C6 - The Rate and Extent of Chemical Change

Question 1

What is the rate of a chemical reaction?

Answer 1

How fast the reactants are changed into products

Question 2

Collision Theory

Answer 2

Explains how reactions happen at different rates (which are the different factors)

Question 3

What are the 2 factors that rate of a chemical reaction are dependant on?

Answer 3

• Collision frequency (how often they collide). The more collisions, the faster the reaction is
• The energy transferred during a collision, particles have to collide with enough energy for the collision to be successful

Question 4

What are the factors affect the rate of reaction?

Answer 4

• Temperature
• Surface area
• Concentration of the solution or pressure of a gas
• Presence of a catalyst

Question 5

Why does temperature increase the reaction rate?

Answer 5

• Particles move faster
• Collide more frequently
• More collisions is more energy

Question 6

Why does increasing the concentration or pressure increase the reaction rate?

Answer 6

• If the solution is more concentrated, it means there are more particles knocking about in the same volume of water
• In a gas, the same concept is applied
• This makes collisions between the reactant particles more frequent

Question 7

Why does increasing the surface area increase the rate?

Answer 7

• Breaking a solid up will increase its surface area to volume ratio
• This means that for the same volume of the solid, the particles around it will have more area to work on
• This means there will be more collisions

Question 8

Why does using a catalyst increase the rate?

Answer 8

• Catalysts work by decreasing the activation energy needed for the reaction to occur
• They do this by providing an alternative reaction pathway with a lower activation energy

Question 9

What is activation energy?

Answer 9

Minimum amount of energy for chemical reactions to happen (through particles)

Question 10

Advantages of Catalysts

Answer 10

Reduces necessary energy

Question 11

Disadvantages of Catalysts

Answer 11

Many are toxic and contaminate ecosystems

Question 12

What are catalysts

Answer 12

Increase the rate of reaction by providing an alternative pathway with a lower activation energy and does not get used in the reaction

Question 13

What are the 3 ways to measure to measure the rate of reaction?

Answer 13

• Precipitation and colour change
• Change in mass
• Volume of a gas given off

Question 14

What are the variables with time and reactants used up in a graph?

Answer 14

y-axis is reactants used up

x-axis is time

Question 15

How do you find the mean reaction rate for a reaction?

Answer 15

Work out the overall change in the y-value and divide it by the total time taken

Question 16

How to work find the reaction rate a certain point on a graph?

Answer 16

• Use a tangent to find out the gradient
• Change in y over change in x
• This gives the gradient (aka the rate in this case)

Question 17

What does this ‘⇌’ symbol show?

Answer 17

That the reaction is reversible and the products can react for form the reactants again

Question 18

What happens to energy at equilibrium?

Answer 18

It is conserved, no energy is gained or lost

Question 19

What is meant by the system being at equilibrium?

Answer 19

• As the reactants react, their concentrations fall - so the forward reaction will slow down. But as more and more products are made and their concentration rise, the backward reaction will speed up
• After a while the forward reaction will be going at exactly the same rate as the backward one

Question 20

What happens at equilibrium?

Answer 20

• Both reactions are still happening, but there’s no overall effect (dynamic equilibrium)
• This means the concetrations of reactants and products have reached a balance and won’t change

Question 21

What is required for equilibrium to be reached?

Answer 21

• The reversible reaction needs to take place in a ‘closed system’

Question 22

What is dynamic equilibrium?

Answer 22

• The rates of forwards and backwards reactions are equal
• Both the forward and backward reactions are still taking place
• The reactants and products do not change when a reaction is at dynamic equilibrium

Question 23

What is a closed system?

Answer 23

None of the reactants or products can escape and nothing else can get in

Question 24

Why is the position of equilibrium important?

Answer 24

• When the reaction is at equilibrium, it doesn’t mean the amount of reactants and products are the same

Question 25

What conditions affect the position of equilibrium?

Answer 25

• Temperature
• Pressure
• Concentration

Question 26

Equilibrium lies at the right (p vs r)

Answer 26

• Temperature decreases

- The concentration of products (exothermic) is greater than that of the reactants (endothermic)

Question 27

Equilibrium lies at the left (p vs r)

Answer 27

• Temperature increases

- The concentration of the reactants (endothermic) is greater than that of the products (exothermic)

Question 28

What happens to the position of equilibrium once you increase pressure?

Answer 28

Shift to the reaction that produces the fewest gas molecules

Question 29

What happens to the position of equilibrium once you decrease pressure?

Answer 29

Shift towards the side of the reaction that produces the most gas molecules

Question 30

How do endothermic and exothermic reactions affect reversible reactions?

Answer 30

• If the reaction is endothermic in one direction, it will be exothermic in the other direction
• The energy transferred from the surroundings by the endothermic reaction is equal to the energy transferred to the surroundings during the exothermic reaction
• A good example is the thermal decomposition of hydrate copper sulphate

Question 31

What is Le Chatelier’s Principle?

Answer 31

• The idea that if you change the conditions of a reversible reaction at equilibriumm the system will try to counteract that change

Question 32

Why can Le Chatelier’s principle be useful?

Answer 32

• It can be used to predicts the effect of any changes you make to a reaction system

Question 33

How Le Chantelier’s principle apply to increasing temperature?

Answer 33

• If you raise the temperature, the equilibrium will move in the endothermic direction to try to decrease it. You’ll now get more products for the endothermic reaction and fewer products for the exothermic reaction

Question 34

How Le Chantelier’s principle apply to decreasing temperature?

Answer 34

• If you decrease the temperature, the equilibrium will move in the exothermic direction to produce more heat. This means you’ll get more products for the exothermic reaction and fewer products for the endothermic reaction

Question 35

How Le Chantelier’s principle apply to increasing pressure?

Answer 35

• If you increase the pressure, the equilibrium shifts to the side with the smaller number of molecules

Question 36

How Le Chantelier’s principle apply to decreasing pressure?

Answer 36

• If you decrease the pressure, the equilibrium shifts to the side with the larger number of molecules

Question 37

How Le Chantelier’s principle apply to decreasing concentration?

Answer 37

• If you increase the concentration of products, the system tries to increase it again by reducing the amount of products

Question 38

How Le Chantelier’s principle apply to increasing concentration of reactants?

Answer 38

• If you increase the concentration of the reactants, the system tries to decrease it by making more products