Formulae & Hazards

Formulae & Hazards

Chemists write everything in a compact code. A formula shows which atoms are in a substance and how many: H2O is two hydrogen atoms and one oxygen atom; CO2 is one carbon and two oxygen. The small subscript number tells you how many of the atom just before it. Get comfortable reading formulae and the rest of chemistry falls into place.

Balancing equations

In a reaction atoms are only rearranged, never created or destroyed, so mass is conserved. A balanced equation therefore has the same number of each atom on both sides. You balance it by putting big numbers in front of the formulae, so 2H2O means two whole water molecules. You must never change the small subscript numbers: H2O and H2O2 are different substances, and rewriting one as the other to make the sums work is not balancing, it is fiction. Work through the atoms one element at a time, and leave oxygen until last: it usually appears in more places than anything else.

Find the number

Balance the equation for making ammonia: N2 + __H2 gives 2NH3. What number must go in front of H2?

Which one balances?

Which of these equations is correctly balanced, with the same number of each atom on both sides?

• 2H2 + O2 gives 2H2O
• H2 + O2 gives H2O
• H2 + O2 gives 2H2O
• 2H2 + 2O2 gives 2H2O

Balance the burn

Methane burns in oxygen: CH4 + __O2 gives CO2 + 2H2O. Count the oxygen atoms on the right, then work out what must go in front of O2.

State symbols

A full equation says what state each substance is in. The symbol goes in brackets straight after the formula, and Edexcel expects them whenever a full equation is asked for:

Which symbol?

A strip of magnesium you could pick up with tongs is written with _____. Pure water at room temperature is written with _____. Hydrogen bubbling out of the flask is written with _____. And the salt that ends up dissolved in the water is written with _____.

Full and ionic equations

For the Higher tier, the same reaction can be written two ways. Neutralising an acid with an alkali is the standard example:

The ions that watch

Silver nitrate solution is mixed with sodium chloride solution, and solid silver chloride appears. The ionic equation is Ag+(aq) + Cl-(aq) gives AgCl(s). Sodium and nitrate ions are still drifting in the solution afterwards, exactly as they were before. What are they, and why leave them out?

• Spectator ions: they are unchanged by the reaction, so writing them on both sides would add nothing
• Reactant ions, left out simply to keep the equation short
• Catalyst ions, which speed the reaction up without being used
• Product ions, because they are formed by the reaction

Working safely

Chemicals carry hazard symbols: black pictures in a white diamond with a red border, so you can read the risk across a bench at a glance. - Flammable: a single flame. It catches fire easily. - Oxidising: a flame burning above a circle. It provides oxygen so other things burn fiercely, which is a different danger from being flammable itself. - Corrosive: liquid pouring from a tube onto a surface being eaten away. It attacks skin and eyes. - Toxic: a skull and crossbones. It can cause death or serious harm if taken in. The pair worth separating is flammable and oxidising. An oxidising agent may not burn at all by itself; the hazard is what it does to everything nearby.

Read the symbols

Four hazard warning diamonds. Tap the one that warns a substance provides oxygen, so that other things burn fiercely.

Run it safely

You are about to heat a flammable liquid and then test the gas it gives off. Work through the precautions.

• The liquid carries the flammable symbol and needs heating. How do you heat it?
• The second bottle carries the corrosive symbol. What matters most?
• A gas comes off and you want to know what it is. What do you do?

Chemistry summary

A balanced equation has the same number of each type of _____ on both sides, because mass is conserved. You balance it by putting numbers in _____ of the formulae, never changing the small numbers. State symbols show whether something is a gas, using _____, or dissolved in water, using _____.