C5 - Monitoring And Controlling Chemical Reactions

Question 1

Describe the yield of a reaction

Answer 1

The yield of a reaction is the mass of the desired products which is made

Question 2

Describe the theoretical yield of a reaction

Answer 2

The theoretical yield is the maximum possible mass of desired products you can make from a given mass of reactants

Question 3

Describe the actual yield of a reaction

Answer 3

The actual yield is the mass of products you actually make

Question 4

How do you calculate percentage yield?

Answer 4

Percentage yield = Actual yield / Theoretical yield x 100

Question 5

What factors affect percentage yield?

Answer 5

• Reactants reacting in an unexpected way
• Reaction not going to completion
• Losing products when separating or moving the substance

Question 6

What is atom economy?

Answer 6

Atom economy is a measure of how many atoms in the products make up the desired product

Question 7

How do you calculate atom economy?

Answer 7

Atom Economy = Mr of desired products / Mr of all products x 100

Question 8

What factors should be considered when making a substance?

Answer 8

• Cost
• Rate of reaction
• Equilibrium

Question 9

What are by products?

Answer 9

By products are the other products formed in a reaction that aren’t the desired products

Question 10

Important conversions for concentration:

Answer 10

1dm³ = 1L

1cm³ = 1ml

cm³ /1000 = dm³

dm³ x 1000 = cm³

Question 11

Equation for concentration:

Answer 11

Concentration = Amount in moles / Volume

Question 12

What are the units for concentration?

Answer 12

g/dm³

mol/dm³

Question 13

How do you calculate a new concentration from a given concentration and volume?

Answer 13

To calculate the new concentration:

C₁V₁ = C₂V₂

Worked example:

20cm³ of 2mol/dm³ HCl was added to 80cm³ of water - what is the new concentration?

1. First assign the values for C₁, V₁, C₂, V₂ - C₁ = 2mol/dm³ V₁ = 0.020 dm³ C₂ = ? V₂ = 0.1dm³
2. Substitute values and rearrange - 2 x 0.02 = C₂ x 0.1 C₂ = 2 x 0.02 / 0.1 = 0.4 mol/dm³

Question 14

What is a titration?

Answer 14

A titration is a method used to find an unknown concentration

Question 15

Define a ‘titre’

Answer 15

Volume of solution added from a burette during a titration

Question 16

What are concordant results?

Answer 16

Titres which are within 0.1cm³ of each other

Question 17

What is molar volume?

Answer 17

1 mol of any substance in the gas state occupies the same volume at the same temperature and pressure

At room temp and pressure, the molar volume is 24dm³

Question 18

What equation links gaseous moles and molar volume?

Answer 18

Amount (mol) = volume (dm³) / 24

Question 19

Define ‘rate of reaction’

Answer 19

How quickly reactants are used up or products are formed

Question 20

How do you calculate rate of reaction?

Answer 20

Rate of reaction = Amount of products produced / time

Rate of reaction = Amount of reactants used / time

Question 21

What is ‘collision theory’?

Answer 21

In order for two particles to react with each other, two things need to happen:

• Reacting particles need to collide
• They need to have enough energy (more than activation energy)

If these two things happen: a successful collision will occur

Question 22

Something will increase the rate of reaction if it:

Answer 22

• Increases the number of collisions
• Increases the percentage of successful collisions

Question 23

When the temperature of a reaction is increased:

Answer 23

1. The number of collisions increase as the particles are moving more quickly
2. The percentage of collisions which are successful increases because all the particles have more energy

This results in an increase in rate of reaction

Question 24

How does an increased concentration affect rate of reaction?

Answer 24

Increasing concentration increases the reaction rate because the particles have become more crowded which increases the number of collisions

Question 25

What do catalysts do?

Answer 25

Catalysts lower the activation energy of a reaction by providing an alternative reaction pathway. They have no effects on the number of collisions however they do increase the percentage of successful collisions.

Question 26

How does pressure affect the rate of reaction?

Answer 26

Increasing pressure increases the reaction rate because the particles have become more crowded which increases the number of collisions

Question 27

How does surface area affect rate of reaction?

Answer 27

Increasing the surface area increases the reaction rate because more particles are available to collide with, increasing the number of collisions.

Question 28

What is a reversible reaction?

Answer 28

A reversible reaction is one in which the products are able to react with each other to change back into the reactants without any additional changes.

Question 29

Define 'equilibrium'

Answer 29

When the forward rate of reaction is equal to the backwards rate of reaction in a reversible reaction.

Question 30

How does the rate change over time during equilibrium?

Answer 30

* The rate of the forward reaction decreases * The rate of the backward reaction increases * Until they are the same - the rate stops changing

Question 31

How does the concentration change over time during equilibrium?

Answer 31

* The concentration of the reactants decreases * The concentration of the products increase * The concentration stops changing until equilibrium is reached

Question 32

What is the position of equilibrium?

Answer 32

The position of equilibrium is related to the ratio of the concentration of products to the concentration of reactants It is on the left if the reactant concentration is greater than the product concentration It is on the right if the product concentration is greater than the reactant concentration

Question 33

Position of equilibrium on the left:

Answer 33

Question 34

Position of equilibrium on the right:

Answer 34

Question 35

What is Le Chatelier's principle?

Answer 35

'Any change made to a reaction which is in equilibrium, will result in the equilibrium position moving to minimise the change made'

Question 36

What effect does temperature have on dynamic equilibrium?

Answer 36

If you decrease the temperature, the equilibrium will move in the exothermic direction to produce more heat If you increase the temperature, the equilibrium will move in the endothermic direction to absorb the extra heat

Question 37

What effect does pressure have on dynamic equilibrium?

Answer 37

If you increase the pressure, the equilibrium will move towards the side that has fewer moles of gas to reduce pressure. If you decrease the pressure, the equilibrium will move towards the side that has more moles of gas to increase pressure.

Question 38

What effect does concentartion have on dynamic equilibrium?

Answer 38

If you increase the concentration of the reactants, the equilibrium will move to the right to use up the reactants. If you increase the concentration of the products, the equilibrium will move to the left to use up the products. Decreasing the concentrations will have the opposite effect.