9.1 Enthalpy changes
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What is enthalpy
A measure of the heat energy in a chemical system Sometimes thought of as the energy stored within bonds Cannot be measured but enthalpy changes can
What does ‘chemical system’ refer to
The atoms, molecules or ions making up the chemicals
The difference in the enthalpies is the enthalpy change ΔH:
ΔH = H(products) - H(reactants)
What is the law of conservation of energy
Energy can neither be created nor destroyed
When a chemical reaction involving an enthalpy change takes place, heat energy is transferred between the system and the surroundings:
The system is the chemicals - the reactants and products The surroundings are the apparatus, the laboratory and anything that isn’t the chemical system The universe is everything and includes both system and surroundings
Can an enthalpy change be determined experientally
Yes, by measuring the energy transfer between the system and the surroundings
Energy transfer can be in either of two directions:
From the system to the surroundings (exothermic) Or from the surroundings to the system (endothermic)
What happens in an exothermic reaction
Energy is transferred FROM the system TO the surroundings Chemical system releases heat energy to surroundings Any energy loss by chemical system is balanced by the same energy gain by these surroundings ΔH is negative Temperature of surroundings increases
What happens in an endothermic reaction
Energy is transferred FROM the surroundings TO the system Chemical system takes in heat energy from surroundings Any energy gain by chemical system is balanced by the same energy loss by surroundings ΔH is positive Temperature of the surroundings decreases as they lose energy
What is activation energy
The energy required to break bonds
In general, reactions with small activation energies take place very ______
Rapidly
Why are reactions with very small activation energies rapid
As the energy needed to break bonds is readily available from the surroundings
Reactions with large activation energies take place very ______
Slowly
Why are reactions with very high activation energies extremely slowly or not at all
They may present such a large energy barrier that not enough energy is available
Why can the enthalpy change vary slightly
Depending on the conditions used
A standard enthalpy change:
ΔH⦵
What is the standard sign
⦵
What is standard pressire
100kPa Very close to one atmosphere (101kPa)
What is standard temperature
Usually 298K (25 degrees C)
What is standard concentration
1 mol dm^-3
What is standard state
The physical state of a substance under standard conditions
What is the standard enthalpy change of reaction
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
What is the standard enthalpy change of formation
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 an enthalpy change of formation of…
0 kJ mol^-1