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Define standard enthalpy of combustion
Define standard enthalpy of formation
Reactions can be endothermic or exothermic.
Enthalpy change (∆H) is the heat energy change measured under conditions of constant pressure.
Standard enthalpy changes refer to standard conditions, ie 100 kPa and a stated temperature
Enthalpy refers to all the heat energy that's stored in a system.
Enthalpy is very difficult to determine - the change in enthalpy is easier.
For example, neutralization reactions release heat energy - measure the temperature before and after then the change in temperature can be used to calculate the enthalpy change, ∆H.
When the enthalpy of the products is less than the reactants, ∆H is negative and the reaction is overall exothermic.
When the enthalpy of the reactants is less than the products, ∆H is positive and the reaction is overall endothermic.
Activation energy (Ea) is the minimum energy required for a reaction to take place. For a reaction to occur, the chemical bonds in the reactant molecules need to be broken – this requires energy, the Ea.
Exothermic Reactions –∆H
Exothermic Reactions +∆H
Enthalpy changes are given under standard conditions
Pressure is 100 kilopascals.
Concentration in mole per cubic decimetres.
Chemical species in their standard states.
Common Enthalpy Changes
The enthalpy change when one mole of a substance reacts completely with oxygen under standard conditions.
The enthalpy change when one mole of water is formed in a neutralization reaction under standard conditions.
(Note: This value is the same, –57kJ per mole, for any reaction of a strong acid with a strong alkali).
The enthalpy change when one mole of a compound is formed from its elements under standard conditions and species in their standard states.
(Remember this is for one mole of product. If you had 4 moles, for example, the value would need to be divided by 4).
The enthalpy change when a reaction takes place. (must be in molar quantities shown by the balanced chemical equation)
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