RP 2 - 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.

Question 2

How can you reduce the uncertainty in the mass measurement?

Answer 2

1) Use a balance with a greater resolution.
2) Use a larger mass.

Question 3

What is percentage uncertainty and how do you calculate it?

Answer 3

Percentage uncertainty in a measurement = 100 x absolute uncertainty / calculated value

Question 4

How do you calculate enthalpy change of a reaction experimentally?

Answer 4

1) q=mc∆T
2) Where m is the mass of the solution that changes temperature, c is the specific heat capacity and ∆T is the temperature change measured using a thermometer and q is the heat energy taken in or released.
3) Divide this number in kilojoules by the number of moles of the limiting reactant
4) Add a sign to show whether enthalpy change is exothermic or endothermic

Question 5

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

Answer 5

• Heat loss to apparatus/surroundings
• Incomplete combustion
• Non-standard conditions
• Evaporation of alcohol/water

Question 6

How do you prevent heat loss to surrounding/apparatus?

Answer 6

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

Question 7

Other than preventing heat loss, how can the accuracy of this experiment be improved?

Answer 7

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

Question 8

What is accuracy?

Answer 8

The more accurate the data the closer is to the actual value