2. Thermochemistry

Question 1

What is enthalpy measured in?

Answer 1

KJmol-1

Question 2

What is the principle of conservation of energy?

Answer 2

All the energy that leaves a system enters the surroundings and vice versa. In an exothermic reactions, energy is not being created nor destroyed in an endothermic reaction. The total energy of the whole system remains constant.

Question 3

What is the standard enthalpy change?

Answer 3

All substances in their standard states, a temperature of 298K and 1atm pressure.

Question 4

What is the standard enthalpy change of formation?

Answer 4

The enthalpy change when one mole of a substance is formed from its constituent elements in their standard states under standard conditions.

Question 5

What is the standard enthalpy change of combustion?

Answer 5

The enthalpy change when one mole of a substance is completely combusted in oxygen under standard conditions.

Question 6

What is Hess’s law?

Answer 6

The total enthalpy change for a reaction is independent of the route from the reactants to the products.

Question 7

What is bond enthalpy?

Answer 7

The average energy required to break one mole of bonds.

Question 8

What is the average bond enthalpy?

Answer 8

The average value of the enthalpy required to break a given type of covalent bond in the molecules of a gaseous species.

Question 9

How can energy change be calculated?

Answer 9

By using the temperature change of a reaction in a calorimeter.

Question 10

What is the equation for energy?

Answer 10

E=mcT

Question 11

What is the equation for enthalpy change?

Answer 11

H=-mcT/n

Question 12

What is enthalpy?

Answer 12

The heat content of a system at constant pressure.

Question 13

What is enthalpy change?

Answer 13

The heat added to a system at a constant pressure.

Question 14

Describe an experiment to determine the enthalpy change of combustion

Answer 14

1. Add water to a metal container and measure the initial temperature of the water
2. Weigh the spirit burner and record the initial mass
3. Allow the fuel to heat the water to a suitable temperature
4. Extinguish the flame and record the final temperature
5. Weigh the final mass of the spirit burner

Question 15

Why can the enthalpy change result be lower than in the books?

Answer 15

Some energy transferred from the burning fuel is lost to heating the surroundings and apparatus
The fuel is not completely combusted

Question 16

Why are values low in an interdict determination of enthalpy change?

Answer 16

Due to the simple type of calorimeter used.

Question 17

Give an example of indirect determination of an enthalpy change

Answer 17

The enthalpy change of reaction of magnesium oxide with carbon dioxide to form magnesium carbonate.

Question 18

Describe an experiment for indirect determination of an enthalpy change

Answer 18

1. Measure an excess of acid using a burette or pipette and place it in the polystyrene cup
2. Use a thermometer to measure the initial temperature of the acid
3. Weigh the solid is powder form and add it to the cup, stirring and starting a stopwatch
4. Keep stirring with the thermometer and record the temperature regularly, stopping when it has fallen for five minutes
5. Plot a graph of temperature over time to calculate the maximum temperature reached
6. Calculate the amount of heat transferred and the enthalpy change