— during a reaction, energy is always taken in or given out (to and from the surroundings)

Exothermic Reactions

— gives out energy after the reaction

reactant → product energy

Endothermic Reactions

— takes in more energy to make the reaction than it gives out

reactant + energy → products

— when energy change (of reaction) > 0 → endothermic reaction because energy is taken in to continue the reaction

— when energy change < 0 → exothermic reaction → because the energy from the reaction is given out

<aside> 📌 energy is needed to break down bonds because atoms are just tempted to stick

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→ solid → liquid → gas ⇒ endothermic

Calculating energy change

— breaking bonds requires energy → endothermic

— forming bonds releases energy → exothermic

Steps

1. write the equation balanced
2. write down the bonds and the number of them, involved in each side of the equation
3. write down the energy needed to create each bond
4. calculate the total energy in both the products' and the reactants' sides
5. calculate the total energy change

e.g the formation of steam

2H2 + O2 → 2H2O

2(H-H) + (O=O) → 4(O-H)

2(436) + 498 → 4(464)

872 + 498 → 1856

1370 → 1856

———— energy change = +486 ————— → exothermic reaction