Energetics
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What is thermochemistry?
Study of heat changes during chemical reactions.
What happens to bonds during a chemical reaction?
Bonds in reactants are broken and new bonds formed in products.
Is energy required to break bonds or form them?
Energy is required to break bonds; energy is released when bonds are formed.
What is an exothermic reaction?
A reaction where energy is given out to the surroundings.
What is an endothermic reaction?
A reaction where energy is taken in from the surroundings.
What happens to temperature in an exothermic reaction?
It usually increases.
What happens to temperature in an endothermic reaction?
It usually decreases.
Give example of exothermic reaction.
Combustion.
Give example of endothermic reaction.
Thermal decomposition.
What unit is energy measured in?
KJ mol-1
What is enthalpy change?
Heat energy transferred during a reaction at constant pressure.
How is enthalpy change calculated?
enthalpy of products- enthalpy of reactants.
What are standard conditions for enthalpy measurements?
100kPa and 298K
What concentration must solutions have under standard conditions?
1.0 mol dm-3
What is standard enthalpy of formation?
Enthalpy change when 1 mole of a compound forms from it elements in standard states.
What is the enthalpy of formation of an element in its standard state?
0
What is standard enthalpy of combustion?
Enthalpy change when 1 mole of a substance is completely burned in oxygen.
Are combustion enthalpies usually positive or negative?
Negative (exothermic)
What substances have zero enthalpy of combustion?
Substances that can’t be combusted e.g. water, carbon dioxide.
Why are state symbols important?
Different physical states have different enthalpy changes.
Is bond breaking endothermic or exothermic?
Endothermic.
What equation is used to calculate heat energy?
Q = mc x change in temp
How are enthalpy changes measured experimentally?
By calorimetry.
How is enthalpy change used to calculate heat energy?
Q/n