Physical 1 Section 1 - Unit 4: Energetics

Question 1

State what is meant by the term mean bond enthalpy (2 marks)

Answer 1

• The enthalpy change required to break a covalent bond

- Averaged over a wide range of compounds

Question 2

Explain what is meant by the term ‘standard enthalpy of formation’ (3 marks)

Answer 2

• Enthalpy change when one mole of a compound
• Is formed from its constituent elements
• Under standard conditions

Question 3

Explain what is meant by the term ‘standard enthalpy of combustion’ (3 marks)

Answer 3

• Enthalpy change when one mole of a compound
• Is completely burnt in oxygen
• Under standard conditions

Question 4

Explain errors that may occur in the calorimetry experiment (3 marks)

Answer 4

• Incomplete combustion
• Heat loss to surroundings
• Approximation of specific heat capacity of solution

Question 5

State Hess’s Law (2 marks)

Answer 5

• The total enthalpy change for a reaction

- Is independent of the route taken

Question 6

Why are mean bond enthalpies inaccurate?

Answer 6

• Bond enthalpies depend on a particular molecule, mean bond enthalpies are general

Question 7

State the meaning of the term enthalpy change (1 mark)

Answer 7

• Heat (energy) change at constant pressure

Question 8

State how you would reduce uncertainty in measuring mass (2 marks)

Answer 8

• Use a balance with a greater resolution

- Use a larger mass

Question 9

State how you would calculate percentage uncertainty (2 marks)

Answer 9

• Percentage Uncertainty = 100 x (absolute uncertainty/calculated value)

Question 10

Explain how you would improve the accuracy of the calorimetry experiment other than by preventing heat loss (2 marks)

Answer 10

• Read the thermometer at eye level
• Stir the solution so that the temperature is evenly distributed
• Use a digital thermometer for accurate readings
• Use greater concentrations and masses so there is a smaller uncertainty

Question 11

How would the value for the formation of water differ if it referred to the formation in the liquid state rather than the gaseous state (2 marks)

Answer 11

• Higher in the liquid state

- More energy would be required to vaporise the water

Question 12

State some possible errors in calorimetry ( marks)

Answer 12

• Energy transfer from / to surroundings
• Approximation in specific heat capacity of solution - assumes all solutions have the heat capacity of water
• Neglects specific heat capacity of the calorimeter - energy absorbed by the apparatus is ignored
• Reaction may be incomplete or slow.
• Density of solution is taken to be the same as density of water