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.
● (For a Hess’s cycle the sum of the clockwise enthalpy changes equals the sum of the anticlockwise enthalpy changes)

Question 2

What is the weighing by difference technique?

Answer 2

● It is a 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

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

Question 5

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

Answer 5

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

Question 6

How do you prevent heat loss to surroundings/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 calorimeter

Question 7

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

Answer 7

● Read the thermometer at 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 it is to the actual value.