Rate of Reactions

Question 1

What is the rate of reaction

Answer 1

The rate of a reaction is a measure of how quickly a reactant is used up, or a product is formed.

Question 2

For a chemical reaction to happen:

Answer 2

reactant particles must collide with each other

the particles must have enough energy for them to react

Question 3

Activation Energy

Answer 3

The minimum amount of energy that colliding particles must have for them to react.

Question 4

Successful collision

Answer 4

A collision between reactant particles that has enough energy for a reaction to happen.

Question 5

The gradient of the line is equal to the rate of reaction:

Answer 5

the steeper the line, the greater the rate of reaction

fast reactions - seen when the line becomes horizontal - finish sooner than slow reactions

Question 6

Mean rate of reaction…

Answer 6

quantity of product or reactant / time taken

Question 7

Measuring mass in a reaction

Answer 7

• The change in mass of a reactant or product can be followed during a reaction.
• This method is useful when carbon dioxide is a product which leaves the reaction container.
• It is not suitable for hydrogen and other gases with a small relative formula mass, Mr.

Question 8

Measuring volume in a reaction

Answer 8

• The change in volume of a reactant or product can be followed during a reaction.
• This method is useful when a gas leaves the reaction container.
• The volume of a gas is measured using a gas syringe, or an upside down burette or measuring cylinder.

Question 9

If the concentration of a reacting solution or the pressure of a reacting gas is increased:

Answer 9

• the reactant particles become more crowded
• the frequency of collisions between reactant particles increases
• the rate of reaction increases

Question 10

The greater the frequency of successful collisions…

Answer 10

the greater the rate of reaction

Question 11

rate of reaction equation on a graph

Answer 11

change in mass or volume of the product / change in time

Question 12

If the temperature of the reaction mixture is increased:

Answer 12

• reactant particles move more quickly
• the energy of the particles increases
• the frequency of successful collisions between reactant particles increases
• the proportion of collisions which are successful increases
• the rate of reaction increases
  harder collision as there is more energy

Question 13

A catalyst is a substance that:

Answer 13

• speeds up the rate of a reaction
• does not alter the products of the reaction
• is not chemically changed or used up at the end of the reaction

Question 14

How catalysts work ?

Answer 14

A catalyst provides an alternative reaction pathway that has a lower activation energy than the un-catalysed reaction. This does not change the frequency of collisions. However, it does increase the frequency of successful collisions because more particles have energy greater than the activation energy, therefore there are more successful collisions.

Question 15

If a large lump is divided or ground into a powder:

Answer 15

its total volume stays the same
the area of exposed surface increases
the surface area to volume ratio increases

Question 16

If the surface area to volume ratio of a reacting solid is increased:

Answer 16

• more reactant particles are exposed at the surface
• the frequency of collisions between reactant particles increases
• the rate of reaction increases
• energy not increased

Question 17

reversible reaction

Answer 17

A chemical reaction which can go both ways.

Question 18

⇌

Answer 18

• the forward reaction is the one that goes to the right

- the backward reaction is the one that goes to the left

Question 19

Energy changes in reversible reaction

Answer 19

If a reaction is exothermic in one direction, it will be endothermic in the other direction. The same amount of energy is transferred in both the forwards and reverse reaction.

Question 20

When a reversible reaction happens in a closed container, it reaches a dynamic equilibrium … when this happens

Answer 20

• the forward and backward reactions are still happening
• the forward and backward reactions have the same rate of reaction
• the concentrations of all the reacting substances remain constant

Question 21

Equilibrium Position

Answer 21

The equilibrium position of a reversible reaction is a measure of the concentrations of the reacting substances at equilibrium.

Question 22

Changes in pressure

Answer 22

If the pressure is increased in a reaction involving gases, the equilibrium position moves in the direction of the fewest molecules of gas, to reduce the pressure.

Question 23

Changing the temperature

Answer 23

In a reversible reaction, if the reaction is exothermic in one direction, it is endothermic in the other direction. If the temperature is increased, the equilibrium position moves in the direction of the endothermic process.

Question 24

Changing the concentration

Answer 24

f the concentration of a reactant (on the left) is increased, the equilibrium position moves in the direction away from this reactant, and so more of the products are produced (on the right). If one of the products is removed from a reaction (on the right), then the position of equilibrium moves to the right to make more of that product.