RP2 - Measuring enthalpy change

Question 1

What is Hess’s Law?

Answer 1

The enthalpy change for a chemical reaction is always the same, regardless of the route from reactants to products.
For a Hess’s cycle the sum of the clockwise
enthalpy changes= sum of anticlockwise enthalpy changes

Question 2

What is the weighing by difference technique?

Answer 2

Method to weigh materials accurately.
Mass of substance = Mass of weighing
dish and substance - Mass of dish after substance has been transferred.

Question 3

How can you reduce the uncertainty in the mass measurement?

Answer 3

Use a balance with a greater resolution.

Use a larger mass.

Question 4

What is percentage uncertainty and how do you calculate it?

Answer 4

absolute uncertainty/ calculated value x100

Question 5

How do you calculate enthalpy change of reaction experimentally?

Answer 5

q = mc𝚫T
Divide this number by the number of moles of the limiting reactant.
Add a sign to show whether 𝚫Hr⦵ is exothermic or endothermic.

Question 6

Why may an experimental value for
enthalpy change be different to the
theoretical value?

Answer 6

1. Heat loss to apparatus/surroundings.
2. Incomplete combustion.
3. Non-standard conditions.
4. Evaporation of alcohol/ water.

Question 7

How do you prevent heat loss to

surroundings /apparatus?

Answer 7

Insulate the beaker by placing it in a
polystyrene cup with a lid.
Avoid large temperature differences between surroundings and calorimeter.
Use bomb calorimeter

Question 8

Other than preventing heat loss, how can the

accuracy of this experiment be improved?

Answer 8

Read thermometer at eye level to avoid parallax errors.
Stir solution so temperature is evenly distributed.
Use digital thermometer for more accurate and faster
readings.
Use greater concentrations and masses, leading to a
greater temperature change and thus smaller uncertainty.

Question 9

What is accuracy?

Answer 9

The more accurate the data, the closer it

is to the actual value.