Chemistry (school) · Part 04
Chemical reactions
What happens in a reaction, how to write it down, how fast it goes and the energy involved.
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- Chemical reactionA change where atoms are rearranged to make one or more new substances. The starting substances are reactants. The new ones are products.
- Conservation of massThe total mass of the substances after a reaction equals the total mass before. Atoms are not made or destroyed, only rearranged.
- Balanced equationA symbol equation with the same number of each type of atom on both sides. You balance it by changing the big numbers in front of formulae, never the small numbers inside.
- State symbolsSmall letters in brackets after a formula that show its state: (s) solid, (l) liquid, (g) gas and (aq) dissolved in water.
- Exothermic reactionA reaction that gives out energy to the surroundings, usually as heat, so the temperature goes up.
- Endothermic reactionA reaction that takes in energy from the surroundings, so the temperature drops.
- Rate of reactionHow fast a reaction happens. It goes up with higher temperature, higher concentration, larger surface area or a catalyst.
- CatalystA substance that speeds up a reaction without being used up. It gives the reaction an easier route.
- Reversible reactionA reaction that can go forwards and backwards. It is shown with a double arrow. In a closed container it can reach equilibrium, where both ways go at the same speed.