9.2 - Measuring enthalpy changes

Question 1

0°C is equal to how many K?

Answer 1

273K

Question 2

How is the energy change of the surroundings calculated?

Answer 2

Energy change of the surroundings is calculated from:

• Mass of the surroundings
• Specific heat capacity
• Temperature change

Question 3

What is meant by the specific heat capacity?

Answer 3

Different materials require different quantities of energy to produce the same temperature change.

The specific heat capacity c is the energy required to raise the temperature of 1g of a substance by 1K.

Conductors have small values of c, insulators have large values of c.

Water has a specific heat capacity of 4.18Jg^-1K^-1

Question 4

What is the equation for calculating energy change?

Answer 4

q = mc**ΔT

Question 5

How would you determine enthalpy change of combustion for fuels? - Method

Answer 5

• Using a spirit burner.

1. Measure out 150cm³ of water. Pour the water into a beaker. Record the initial temperature of the water to the nearest 0.5°C.
2. Add methanol to the spirit burner. Weigh the spirit burner containing methanol.
3. Place the spirit burner under the beaker. Light the burner and burn the methanol whilst stirring whilst stirring the water with the thermometer.
4. After 3 minutes extinguish flame. Immediately record maximum temperature reached by water,
5. Re-weigh the spirit burner containing the methanol. Assume the wick has not been burnt.

Question 6

How would you determine enthalpy change of combustion? - equation

Answer 6

q = mcΔT

• Calculate energy change q of the water (surroundings)
• Calculate the mol of fuel burnt,
• Calculate Δ_cH by scaling up mol of fuel to 1 mol.

Question 7

What factors can affect the accuracy of the experimental Δ_cH value?

Answer 7

• Heat loss to the surroundings other than water.
• Incomplete combustion of methanol - means CO or C would be produced, not CO2. This isn’t part of the combustion equation we’re calculating.
• Evaporation of methanol from the wick before weighing.
• Non-standard conditions - in data book, value is standard value. Conditions for experiment will be different from standard conditions.

These would lead to a value of Δ_cH that is less exothermic than expected.

Question 8

How could you mitigate factors that affect the experimental Δ_cH value?

Answer 8

• Use of draught screens
• An input of oxygen gas