S4 - Energy transfer in reactions
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What happens during a chemical reaction?
Old bonds are broken and new bonds are formed
What process describes breaking bonds?
Endothermic - energy must be supplied to break existing bonds
What process describes the formation of bonds?
Exothermic - energy is releases when new bonds are formed
In terms of energy what happens in an exothermic reaction?
The energy released in bond formation is greater than the energy used in breaking old bonds
What is an exothermic reaction?
One which gives out energy to the surroundings usually in the form of heat shown by a rise in temperature
In terms of energy what happens in an endothermic reaction?
The energy required to break bonds is greater than the energy released when new bonds are formed
What is an endothermic reaction?
One which takes in energy from the surroundings usually in the form of heat shown by a fall in temperature
What is the overall change in energy in a reaction called?
The enthalpy change (symbol = ΔH)
What are the units of ΔH?
KJ/mol - its the amount of energy in kilojoules per mole of reactant
What value can the enthalpy change be?
Either positive or negative
If the reaction is exothermic, what is the value of the enthalpy change?
Negative because the reaction is giving out energy
If the reaction is endothermic, what is the value of the enthalpy change?
Positive because the reaction takes in energy