Energy and Equilibria

Question 1

Exothermic

Answer 1

Temperature increases, enthalpy decreases

Question 2

Endothermic

Answer 2

Temperature decreases, enthalpy increases

Question 3

Enthalpy

Answer 3

Chemical potential energy, measured in joules (J)

Question 4

Examples of exothermic reactions

Answer 4

1. Neutralisation

2. Acid + metal reactions

Question 5

An example of an endothermic reaction

Answer 5

Thermal decomposition

Question 6

1. How can enthalpy change be expressed in terms of bond energies?
2. Which values represent an endothermic reaction?
3. Exothermic?

Answer 6

1. Bonds broken - bonds made
2. Positive
3. Negative

Question 7

1. The breaking of bonds is:

2. The making of bonds is:

Answer 7

1. Endothermic

2. Exothermic

Question 8

Steps to calculate enthalpy change of a reaction

Answer 8

1. Balance the equation
2. Show all the bonds
3. Show all bond energies
4. Add the bond energies together
5. ∆H = bonds broken - bonds made

Question 9

What is the unit for bond energies?

Answer 9

kJ/mol

Question 10

Equation for enthalpy released

Answer 10

Energy released = mass of liquid * specific heat capacity * change in temperature
OR
Q = m * c * ∆T

Question 11

What is:

1. The mass of 100ml of water?
2. Water’s specific heat capacity*?

*Don’t need to know

Answer 11

1. 100g

2. 4.18 kJ/kg/°C

Question 12

Energy released per gram of fuel

Answer 12

= Enthalpy / mass of fuel

Question 13

Calorimetry experiment steps

Answer 13

1. Cold water is measured into a copper calorimeter (small metal cup)
2. Starting temperature of the water is recorded using a thermometer
3. The water is heated using the flame from the burning fuel
4. The final temperature of the water is recorded
   THEN Q = mc∆T

To find the mass of fuel burned, weigh the spirit burner before and after the experiment

Question 14

What must be kept the same in the calorimetry experiment to ensure a fair test?

Answer 14

1. Mass of water
2. Distance between spirit burner and calorimeter
3. Concentration of fuel
4. Starting temperature of the water

Question 15

How can you tell if a temperature change reaction is complete?

Answer 15

The reaction should have reached a minimum temperature

Question 16

Reversible reaction

Answer 16

A reaction where the reactants form products, which, in turn, react together to form the reactants again

Question 17

What symbol indicates a reversible reaction?

Answer 17

⇌

Question 18

Examples of reversible reactions

Answer 18

1. Blue copper (II) sulphate crystals ⇌ white anhydrous copper (II) sulphate
   - when heating the former, the crystals turn into white powder and the water driven off
   - loss of water of crystallisation takes place
2. Ammonium chloride ⇌ ammonia + hydrogen chloride
   - when heating the former, white crystals appear further up test tube
   - recombination takes place further up, where it is cooler

Question 19

Dynamic equilibrium

Answer 19

When the forwards and backwards reactions take place at the same rate, meaning that there is no net change in the ratio of reactants and products

Question 20

Le Chatelier’s Principle

Answer 20

For any change imposed on an equilibrium, the position of the equilibrium will move to oppose that change

Question 21

What are the 3 things you can alter to change the position of equilibrium?

Answer 21

1. Concentration
2. Temperature
3. Pressure

Question 22

What happens if you increase the concentration of the reactants?

Answer 22

1. Increase concentration of reactants
2. System opposes change
3. Forwards reaction occurs more frequently
4. Equilibrium shifts to the right
5. Yield of products increases

Question 23

What happens if you decrease the concentration of the reactants?

Answer 23

1. Decrease concentration of reactants
2. System opposes change
3. Backwards reaction occurs more frequently
4. Equilibrium shifts to the left
5. Yield of reactants increases

Question 24

What happens if you increase the temperature and the forwards reaction is exothermic?

Answer 24

1. Increase temperature
2. System opposes change
3. Backwards reaction (endothermic - takes up heat) occurs more frequently
4. Equilibrium shifts to the left
5. Yield of reactants increases

Question 25

What happens if you decrease the temperature and the forwards reaction is endothermic?

Answer 25

1. Decrease temperature 2. System opposes change 3. Backwards reaction (exothermic - gives off heat) occurs more frequently 4. Equilibrium shifts to the left 5. Yield of reactants increases

Question 26

What happens if you increase the pressure and the products have more molecules?

Answer 26

1. Increase pressure → volume decreased 2. System opposes change 3. Equilibrium shifts to left (side with fewer molecules) 4. Yield of reactants increases

Question 27

What happens if you decrease the pressure and the products have fewer molecules?

Answer 27

1. Decrease pressure → volume increased 2. System opposes change 3. Equilibrium shifts to left (side with more molecules) 4. Yield of reactants increases

Question 28

What effect does the addition of a catalyst have on the yield of products and reactants?

Answer 28

Nothing - catalysts only affect the rate of reaction