C12: Energy Changes in Chemical Reactions
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Exothermic
A reaction that gives out energy to the surroundings.
Endothermic
A reaction that takes in energy from the surroundings.
Sign for enthalpy change
△H
Are the products of exothermic energy calculations positive or negative?
Negative
Uses of exothermic reactions
> Hand warmers > Self-heating cans
Use of endothermic reactions
> Cold packs for sports injuries.
Test for water
Anhydrous copper sulfate (white) + Water = Hydrated copper sulfate (blue)
If a reaction releases energy when it goes in one direction…
… It will absorb exactly the same amount of energy when it goes in the opposite direction.
energy released =
mass of object heated x specific heat capacity x change in temperature
What is the unit of energy?
Joules (although calories are still used sometimes)
Calorimeter
A machine used to measure the amount of energy released from an object. They can be made inaccurate by energy losses.
How do catalysts increase the rate of reaction?
By providing an alternative pathway for the products which has a lower activation energy. This means that a greater proportion go reactant particles have enough energy to react.
Breaking bonds…
aBsorbs energy (endothermic)
foRming bonds…
Releases energy (exothermic)