3.1.4- Energetics

Question 1

What is enthalpy?

Answer 1

Another word for energy or heat.

Question 2

What is enthalpy change (∆H)?

Answer 2

• The heat energy change.
• Measured at constant pressure.

Question 3

What are the two types of reactions?

Answer 3

• Endothermic
• Exothermic

Question 4

What is happens to the bonds in an endothermic reaction?

Answer 4

Bonding breaking.

Question 5

What happens to enthalpy change in an endothermic reaction?

Answer 5

• Enthalpy change has a positive sign (+).
• Products have more energy than reactants.

Question 6

What happens to temperature in an endothermic reaction?

Answer 6

• As heat is taken in.
• Temperature decreases.
  OR
• Takes in energy.
• The temperature of the surroundings decreases.

Question 7

What is happens to the bonds in an exothermic reaction?

Answer 7

Bond making.

Question 8

What happens to enthalpy change in an exothermic reaction?

Answer 8

• Enthalpy change has a negative sign (-).
• Reactants have more energy than products.

Question 9

What happens to temperature in an exothermic reaction?

Answer 9

• As heat is given out.
• Temperature increases.
  OR
• Transfers energy to the surroundings.
• The temperature of the surroundings increases.

Question 10

What are the standard conditions in standard enthalpy change?

Answer 10

• Pressure of 100kPa.
• Temperature of 298K (∆H298Ɵ).

Question 11

What is enthalpy of combustion (∆cHƟ)?

Answer 11

• Enthalpy change when one moles of a substance is completely burned.
• In excess oxgen.

Question 12

What is enthalpy of formation (∆fHƟ)?

Answer 12

• Enthalpy change when one mole of a compound is formed.
• From its elements in their standard states.
• Under standard conditions.

Question 13

What is calorimetry?

Answer 13

A way to measure the heat change of a chemical reaction.

Question 14

What are example of calorimetry experiments?

Answer 14

Measuring the temperature change:
- Of water in a simple reaction (e.g. NaOH and HCl), where a polystyrene cup with a lid is used.
- When a fuel (e.g. ethanol) is burned (combustion) in a spirit burner, to heat up water.
Using a thermometer.

Question 15

Why is a beaker not used in a calorimetry practical?

Answer 15

• A beaker with conduct heat.
• Leading to heat loss.

Question 16

Why is a polystyrene cup with a lid used in a calorimetry practical?

Answer 16

• It is better at retaining heat.
• It can minimise heat loss.

Question 17

How to calculate enthalpy change (kJ mol-1)?

Answer 17

Q = mc∆T
- Q= heat change in joules
- m= mass of water in grams
- c= specific heat capacity of water (4.18 J g-1 K-1)
- ∆T= temperature change measured (°C or K)

Question 18

How to calculate enthalpy change using equation?

Answer 18

1. Calculate Q.
2. Calculate moles.
3. Q divided by moles.
4. Divide by 1,000 to convert from J to kJ.

REMEMBER:
- For water g=cm3
- Temperature decrease in question means it is endothermic (+).
- Temperature increase means it is exothermic, so include (-) in answer.

Question 19

What is the main reason why the experimental and theoretical values differ in calorimetry?

Answer 19

Heat loss.

Question 20

Why might the experimental and theoretical values differ in calorimetry?

Answer 20

• Incomplete combustion of fuel.
• Experiment not under standard conditions.
• Heat capacity of water my not be exactly 4.18, as water is not pure.
• Heat of the flame may heat the surroundings.
• Some fuel in spirit burner may evaporate.
• Solid used may not be 100% pure (impurities).
• Thermometer may have small reading errors.
• Volume may change when adding solid.