Enthalpy definition
A measure of the heat content stored in a chemical system at constant pressure
Enthalpy change definition
heat energy change measured under conditions of constant pressure
What are standard enthalpy changes?
The standard conditions
Temp=298 K
Pressure=100kPa
How to calculate enthalpy change
Triangle H=enthalpy products — enthalpy reactants
Standard enthalpy change of formation definition
Enthalpy change when 1 mole of a compound is formed from its constituent elements under standard conditions where all reactants and products are in their standard state
Standard enthalpy change of combustion definition
Enthalpy change when 1 mole of a substance is completely burnt in oxygen under standard conditions where all reactants and products are in their standard state
Standard enthalpy change of neutralisation definition
Enthalpy change when solutions of an acid and an alkali react tgt under standard conditions to produce 1 mole of water
Standard enthalpy change of reaction
Enthalpy change which occurs when equation molar quantities of materials react under standard where reactants+products are in their standard state
Heat change equation
q=mcΔT
m - mass of water
c - shc of water
T - in C/K
Hess’s law
The overall standard enthalpy change of a reaction is the same regardless of the path it takes
Hess cycles/equation
- *using enthalpies of formation**
ΔH⦵r = ΣΔH⦵f(products) - ΣΔH⦵f(reactants)
Arrows pointing away from elements - using enthalpies of combustion
ΔH⦵r = ΣΔH⦵c(reactants) ΣΔH⦵c(products)
Arrows pointing towards combustion products
Mean bond enthalpy definition
energy required to break one mole of a specified type of covalent bond in a gaseous molecule
How to calculate ΔHr using bond enthalpies
ΔHr = Σ(bond enthalpies of bonds broken=reactants)-Σ(bond enthalpies of bonds formed=products)
Problems calculating ΔHr using bond enthalpies
- less accurate than Hess cycles=mean bond enthalpies x consider unique molecular environment of each bond in a specific molecule
- Hess cycles consider actual enthalpy changes associated with the specific bonds=more precise estimate of ΔHr
