Energy Changes in Chemistry - GCSE Unit 5 Summary Notes

Energy Conservation and Reaction Types

Here's something brilliant about energy - it never disappears, it just moves around! Energy conservation means the total amount of energy stays exactly the same during any chemical reaction, but it can transfer between chemicals and their surroundings.

Exothermic reactions are like chemical heaters - they release energy to their surroundings, making everything around them warmer. You've probably used these without realising: hand warmers, self-heating food cans, and even burning fuel are all exothermic. Common examples include combustion, oxidation reactions, and neutralisation reactions.

Endothermic reactions work the opposite way - they're energy thieves that absorb heat from their surroundings, making things cooler. Sports injury ice packs often use endothermic reactions. Thermal decomposition and mixing citric acid with sodium hydrogencarbonate are classic endothermic examples.

Quick Tip: Remember ENDO = ENergy Goes IN, EXO = EXits OUT!

Reaction Profiles and Bond Energy

Reaction profiles are like energy maps that show you the journey from reactants to products. They reveal the activation energy - the minimum energy boost particles need before they can react (think of it as the energy hurdle they must jump over).

In reaction profiles, exothermic reactions end up lower than they started (energy released), while endothermic reactions climb uphill (energy absorbed). The peak always shows the activation energy barrier that must be overcome.

Here's the key to understanding energy changes: breaking bonds requires energy input, but making new bonds releases energy. In exothermic reactions, more energy comes out from making new bonds than goes into breaking old ones. In endothermic reactions, it's the opposite.

Bond energy calculations let you work out exactly how much energy a reaction will release or absorb. Simply subtract the energy needed to break reactant bonds from the energy released when product bonds form.

Exam Success: Energy change = bonds broken - bonds made. This formula is your best friend in calculations!

Cells, Batteries and Fuel Cells

Your phone battery is basically a portable chemistry lab! Cells use chemical reactions to generate electricity, and when you connect multiple cells together, you get a battery with higher voltage.

The voltage depends on how different your two metals are - the bigger the reactivity gap, the more electrical punch you get. Non-rechargeable batteries (like alkaline ones) stop working when the chemicals run out, but rechargeable batteries can reverse their chemical reactions when you plug them in.

Hydrogen fuel cells are the future of clean energy. They combine hydrogen fuel with oxygen from air to produce electricity, with water as the only waste product. At the negative electrode, hydrogen makes electrons (which create the electric current), whilst at the positive electrode, oxygen uses up those electrons.

Fuel cells never need recharging and produce zero pollution, but hydrogen is tricky to store safely and filling stations are rare. They're renewable only if the hydrogen comes from clean electricity sources.

Future Focus: Fuel cells could power everything from cars to homes - understanding them now puts you ahead of the curve!