Rp-02 Measuring Enthalpy Change

Question 1

What’s Hess’ law?

Answer 1

• enthalpy change for a reaction is always the same regardless of the route taken
• sum of the clockwise enthalpy change equals the same of the anti-clockwise enthalpy changes

Question 2

What is percentage uncertainty and how to calculate it?

Answer 2

100 x absolute uncertainty/calculated value

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

How do you calculate enthalpy change of reaction experimentally?

Answer 4

Q= mcDeltaT

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 surroundings/apparatus?

Answer 6

• insulate beaker by placing in a polystyrene cup with a lid
• avoid large temp 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 thermometer at eye level to avoid parallax error
• stir solution so temp is evenly distributed
• use digital thermometer for accurate quicker readings
• use greater concentrations and masses leading to greater temperature and thus smaller uncertainty