Question 21

How do you find the initial rate of reaction on a concentration-time graph?

Answer 21

Draw tangent at time = 0 and calculate the gradient