The rate and extent of chemical change

Question 1

What is meant by rate of reaction

Answer 1

Measure of how quickly a reactant is used up or a product is formed in a reaction per unit of time

Question 2

How do you determine the rate of reaction

Answer 2

Measuring the products made or the reactants consumed, and the time it takes

Question 3

What are the two equations for rate of reaction; give the units

Answer 3

1. Quantity of reactant used (g or cm^3)/time taken (seconds)
2. quantity of product formed (g or cm^3)/time taken (seconds)

Question 4

What are the units which can be used to measure rate of reaction

Answer 4

g/s or cm^3/s

Question 5

What are limiting reactants

Answer 5

Chemical used up in a reaction that limits the amount of product formed

Question 6

How can you measure the rate of reaction at a specific point on a graph

Answer 6

• Draw a tangent from the point
• Measure the rise and the run
• Do rise/run (quantity formed/time taken)

Question 7

What factors affect the rate of reaction

Answer 7

• Concentration
• Pressure
• Surface area
• Temperature
• Presence of catalysts

Question 8

How does temperature affect the rate of reaction

Answer 8

As temp increases, particles gain more energy therefore they move faster, so particles will collide more frequently and with more energy each time (so they will more likely exceed the activation energy), which increases the rate of reaction

Question 9

How does concentration affect the rate of reaction

Answer 9

As concentration increases, there are more particles per unit of volume, therefore collisions are more frequent, which increases the rate of reaction

Question 10

How does pressure affect the rate of reaction

Answer 10

As pressure increases, there are more particles per unit of volume, therefore collisions are more frequent, which increases the rate of reaction

Question 11

How does surface area affect the rate of reaction

Answer 11

As surface area increases, frequency of collisions increases, therefore rate of reaction increases

Question 12

How does the presence of a catalyst affect the rate of reaction

Answer 12

Increases the rate of reaction by lowering the activation energy, therefore less energy is needed for a reaction to occur as less successful collisions are required for a reaction to occur

Question 13

What is collision frequency

Answer 13

Number of successful collisions between reactant particles that happen each second

Question 14

At any point in a reaction, what will the rate of reaction depend on

Answer 14

• Amount of energy that the particles have; the more energy they have the more energy they transfer during collisions, making it more likely to surpass the activation energy
• Frequency of collisions; how often particles collide with each other. The more often they collide, the more successful collisions there will be over all

Question 15

What does the collision theory state

Answer 15

For particles to react, they have to collide with sufficient energy (activation energy)

Question 16

What is a hypothesis

Answer 16

A proposal that could explain a fact or an observation

Question 17

Describe the practical to discover how concentration affects the rate of reaction

Answer 17

1. Measure 50cm3 of 2mol/dm3 hydrochloric acid using a measuring cylinder and pour it into a 100cm3 conical flask
2. Add a 3cm strip of magnesium ribbon to the flask and put the bung attached to a gas syringe into the flask
3. Record the volume of hydrogen gas given off every 10 seconds up to 100 seconds
4. Repeat this using 1mol/dm3 hydrochloric acid and 0.5mol/dm3

Question 18

What is a catalyst

Answer 18

A substance which increases the rate of chemical reactions but are not used up during the reaction

Question 19

How do catalysts work

Answer 19

They increase the rate of reaction by providing a different pathway for the reaction that has a lower activation energy

Question 20

How does the presence of a catalyst affect how an energy profile graph looks

Answer 20

• The activation energy with the catalyst is lower than it would be without the catalyst, so the curve is lower on the diagram

Question 21

What is a reversible reaction

Answer 21

A reaction where the products can react together to reform the original reactants

Question 22

How can you change the direction of a reversible reaction

Answer 22

By changing the conditions

Question 23

What is a reversible reaction represented by

Answer 23

A + B <–> C + D

Question 24

Give 2 examples of reversible reactions

Answer 24

• Ammonium chloride -> ammonia + hydrogen chloride
• Thermal composition of hydrated copper sulphate; Hydrated copper sulphate (blue) -> anhydrous copper sulphate (white) + water

Question 25

In a reversible reaction, what reaction will either the forward or backward reaction be

Answer 25

Exothermic or endothermic

Question 26

If the forward reaction is exothermic, what will the backwards reaction be

Answer 26

endothermic

Question 27

In the reversible reaction of ammonium chloride, which direction of reaction is heating or cooling and exothermic or endothermic

Answer 27

• Forwards reaction is heating and is endothermic
• Backwards reaction is cooling and is exothermic

Question 28

In the thermal decomposition of hydrated copper sulphate, which direction of reaction is endothermic or exothermic

Answer 28

• The forwards reaction is endothermic
• The backwards reaction is exothermic

Question 29

What is equilibrium

Answer 29

When the forward and reverse reactions occur at exactly the same rate

Question 30

What is the concentration of reactants and products at equilibrium

Answer 30

They stay the same and are constant, but this does not mean they have the same concentration

Question 31

What is meant by the position of equilibrium moving to the left

Answer 31

More reactant and fewer products

Question 32

What is meant by the position of equilibrium moving to the right

Answer 32

More products and less reactants

Question 33

What condition must there be for equilibrium to be reached

Answer 33

The reversible reaction must be done in a closed system, which means a sealed environment where none of the reactants or products can escape (if products escaped you would not reach equilibrium)

Question 34

How can the position of equilibrium change

Answer 34

By changing the conditions; if a system is at equilibrium and a change is made to any of the conditions, then the system responds to counteract the change

Question 35

What is Le Chatelier’s principle

Answer 35

The idea that if you change the conditions of a reversible reaction at equilibrium, then the position of equilibrium will shift to try and counteract the change

Question 36

What changing conditions on a system at equilibrium can be applied to Le Chatelier’s principle

Answer 36

• Changing concentration
• Changing temperature
• Changing pressure

Question 37

How does changing concentration effect equilibrium

Answer 37

• If the concentration of one of the reactants or products is changed, the system is no longer at equilibrium and the concentration of all the substances will change until equilibrium is reached again
• If the concentration of a reactant is increased, more products will be formed until equilibrium is reached again, so the position of equilibrium will move to the right
• If the concentration of products is increased, the system will counteract this until equilibrium is reached again, so the position of equilibrium will move to the left
• If the concentration of a product is decreased, more reactants will react until equilibrium is reached again, so the position of equilibrium will move to the right

Question 38

How does increasing temperature effect equilibrium

Answer 38

• Equilibrium shifts to favour the endothermic reaction to reduce the temperature, as energy is taken in causing the temperature to fall
• The relative amount of products at equilibrium increases for an endothermic reaction
• The relative amount of products at equilibrium decreases for an exothermic reaction

Question 39

How does decreasing temperature effect equilibrium

Answer 39

• Equilibrium shifts to favour the exothermic reaction to increase the temperature as energy is released, causing the temperature to increase
• Relative amount of products at equilibrium decreases for an endothermic reaction
• The relative amount of products at equilibrium increases for an exothermic reaction

Question 40

How does increasing pressure effect equilibrium

Answer 40

The position of equilibrium shifts to the side with the smaller number of molecules in order to lower pressure (this is because pressure measures how many particles there are per unit of volume, so to lower the volume the position of equilibrium will shift to the side with the least number of molecules)

Question 41

How does decreasing pressure effect equilibrium

Answer 41

The position of the equilibrium shifts to the side with the larger number of molecules in order to increase pressure

Question 42

How does pressure effect a system where the number of molecules on both sides is the same

Answer 42

Changing pressure has no effect on the position of equilibrium