Enthalpy: H. The heat content that is stored in a chemical system.
• Enthalpy Change: ∆ = − .
• Conservation of Energy: Energy cannot be created or destroyed. Heat transferred between
system and surroundings.
• (Chemical) System: The chemical compounds being studied- the reactants and products.
• Surroundings: Everything that is not the chemical system. Includes apparatus. Universe is term
given to describe both system and surroundings.
Measuring: Enthalpy change can be determined experimentally by measuring energy transfer.
• Enthalpy Profile Diagrams: Label with reactants and products. And ∆H and Ea. Don’t have sign
before triangle H. Keep reactants and products as they are in chemical equation.
• Exothermic Reaction: It is a reaction in which the enthalpy of the products is smaller than the
enthalpy of the reactants. ∆H is negative. Same amount of energy transferred out of system to
surroundings. Temperature of surroundings increases.
• Endothermic Reaction: It is a reaction in which the enthalpy of the products is greater than the
enthalpy of the reactants. ∆ is positive. Same amount of energy transferred from surroundings
to system. Temperature of surroundings decreases.
• Activation Energy: Ea The minimum required energy to start a reaction. New bonds in products
can then form to complete reaction. Reactions with large activation energy take place slowly or
not at all.
• Standard Conditions: Enthalpy change for reaction varies depending on conditions used. So data
tables taken under standard conditions. A standard value shown using standard sign o
. A standard
enthalpy change ∆H
o
. 100 kPa. 298 K or 25oC. 1 mol dm-3
. Only relevant to solutions.
• Standard State: The physical state of a substance under standard conditions.
• Standard Enthalpy Change of Reaction: ∆rH
o
the enthalpy change that accompanies a reaction in
the molar quantities shown in a chemical equation under standard conditions, with all reactants
and products in their standard states.
• 2.5 mol product given and equation has enthalpy change as 909. So divide 2.5 mol with balancing
number in equation. Then multiply by 909 to get energy released. Moles x enthalpy change =
energy released.
Standard Enthalpy Change of Formation: ∆fH
o
the enthalpy change that takes place when one
mole of a compound is formed from its elements under standard conditions, with all reactants
and products in their standard states.
– All elements have formation ∆fH
o of 0 kJ mol-1
.
• If moles of product is 2, divide enthalpy change of reaction by two to find enthalpy change of
formation.
• Standard Enthalpy Change of Combustion: ∆cH
o
the enthalpy change that takes place when one
mole of substance reacts completely with oxygen under standard conditions, with all reactants
and products in their standard states. The organic molecule should be one mole.
• Standard Enthalpy Change of Neutralisation: ∆neutH
o
the enthalpy change that accompanies the
reaction of an acid by a base to form one mole of H2O, under standard conditions, with all
reactants and products in their standard states.
• Standard state of water is (l).
• Don’t include standard stuff at end, if just says define enthalpy change of reaction.
• Activation energy for backwards/ reverse uncatalysed reaction would be 250 – (+)92. Activation
energy – enthalpy change.
• Need to calculate moles of substance and compare it what it says on diagram. Whatever factor
multiplied to get actual moles from moles in Q, multiply by enthalpy change in Q.