Energetics Flashcards
What is enthalpy change?
The heat energy change at constant pressure.
What mean bond enthalpy?
The heat energy required to break one mole of a covalent bond, averaged for that type in a range of different compounds in the gaseous state.
How do you calculate enthalpy change with mean bond enthalpies?
(Sum of all bonds broken) — (Sum of all bonds formed)
What are the limitations of bond energy calculations?
- Bond enthalpies only apply to reactions in gas phase.
- Bond enthalpies are averaged over a series of compounds.
What are the standard conditions of enthalpy change?
• Temperature:
- 298K
- Or 25°C
• Pressure:
- 100kPa
- Or 1atm
What is standard enthalpy of formation?
The enthalpy change that occurs when one mole of a compound is formed from its constituent elements with all reactants and products in their standard states.
What is standard enthalpy of combustion?
The enthalpy change that occurs when one mole of a compound reacts completely in oxygen with all reactants and products in their standard states.
What is the equation to calculate heat energy change?
Heat energy change = mass of water X specific heat capacity X Temperature
q = mcAT
Why will the experimental value be different to the data book value?
- Heat loss to surroundings and equipment.
- Not standard condition/states.
- Some incomplete combustion may occur.
What is Hess’ Law?
The enthalpy change for a chemical reaction is independent of the route taken.
Why is not always possible to measure the enthalpy change of a reaction directly?
- Reaction may be too slow.
- Reaction may have a very high activation energy.
- Multiple products may form.
How do you use Hess’ Law in a calculation?
- Write a balanced equation for the reaction in the question.
- Complete the energy cycle by identifying how the species involved in the equation relate to the data provided.
- Identify the two routes around the cycle and apply Hess’ Law
- Re-arrange to find the enthalpy change required.
