Chemical Reactions and Substances
Chemical reactions and equations
Balance a skeletal equation by counting atoms, name the five kinds of reaction the chapter sets out, and say which substance has been oxidised and which reduced.
Milk left out on a summer afternoon turns. An iron gate left through the monsoon grows a reddish brown coat. Grapes ferment; food is cooked; food is digested. In each of these the substance you started with is not the substance you end with, and that is what a chemical reaction is. Four signs give one away: the state changes, the colour changes, a gas comes off, or the temperature changes.
Why an equation gets balanced
Atoms are not made and not destroyed in a reaction — the mass on the two sides has to agree. So the number of atoms of each element must come out the same on the left of the arrow as on the right. Until it does, what you have written is a skeletal equation. You fix it by putting a coefficient in front of a whole formula, and you leave the formula itself alone: four molecules of water are written 4H₂O, and writing H₂O₄ would invent a different substance.
Worked example
Read what this equation tells you: 3Fe(s) + 4H₂O(g) → Fe₃O₄(s) + 4H₂(g)
Iron and water are the reactants; iron oxide and hydrogen are the products.
Reactants stand to the left of the arrow, products to the right, and the arrowhead points the way the reaction runs.
Try it together
Balance Fe + H₂O → Fe₃O₄ + H₂, the way the chapter works it through.
Answer with a bare number each time.
1.Start with the compound holding the most atoms, and with its commonest element. That is Fe₃O₄ and oxygen. How many oxygen atoms does Fe₃O₄ hold?
Have a go
Have a go on your own. In 2H₂ + O₂ → 2H₂O, how many oxygen atoms stand on the left-hand side? Answer with a number.
Ready to practise?
Eight questions on what you have just read. Nothing is timed, and you can play as many times as you like.