Rate and extent of chemical change

Question 1

Give the equations for the mean rate of reaction

Answer 1

Mean rate of reaction = quantity of product (or reactants) / time

units could be cm3/s, g/s or mol/s

Question 2

How do you use a tangent to calculate the rate of reaction from a graph

Answer 2

1. Use a clear ruler to draw a tangent against the curve at a specific point in time.
2. Draw extension lines from 2 points on your tangent which cut through both of the axes.
3. Use the equation change in y axis/change in x axis to calculate the rate of reaction at that point in time.

Question 3

What is meant by the the term activation energy?

Answer 3

The minimum amount of energy required for particles to react.

Question 4

Explain collision theory

Answer 4

If particles have sufficient energy (activation energy) then they can collide with each other.
Successful collisions result in a reaction between the particles.
The rate of reaction is dependent on the frequency of successful reactions.

Question 5

What factors affect the rate of reaction?

Answer 5

1. Temperature
2. Concentration of reactants
3. Pressure of reacting gases
4. Surface area of solid reactants
5. Presence of catalysts

Question 6

Describe how temperature affects the rate of a reaction

Answer 6

The higher the temperature more energy the particles have
therefore - the particles are moving faster
therefore - they collide more frequently
therefore - there are more successful collisions
therefore - increased rate of reaction

Question 7

Describe how the concentration of reactants in solution affects the rate of a reaction

Answer 7

1The higher the concentration of the reactants the more particles of solute are present.
therefore - there are more particles available to collide with each other
therefore- there are more successful collisions
therefore - increased rate of reaction

Question 8

Describe how pressure affects the rate of a reaction in gases

Answer 8

The higher the pressure of the gases the closer together are the reactant particles
therefore - there are more successful collisions
therefore the rate of reaction increases

Question 9

Explain how the surface area of a solid affects the rate of reaction.

Answer 9

The more pieces of a solid there are the bigger the surface area. The higher the surface area to volume ratio the more particles are on the surface, therefore the more collisions there are, therefore the greater the rate of reaction. A powder will react more quickly than a lump of solid.

Remember if you are comparing surface area to volume ratios then check that the units are the same.

Question 10

What is a catalyst?

Answer 10

A substance which seeds up a chemical reaction but does not get used up.

It provides a different pathway for the reaction that has a lower activation energy.

It is not written in an equation because it isn’t part of the reaction. It can be shown by writing it’s symbol above the arrow

Question 11

Give some examples of catalysts

Answer 11

1. Iron acts as a catalyst in the reaction between nitrogen and hydrogen to create ammonia in the Haber process
2. Nickel in the reaction of vegetable oil and hydrogen to make margarine
3. Platinum in the catalytic converters of an exhaust which helps remove pollutant gases.

Question 12

What are enzymes?

Answer 12

They are molecules which act as catalysts in biological systems.

They are used to break down food molecules in digestion
eg Amylase is the enzyme used in the breakdown of starch to sugar
Protease is the enzyme used in the breakdown of proteins to amino acids

Biological washing powder contains artificial proteases which breakdown proteins in dirt on clothes.

Question 13

Some natural catalysts are found in the body, what are they called and how do they work?

Answer 13

Enzymes are molecules which act as catalysts in biological systems.

They are used to break down molecules in digestion and respiration
eg Amylase is the enzyme used in the breakdown of starch to sugar
Protease is the enzyme used in the breakdown of proteins to amino acids

Question 14

Describe the shape of the rate of reaction graphs for temperature, concentration, gas pressure and surface area of solids

Answer 14

1. temperature- graph is a curve because the rate of reaction is not proportional to temperature- a small increase in temperature can result in a big effect on the rate of reaction
   2 concentration of solution- graph is a straight line, the rate of reaction is directly proportional to the concentration
2. Gas pressure- graph is a straight line, the rate of reaction is directly proportional to the pressure
3. Surface area of solid- graph is a straight line, the rate of reaction is directly proportional to the surface area

Question 15

What is a reversible reaction?

Answer 15

A reaction which can be reversed, the reactants can be reformed from the products.
EG anhydrous copper sulphate which is white reacts with water to form hydrated copper sulphate which is blue. When it is heated it breaks back down into the white anhydrous copper sulphate and water.

CuSO4 + 5H2O ⇌ CuSO4.5H20

Question 16

What symbol is used to show a reversible reaction?

Answer 16

⇌

The sign for an equlibrium

Question 17

Explain energy changes in a reversible reaction

Answer 17

If a reaction is endothermic in one direction it will be exothermic in the other.
The amount of energy transferred is the same in each direction, due to the law of conservation of energy.

Question 18

What is meant by a closed system?

Answer 18

A system where no substances can get in or out.

Question 19

What is a dynamic equilibrium?

Answer 19

When a reversible reaction takes place In a closed system where nothing can get in or out the forward and backward reactions in a reversible reaction take place simultaneously and at exactly the same rate of reaction.

Question 20

Explain Le Chatelier’s principle

Answer 20

A dynamic equilibrium can contain mainly reactants, equal amounts of both reactants and products or mainly products. The position of the equilibrium is described as lying to the left, centre or right of the equation.
Wherabouts the equilibrium lies depends on the conditions.
If the conditions change then the position of the equilibrium changes and the relative amounts of reactants and products changes with it.

Question 21

Describe how thee position of the equilibrium changes as the concentration of reactants and products are increased or decreased

Answer 21

1 Reactant concentration increases /Equilibrium moves right/To reduce the concentration of the reactant
2. Reactant concentration
decreases/Equilibrium moves left/To increase the concentration of the reactant
3. Product concentration
increases/Equilibrium moves left /To reduce the concentration of the reactant
4. Product concentration
decreases/Equilibrium moves right/To increase the concentration of the reactant

Question 22

Describe the effect of changes in temperature on the position of an equilibrium

Answer 22

1. If the forward reaction is exothermic then an increase in temperature moves the equilibrium left to lower the temperature.
2. If the forward reaction is exothermic then a decrease in temperature moves the equilibrium right to raise the temperature.
3. If the forward reaction is endothermic then a decrease in temperature moves the equilibrium left to raise the temperature.
4. If the forward reaction is endothermic then an increase in temperature moves the equilibrium right to lower the temperature.

Question 23

Describe the effect of changes in pressure on the position of an equilibrium.

Answer 23

1. If there are more molecules of gas in the reactants than there are in the products then an increase in pressure moves the equilibrium right to decrease the pressure and a decrease in pressure does the opposite, moves it left to increase the pressure.
2. If the number of gas molecules in the reactants and products are the same then the equilibrium doesn’t move.
3. If there are more molecules of gas in the products than there are in the reactants then an increase in pressure moves the equilibrium left to decrease the pressure and a decrease in pressure does the opposite, moves it right to increase the pressure.