2.1 Thermochemistry

Question 1

What is enthalpy change?

Answer 1

The heat energy change measured at constant pressure.

Question 2

What are standard conditions?

Answer 2

Pressure - 100 kPa
Temperature - 298 K

Question 3

What is an exothermic reaction?

Answer 3

A reaction in which energy is released to the surroundings.

Question 4

What is an endothermic reaction?

Answer 4

A reaction in which energy is taken in from the surroundings.

Question 5

What is the sign of 𝚫H for exothermic and endothermic reactions?

Answer 5

Exothermic: 𝚫H is negative
Endothermic: 𝚫H is positive

Question 6

What is the definition for the standard enthalpy change of reaction, 𝚫ᵣH⁰?

Answer 6

The enthalpy change that occurs when the quantities of reactants shown in the equation react under standard conditions, with all reactants and products in their standard states.

Question 7

What is the definition for the standard enthalpy change of formation, 𝚫fH⁰

Answer 7

The enthalpy change when one mole of product is formed from its constituent elements with all reactants and products in their standard states, under standard conditions.

Question 8

What is the definition for the standard enthalpy change of combustion, 𝚫cH⁰

Answer 8

The enthalpy change that occurs when one mole of compound is completely reacted with excess oxygen under standard conditions with all reactants and products in their standard states.

Question 9

What is Hess’ law?

Answer 9

The enthalpy change of a reaction is independent of the route taken.

Question 10

What is mean bond enthalpy?

Answer 10

The energy needed to break a specific type of covalent bond, averaged out across a wide variety of different compounds.

Question 11

Why do calculation results involving formation and combustion enthalpies differ to calculation results involving mean bond enthalpies?

Answer 11

The calculations using formation and combustion enthalpies are much more accurate. The mean bond enthalpies are averaged out over all the different types of compounds so they are not specific to the compound being investigated.

Question 12

Which equation can be used to work out the energy transferred in a reaction?

Answer 12

q = mc𝚫T

q = energy released/absorbed ( J)
m = mass (g)
c = specific heat capacity ( J g⁻¹ °C⁻¹)
𝚫T = temperature change (°C)

Question 13

Why are energy change reactions carried out in an insulated container?

Answer 13

To prevent heat energy loss to the surroundings.

Question 14

Describe an experiment that can be conducted to investigate the enthalpy change for the thermal decomposition of potassium hydrogencarbonate

Answer 14

1. Weigh 3 g of potassium carbonate into a test tube.
2. Use a burette to add 30 cm³ of 2 mol dm⁻³ hydrochloric acid to a polystyrene cup in a beaker.
3. Measure the initial temperature of the acid.
4. Add the potassium carbonate to the cup. Continually measure the temperature and stir. Record the highest temperature reached.
5. Use the equation q=mc𝚫T to calculate the enthalpy change.

Question 15

Define enthalpy change

Answer 15

Enthalpy change is the heat energy evolved or absorbed in a reaction at constant pressure.

Question 16

Define standard enthalpy change of combustion

Answer 16

The enthalpy change when one mole of a compound is burned completely in oxygen under standard conditions in their standard states.

Question 17

What type of reaction is a combustion reaction?

Answer 17

Exothermic reaction:
Energy is transferred to the surroundings.

Question 18

What apparatus is required to investigate the enthalpy change of combustion?

Answer 18

• Digital mass balance
• Conical flask
• Spirit burner
• Heatproof mat
• Thermometer
• Clamp and stand

Question 19

Give the experimental procedure to investigate the enthalpy change of combustion of methanol

Answer 19

1. Add 100 cm³ of deionised water to a 250 cm³ conical flask.
2. Place the conical flask in the clamp and position it on the stand at a height so that the spirit burner can fit underneath it.
3. Weigh a spirit burner and lid containing methanol. Record the mass.
4. Record the initial temperature of the water.
5. Place the spirit burner under the conical flask and light the wick.
6. Allow the flame to heat the water to around 40 °C.
7. Extinguish the flame and record the final temperature of the water.
8. Re-weigh the spirit burner and lid and record the mass. Calculate the mass of methanol used.
9. Determine the energy released by methanol and calculate the enthalpy change of combustion.

Question 20

Give the equation used to calculate enthalpy change

Answer 20

q = mc𝚫T

q = enthalpy change ( J)
m = mass of the solution which changes temperature (g)
c = specific heat capacity ( J g⁻¹ K⁻¹)
𝚫T = change in temperature of the solution (K)

Question 21

0.23 moles of a fuel raised the temperature of 100g of water from 293 K to 357 K.
What is the standard enthalpy of combustion of the fuel?
The specific heat capacity of water is 4.18 J g⁻¹ K⁻¹

Answer 21

First calculate the amount of heat given off by the fuel:
q= mc𝚫T = 100 x 4.18 x (357-293) = 26752 J = 26.8 kJ

Now find the energy to burn one mole and since it is an exothermic reaction, the final value must be negative:

𝚫cH⁰ = -(26.8 ÷ 0.023) = -1170 kJ mol⁻¹
(3 s.f.)