3.3 Enthalpy
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What is enthalpy ?
Enthalpy is a measure of the heat energy in a chemical system.
Enthalpy change
^H = H(products) - H(reactants) Can be positive or negative depending if reaction is endothermic or exothermic.
Exothermic
HEAT LOST TO SURROUNDINGS. ^H is negative Reactants are higher than products.
Endothermic
HEAT INTAKE FROM SURROUNDINGS. ^H is positive. Reactants are lower than products.
Activation Energy
Activation energy is the energy required to break bonds for a reaction to take place.
What is the standard enthalpy change of reaction ?
The enthalpy change in a reaction on the molar quantities shown in chemical equation under standard conditions, with all reactants and products in standard state.
What is the standard enthalpy change of formation ?
Then enthalpy change when 1 mole of a compound is formed from the elements under standard conditions, with all reactants and products in their standard states.
What is the standard enthalpy change of combustion ?
The enthalpy when 1 mole of a substance reacts completely with oxygen under standard conditions, will all reactants and products in their standard states.
What is the standard enthalpy change of neutralisation ?
The enthalpy that accompanies reaction of an acid and base to form 1 mole of H2O under standard conditions, with all reactants and products in their standard states.
How to find temperature change ?
^T = T(final) - T (initial)
How to calculate energy change (q) ?
q = m c ^T -q= heat energy /J - m = mass of surroundings that changes temperature /g - c = specific heat capacity ( 4.18 KJ mol -1) -^T = temperature change of surroundings
What is the specific heat capacity ?
Energy requires to raise temperature of 1g of substance by 1K / 1*C
How to work out standard enthalpy change ?
q/n -q= heat energy /J -n= number of moles of limiting reactant.
What is the average bond enthalpy ?
Is the energy required to break 1 mole of a specified covalent bond (by homolytic fission) in a gaseous molecule.
What do bond enthalpies always have ?
Always have a positive enthalpy value and are always endothermic.
What energy is greater in bond enthalpies in an endothermic reaction ?
Energy needed to break bonds is greater than energy need to make bonds.
What energy is greater in terms of bond enthalpies in exothermic reactions ?
Energy needed to make bonds is greater than energy needed to brake bonds.
How to work out standard enthalpy change of a reaction ?
(Sum of enthalpy of reactants) - (sum of enthalpy of products) Break-Make
What are some limitations of average bond enthalpies ?
-All molecules must be gaseous in both reactants and products. -Uses average bond enthalpy.
What is Hess’ Law ?
For a chemical reaction, the enthalpy change is independent of the route taken, provided the initial and final states remain the same.
What is an allotrope ?
Different form of the same element.
What is indirect determination of enthalpy changes ?
Finding enthalpy change by using 2 common reactions, with same products and finding enthalpy of each reaction to find overall enthalpy indirectly.
What are 2 methods of Hess’ Law?
1) Use 2 equations and rearrange to find enthalpy. 2) Use energy profile t find enthalpy change indirectly.