2.1 Thermochemistry

Question 1

What are the standard conditions of enthalpy change?

Answer 1

1. All substances in their standard states
2. A temperature of 298k
3. A pressure of 1 atm

Question 2

Define the term, standard enthalpy of combustion:

Answer 2

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

Question 3

Define the term, standard enthalpy of formation:

Answer 3

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

Question 4

Define the term, mean bond enthalpy

Answer 4

Average value of enthalpy required to break a given type of covalent bond in the molecule of a gaseous species

Question 5

Describe the experimental procedures of combustion:

Answer 5

1. Allow a suitable gap between the base of the metal container and the top of the spirit burner
2. Accurately measure the amount of water being added to metal container
3. use an accurate thermometer to measure initial temp
4. weigh the spirit burner containing the alcohol and record the initial mass
5. allow the alcohol to heat the water to a suitable temp
6. extinguish the flame and record the final maximum temperature
7. once cooled weigh the spirit burner and record the mass

Question 6

Give two reasons why the value of mass will be much lower than the book value in experimental procedures of combustion:

Answer 6

1. some energy will be transferred to surroundings
2. The alcohol is not completely combusted, soot on the bottom of the calorimeter will show if this has happened

Question 7

Describe the experimental procedures for indirect determination of enthalpy change:

Answer 7

1. measure an appropriate volume of acid using a pipette and place it in a polystyrene cup
2. use a thermometer to measure the initial temperature of acid
3. accurately weight out solid, in powder form in a suitable container
4. add all the solid to the cup and stir the mixture well and start the stop watch
5. keep stirring and record the temp regularly, stop recording the temp when its fallen for 5 mins
6. Re-weigh the weighing container to ensure that the correct mass of solid added is recorded
7. plot a graph to calculate the maximum temperature reached
8. calculate enthalpy change for the reaction
9. repeat the procedure with other solid