9.1 Enthalpy changes Flashcards
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
