Chemical reactions, Year 9: equations, catalysts and concentration

2

提示Count the magnesium atoms on the right-hand side, then make the left agree.

为什么The right-hand side holds two magnesium atoms and two oxygen atoms. O2 already supplies both oxygens, so only the magnesium needs a big number in front to bring the left-hand side up to match.

2; 2

提示Start with the oxygen: O2 brings a pair of atoms, and each water molecule takes only one.

为什么O2 supplies two oxygen atoms and each water molecule holds one, so two water molecules must form. Between them those contain four hydrogen atoms, which takes two molecules of H2.

4

提示The big number multiplies everything in the formula that follows it.

为什么H2O has two hydrogen atoms, and the big 2 in front means two whole molecules, so the count is two lots of two. The same 2H2O holds two oxygen atoms. Counting like this on both sides is how you check that an equation balances.

H2O2 is a different substance, not water

提示Ask whether the reaction is still making the same product as before.

为什么A formula is the identity of a substance. H2O2 is hydrogen peroxide, a bleach, so the altered equation describes a reaction that does not happen. To balance, change only how many of each particle there are: the big numbers in front.

A catalyst

提示The exhaust system of a car contains one, to help clean up the fumes.

为什么Because it is not used up, a small amount can go on working again and again. Different reactions need different ones, so chemists have to find the right one for each job.

1

提示Decide whether the powder is a reactant or something that only helps the reaction along.

为什么The powder is a catalyst, so none of it is used up and all of it can be recovered. Weighing it before and after is how you show that a substance acted as a catalyst and not as a reactant.

An enzyme

提示Biological washing powders contain them to break down stains.

为什么Your body runs thousands of reactions that would be far too slow on their own at body temperature. Each is speeded up by its own enzyme, which is not used up and can work over and over again.

50 cm³, the same as before

提示A catalyst changes how quickly things happen, not how much there is to react.

为什么The amount of product depends only on how much of the reactants there is to start with. The catalyst lets the reaction finish sooner, but it adds no atoms of its own to the products.

platinum

提示A silvery metal, costlier than silver, that is also used for expensive rings.

为什么Exhaust gases pass over a honeycomb thinly coated with the metal, which gives a huge surface for the reactions to happen on. Because a catalyst is not used up, the same converter can last for years.

The same

提示Concentration is about how crowded the dissolved particles are, not how many there are altogether.

为什么Concentration compares the amount of dissolved substance with the volume it is dissolved in. The jug holds more squash particles, but it holds more water in exactly the same proportion, so every mouthful tastes alike.

It goes down

提示The same salt particles now have more liquid to spread through.

为什么Adding water is called diluting. The number of dissolved particles has not changed, but they are spread through a bigger volume, so there are fewer of them in each cubic centimetre.

concentrated

提示Its opposite is "dilute".

为什么Two spoonfuls of sugar in a mug of tea make a stronger solution than one spoonful in the same mug: more sugar particles are packed into the same volume of liquid.