8: Energetics I
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What are standard conditions?
100 kPa 298 K
State what is meant by the enthalpy change?
The heat energy change measured at constant pressure
Define standard enthalpy change of reaction
The enthalpy change when - Substances react together - In the mole quantities given in the equation - Under standard conditions (100 kPa, 298 K)
Define standard enthalpy change of formation
The enthalpy change when - 1 mole of a substance is formed - from its elements in their standard states - Under standard conditions (100 kPa, 298 K)
Define standard enthalpy change of combustion
The enthalpy change when - 1 mole of a substance - is burnt completely in excess oxygen - Under standard conditions (100 kPa, 298 K)
Define standard enthalpy change of neutralisation
The enthalpy change when - 1 mole of water is formed - from the reaction between an acid and an alkali - Under standard conditions (100 kPa, 298 K)
When measuring standard enthalpy change of combustion, what reasons could cause a difference in the experimental value and the theoretical value?
Heat losses to the surroundings Incomplete combustion
What assumptions are made when measuring enthalpy change?
• No energy is lost to the surroundings • The density of a solution is 1.00 g cm^-3 • The SHC of a solution is 4.18
State Hess’s Law.
The enthalpy change for a reaction is independent of the route taken.
What is bond enthalpy?
The energy needed to break a bond (The same as the energy released when the bond is formed)
How do you calculate the enthalpy change of a reaction using the mean bond enthalpies?
Enthalpy change = bonds broken - bonds formed