Physical 1 Section 1 - Unit 4: Energetics
No description yet. This curated deck is ready for daily study.
Card preview (12)
State what is meant by the term mean bond enthalpy (2 marks)
• The enthalpy change required to break a covalent bond - Averaged over a wide range of compounds
Explain what is meant by the term ‘standard enthalpy of formation’ (3 marks)
• Enthalpy change when one mole of a compound • Is formed from its constituent elements • Under standard conditions
Explain what is meant by the term ‘standard enthalpy of combustion’ (3 marks)
• Enthalpy change when one mole of a compound • Is completely burnt in oxygen • Under standard conditions
Explain errors that may occur in the calorimetry experiment (3 marks)
• Incomplete combustion • Heat loss to surroundings • Approximation of specific heat capacity of solution
State Hess’s Law (2 marks)
• The total enthalpy change for a reaction - Is independent of the route taken
Why are mean bond enthalpies inaccurate?
• Bond enthalpies depend on a particular molecule, mean bond enthalpies are general
State the meaning of the term enthalpy change (1 mark)
• Heat (energy) change at constant pressure
State how you would reduce uncertainty in measuring mass (2 marks)
• Use a balance with a greater resolution - Use a larger mass
State how you would calculate percentage uncertainty (2 marks)
• Percentage Uncertainty = 100 x (absolute uncertainty/calculated value)
Explain how you would improve the accuracy of the calorimetry experiment other than by preventing heat loss (2 marks)
• 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
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)
• Higher in the liquid state - More energy would be required to vaporise the water
State some possible errors in calorimetry ( marks)
• 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