Controlling the Rate

Question 1

What risks do chemists need to weigh up with the speed of a reaction?

Answer 1

Too slow = unlikely to be profitable.

Too fast = risk of an explosion.

Question 2

What is collision theory?

Answer 2

For reactants to form products they must collide.

Question 3

Effect of concentration?

Answer 3

Higher the concentration, the faster the reaction. This is because there are more particles and therefore more collisions and a greater chance of a successful reaction. So, higher concentration = more collisions = faster reaction.

Question 4

Effect of pressure?

Answer 4

Increasing pressure, in reactions involving gas reactants, compresses the molecules into a smaller space, and therefore there is a greater chance of them colliding and successfully producing products. Higher pressure = more collisions = faster rate.

Question 5

Effect of particle size (surface area)?

Answer 5

Smaller the particle size, the faster the reaction. This is because there is a larger surface area to react with increasing the rate of reaction.

Question 6

What is collision geometry?

Answer 6

The angle at which molecules collide. This must be correct for a successful collision to occur.

Question 7

Effect of temperature?

Answer 7

Increasing temperature will increase the rate of reaction - for most reactions. This is because the particles will have more energy to collide with and therefore speeding up the chance of a successful collision.

Question 8

What is temperature?

Answer 8

The measure of the average kinetic energy ( Ea) of the particles in a substance.

Question 9

What is activation energy?

Answer 9

The minimum energy required by colliding particles to overcome the repulsion of the outer electrons and form an activated complex.

This explains why the rate of some reactions is very slow unless energy is supplied.

Question 10

Relative rate formula

Answer 10

1/t(ime)

Question 11

What does the slope of a rate graph indicate?

Answer 11

How fast a reaction is going - the steeper the slope, the faster the reaction.

Question 12

What does ΔH mean?

Answer 12

Enthalpy change - the change in energy produced from a chemical reaction.

Question 13

What is ΔH measured in?

Answer 13

kJ or kJ mol-1

Question 14

What is an exothermic reaction?

Answer 14

Energy is RELEASED into the surrounding area, usually as heat, resulting in -ve ΔH.

Question 15

Where does the energy come from in an exothermic reaction?

Answer 15

It comes from the reactants which have potential energy (PE).

Question 16

What are examples of exothermic reactions?

Answer 16

Combustion and neutralisation.

Question 17

What is an endothermic reaction?

Answer 17

When a reaction TAKES IN energy from the surrounding area, resulting in a +ve ΔH.

Question 18

How do you calculate enthalpy change?

Answer 18

ΔH = H (products) - H (reactants)

Question 19

How do you calculate activation energy?

Answer 19

The value of the top of the graph - H (reactants)

Question 20

What is the activated complex?

Answer 20

An unstable arrangement of atoms formed (intermediate product) at the maximum of the potential energy barrier, during a reaction. It loses energy by going on to form products.

Question 21

How are the activated complex and activation energy linked?

Answer 21

Activation energy is the minimum energy needed by colliding particles to form the activated complex.

Question 22

Describe the activated complex.

Answer 22

A very unstable stage of a reaction that only exists for a short time. Not all activated complexes are successful and some break down to reform the reactants again.

Question 23

What is a catalyst?

Answer 23

A substance that increases the rate of a particular reaction, without being used up in the reaction. The catalyst acts as a site for the reaction to take place.

Question 24

N2 + 3H2 = 2NH3?

Answer 24

Haber process - ammonia - uses iron as a catalyst

Question 25

4NH3 + 5O2 = 4NO + 6H2O?

Answer 25

Ostwald process - nitric acid - uses platinum as a catalyst

Question 26

Unsaturated oils + H2 = saturated fats

Answer 26

Hydrogenation - margarine - uses nickel as a catalyst

Question 27

How does a catalyst help a reaction?

Answer 27

Catalysts provide an alternative reaction pathway for a reaction to take place which has lower activation energy, meaning more molecules will have enough energy to react.

Question 28

What is adsorption?

Answer 28

When reactants attach to the surface of the catalyst temporarily, providing an area for collisions.

Question 29

What is enthalpy?

Answer 29

The measure of chemical energy in a substance.

Question 30

How may exothermic and endothermic reactions affect industry processes?

Answer 30

Exothermic reactions may require heat to be removed to prevent the temperature rising. Endothermic reactions may cost more as heat may need to be supplied to maintain the reaction rate.