Energy Changes
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When chemical reaction occurs, what happens to energy?
It’s transferred to or from the surroundings
What is an exothermic reaction?
One that transfers energy to the surroundings, usually by heating
How is an exothermic reaction shown?
By a rise in temperature
What is an endothermic reaction?
One which takes in energy from the surroundings
How is an endothermic reaction shown?
By a fall in temperature
Give three examples of an exothermic reaction
• Combustion 2. Neutralisation reactions 3. Oxidation reactions
Give 2 everyday uses of exothermic reactions
• Hand warmers 2. Self-heating cans
Explain how hand warmers use exothermic reactions
Use exothermic oxidation of iron in air (with a salt solution catalyst) to release energy
What do self-heating cans rely on?
Exothermic reactions between chemicals in their bases
Give an example of an endothermic reaction
Thermal decomposition
Give a everyday use of an endothermic reaction
In some sports injury packs
Explain how sports injury packs work
Chemical (endothermic) reaction allows pack to become instantly cooler without having to put it in the freezer
Give an example of a reversible reaction (that has the forward endothermic reaction and backwards exothermic reaction)
Hydrated copper sulfate (blue) ⇌ Anhydrous copper sulfate (white) + water
How can you measure the energy released when a substance is burnt?
Via simple calorimetry
Why is calorimetry used?
Used to compare amounts of energy released
What is the bad thing about calorimetry and why?
Not accurate because of energy losses
What equation can you use to calculate and compare amount of energy released by different fuels?
Q = mc ΔT
How is energy measured?
In J or kJ for a given mass/mole e.g. J/mol
How can the amount of energy produced by a chemical reaction in solution be calculated + when can this method be used?
Can be calculated from measured temperature change of the solution when the reagents are mixed in an insulated container Method can be used for reactions of solids with water or for neutralisation reactions
When doing bond energy calculations, what are the 3 things you need to remember?
• Reactants − Products 2. − = exothermic 3. + = endothermic
In endothermic reactions why must energy must be supplied?
To break existing bonds
In exothermic reactions when is energy released?
When new bonds are formed
In endothermic reactions: (comment on energy + bonds = comparison)
Energy used to break bonds > energy released by forming them
In exothermic reactions: (comment on energy + bonds = comparison)
Energy released by forming bonds > energy used to break them