Measuring enthalpy changes

Question 1

What is the general name given to the enthalpy change for a reaction? What are its units?

Answer 1

Standard molar enthalpy change kilojoules per mole

Question 2

What is the stnadard molar enthalpy of formation (Δ_fH^θ)

Answer 2

The enthalpy change when one mole of substance is formed from its constituent elements, under standard conditions.

All reactants and products being in their standard states.

Question 3

What is standard molar enthalpy of combustion (Δ_cH^θ)?

Answer 3

The enthalpy change when one mole of substance is completely burnt in oxygen under standard conditions, all reactants and products being in their standard states.

Question 4

What is the difference between temperature and heat?

Answer 4

Temperature:

• average KE of particles in a system, as particles move faster their average KE increases and temp rises.
• It does not matter how many particles there are - temperature is independant of the number present.
• Measured with a thermometer

Heat:

• The measure of the total energy of all the particles present in a given amount of substance.
• It does depend on how much of the substance is present.
• The energy of every particle is included.
• Therefore a lukewarm bath has more heat than a hot nail.
• Heat always flow from high to low temperature.

Question 5

What is needed to measure the enthalpy change of a reaction?

Answer 5

There is no instrument to directly measure heat.

You need:

1. Mass of substance that is being heated up or colled down
2. To measure the temperature change.
3. The specific heat capacity of the substance.

Question 6

What is specific heat capacity?

What is the heat capacity of water?

Answer 6

The amount of heat needed to raise the temperature of 1g of a substance by 1K, its units are J g^-1 K^-1.

For water it is 4.18

Question 7

What is the equation for enthalpy change, Q?

Answer 7

Q = m x c x ΔT

mass x specific heat capacity x temperature change

Question 8

What is the apparatus used to measure enthalpy change using a simple calorimeter? How is the apparatus used?

Answer 8

Spirit burner

Water

Thermometer

Calorimeter

Use the fuel to heat a known mass of water and then measure the temperature rise of the water.

You assume that all the heat from the fuel goes into the water.

Question 9

What are the uses of the simple calorimiter?

Answer 9

Can be used to compare the Δ_cH values for a series of similar compounds, because the errors will be similar for every experiment.

Question 10

How can heat loss be reduced when using a simple calorimeter?

Answer 10

• Using draught screens and mineral wool to insulate the burner.
• Reduce distance
• sleeve around flame - reduce draft
• lid
• copper calorimiter

Question 11

How does a flame calorimiter reduce heat loss?

(theres a whole subtitle in the text book, not 100% if we need to know this, or maybe Jarv missed it)

Answer 11

Has a spiral chimney made from copper

Flame is enclosed

Fuel burns in pure oxygen rather than air