Tackling word equations and balanced symbol equations in Chemistry exam papers
Word equations and balanced symbol equations
A reaction may happen when two or more chemicals are mixed. Sometimes you have to add heat (energy) before the reaction will occur.
Not all chemicals react together when they are mixed. We can tell a reaction has taken place if we see bubbles of gas, a change in colour, a change in temperature or a precipitate (a layer of solid in the bottom of the test tube that wasn’t there when you started). So vinegar (an acid) reacts with sodium bicarbonate – there are bubbles of gas and the test tube gets warm – but it doesn’t react with table salt (sodium chloride). The salt dissolves but nothing else happens, even if you heat the mixture.
A word equation helps us work out what has happened during a reaction.
Let’s look at the word equation for the reaction between sulphuric acid and calcium carbonate (limestone or marble).
We know a reaction happens because we see bubbles of gas and the test tube gets warm.
The clue to what the gas is is in the name of the reactants; if you add acid to a carbonate you always get carbon dioxide. (You could also test the gas; we know it is carbon dioxide because if we bubble it through limewater, the limewater goes ‘milky’.) In addition, if you react an acid with an alkali or base, like a carbonate, an oxide or a hydroxide, you always get water. (You need to know this for the chemistry exam.) The name of the acid gives you a clue to name of the salt it forms.
Calcium carbonate + sulphuric acid→ calcium sulfate + carbon dioxide + water
Let’s look at another reaction. This is one you learn about in biology; respiration.
The reactants are glucose and oxygen, the products are carbon dioxide and water.
Glucose + oxygen→ carbon dioxide + water (+energy)
(This is almost the same as burning a hydrocarbon; you get the same products.)
Some reactions are very hard to describe in any other way than by a word equation. Not all substances have a chemical formula you can write down, for example wood or a chocolate biscuit. Or the chemical formula may be one you don’t know, such as vinegar which is mainly a solution of something called ethanoic acid.
Word equations describe what is happening, but it is often more useful to know what each of the atoms, molecules or ions is doing. To do this, we use a balanced symbol equation. Sometimes you will have to look up the formulas for the compounds or even work them out using the Periodic Table.
‘Balancing’ just means making sure you have the same number of each sort of atom on each side.
This is important. We do not lose or gain mass during a chemical reaction. This is called ‘Conservation of mass’. You may have come across this idea already in Physics.
Let’s see how to write a balanced formula equation for the reaction between magnesium and hydrochloric acid. You know a reaction happens because there is fizzing. If you test the gas with a lighted splint, you get a ‘squeaky pop’, telling you hydrogen has been produced.
The word equation is
magnesium + hydrochloric acid → hydrogen + magnesium chloride.
If you look in the table of ions, you will see that magnesium makes ++ ions and chloride ions are – , so you need 2 chlorine ions to each ion of magnesium. You could also work this out by looking at the Periodic Table. Magnesium is in the second column of the Periodic table and chlorine is in the next-to-last column, so magnesium has 2 electrons in its outer shell to give away, and chlorine has 7 electrons in its outer shell, so needs 1 to make a full shell.
To write the symbol equation, first write down the reactants and products in symbol form: Mg + HCl → MgCI2 + H2
This doesn’t balance; we have 1 H and 1 Cl on the left hand side and two of each on the right hand side. So we need to add another HCl to make the equation balance:
Mg + 2HCl →MgCI2 + H2
The hydrochloric acid is in solution, and so is the magnesium chloride. You can show this by writing a little ‘aq’ (short for aqueous, which means ‘in water’) under the HCl and MgCI2. You could also show that the magnesium chloride is ionic and in solution by writing Mg++ (Cl-)2 instead of MgCl.
You have probably seen the reaction between a group I metal and water; for example sodium and water. The sodium races around fizzing and there is a yellow flame too.
The fizzing tells us there is a gas being produced. The flame tells us there is also a lot of heat being produced; enough to ignite the gas. There is definitely a reaction going on!
The only substances involved are sodium and water and if you test the water afterwards with pH or litmus paper, you will find it is alkaline. The gas gives a ‘squeaky pop’ if it is tested with a lighted splint; it must be hydrogen.
Sodium + water→ hydrogen + sodium hydroxide.
Na + H2O → H2 + NaOH
This doesn’t balance, there are too many H’s on the right hand side. If we add another Na, we also need another H2O to make sure we have enough OH.
So we have:
2Na + 2H2O → 2Na OH + H2
This is an example of a very important reaction; make sure you learn it, and the one for Group II metals. You’ll find that in the ‘some to try for yourself’ section.
Another reaction you might have seen or done is the reaction between copper II oxide and dilute sulphuric acid. This is a reaction requiring heat (and a lot of stirring) to make it happen. Copper II oxide (CuO) is a black insoluble powder. Dilute sulphuric is a clear liquid. The products are copper II sulfate and water. Copper II sulfate is a pretty blue colour. You can tell this reaction has happened because you have a change in colour.
Copper II oxide + dilute sulphuric acid → copper sulfate + water
Write the symbols:
Cu O + H2SO4 → CuSO4 + H2O
There are the same number of atoms on each side; the equation is balanced. The dilute sulphuric acid and copper sulfate are both in solution, so you could write ‘aq’ underneath or show them as ions.
Let’s look at something which needs more balancing: the reaction between iron and chlorine. These are both elements and you aren’t adding anything else, so the product will be iron III chloride, as long as you have lots of chlorine. Chlorine makes Cl- ions and iron III ions are +++.
Chlorine always comes as Cl2 molecules.
Iron + chlorine → iron III chloride
Write a formula equation:
Fe + CI2 → FeCI3
This doesn’t balance: you have 2 chlorines on the left hand side and 3 on the right hand side. You need to use a bit of clever maths here; 2 x 3 = 6. If you put a 3 in front of the CI2 and a 2 in front of the FeCI3, you have 6 chlorines on both sides. However, you now also have to put a 2 in front of the Fe on the left hand side because you have 2 irons on the right hand side:
2Fe + 3CI2 → 2FeCI3
You will find this ‘balancing trick’ with 2 and 3 useful for a lot of iron and also aluminium compounds. (The only — ion you are likely to come across is phosphate.) There is one to try in the ‘some to try for yourself’ section. But first here’s another example.
Let’s look at the reaction between aluminium hydroxide and sulphuric acid.
It’s a reaction between an alkali and an acid, so we will get water as well as a salt.
The salt is aluminium sulfate, and we can use the balancing trick to work out it is AI2 (SO4)3.
Aluminium hydroxide + sulphuric acid → aluminium sulfate + water.
Begin by writing down the symbols:
Al (OH)3 + H2SO4→ Al2 (SO4)3 + H2O
We can see straight away that we need 2 aluminium ions and 3 molecules of sulphuric acid on the left hand side to make the aluminium sulfate.
2Al (OH)3 + 3H2SO4→ Al2 (SO4)3 + H2O
Now we have 6 OH’s and 6 H’s on the left. Each O needs 2 H’s to make water, so we have the right amount to make 6 H2O with nothing left over.
The final balanced equation is:
2Al (OH)3 + 3H2SO4→ Al2 (SO4)3 + 6H2O
Take your time and remember to make sure you have the same number of each element or ion on each side.
You might find it helps to write the symbols in groups and cross them off as you use them:
These symbol equations all involve elements and ions, but we can write balanced symbol equations for other reactions, too.
If we burn the hydrocarbon gas propane in oxygen (or in air), we get carbon dioxide and water.
Propane + oxygen → carbon dioxide + water
The formula for propane is C3H8.
Write down a symbol equation:
C3H8 + O2→ CO2 + H2O
You can see straight away that there are 3C’s on the left and 1 on the right, so you know there must be 3 CO2’s. The 8 H’s will make 4 water molecules. So you need 4O’s for the water and 6 for the CO2’s; 10 O’s or 5 O2’s.
The balanced equation is:
C3H8 + 5O2→ 3CO2 + 4H2O
Sometimes you end up with an odd number of O’s or H’s. If this happens, you just double everything up.
Propane’s cousin ethane C2H6 needs this:
2C2H6 + 7O2 → 4CO2 + 6H2O
Some to try for yourself
- Write a word equation for the reaction that occurs when a candle is burned. (Candle wax is a hydrocarbon.)
- Write a word equation for the reaction between potassium and water.
- Write a balanced formula equation for the reaction between calcium metal and water.
- Write a balanced formula equation for the reaction between magnesium carbonate (MgCO3 ) and sulphuric acid (H2SO4).
- Write a balanced formula equation for the reaction between iron II oxide (FeO) and nitric acid (HNO3).
- Write a balanced formula equation for the reaction between sodium carbonate and hydrochloric acid.
- Ammonia (NH3) is made by reacting together nitrogen and hydrogen.
- Write a balanced formula equation for this reaction.
- Iron is extracted from haematite iron ore (Iron III oxide) by heating with carbon. Write a balanced formula equation for this reaction. Write a balanced formula equation for the reaction between aluminium oxide and nitric acid.
- Quicklime is made by heating limestone, calcium carbonate. A gas is given off during the reaction. Quicklime is alkaline and reacts with water to form calcium hydroxide. Write balanced formula equations for these two reactions?
Remember to write a word equation before you write the balanced formula equation to make sure you don’t forget anything.
candle wax + oxygen → carbon dioxide + water
potassium + water → potassium hydroxide + hydrogen
calcium + water → calcium hydroxide + hydrogen
Ca + 2H2O→ Ca(OH)2 + H2
Calcium is Group II so it is ++. Otherwise its the same as the reaction between sodium and water. The calcium fizzes and the test tube gets warm.
magnesium carbonate + sulphuric acid → magnesium sulfate + carbon dioxide + water
MgCO3 + H2SO4 → MgSO4 + CO2 + H2O
In the acid/carbonate reaction, the carbonate ion gives an oxygen atom to the hydrogen from the acid to form a water molecule.
Magnesium carbonate is the chemical name for gypsum, a soft white rock.
Iron II oxide + nitric acid → iron II nitrate + water
FeO + 2HNO3 → Fe(NO3)2 + H2O
Transition metals like iron and copper often have more than one ‘oxidation state’ – they can have different numbers of +. Watch out for things like iron II and copper I.
Sodium carbonate + hydrochloric acid → sodium chloride + carbon dioxide + water
Na2 CO3 + 2HCl → 2NaCl + CO2 + H2O
Something alkaline + an acid gives a salt. This reaction gives ‘table salt’. Sodium carbonate is washing soda.
N2 + 3H2 → 2NH3
This is an important reaction because it is used to make ammonia for fertiliser.
It is what is called an ‘equilibrium’ reaction; not all the reactants are turned into product. You will learn more about this reaction in a later unit of your course.
Iron III oxide + carbon → iron + carbon dioxide
2Fe2O3 + 3C → 2Fe + 3CO2
You will learn about the extraction of metals later in your course.
Aluminium oxide + nitric acid → aluminium nitrate + water
Al2O3 + 6HNO3 → 2Al (NO3)3 + 3H2O
Aluminium reacts in a similar way to group II metals, but forms +++ ions.
Since there are 2 Al ions there need to be 6NO3 ions.
The gas given off is carbon dioxide. This means that the first reaction is
CaCO3 → CO2 + CaO
This tells you that quicklime is calcium oxide.
The calcium oxide is reacted with water. Nothing is given off, so the product must be calcium hydroxide. This is known as slaked lime.
CaO + H2O → Ca (OH)2
These are very important reactions.
Calcium oxide is used in the manufacture of cement, concrete and mortar (used to stick bricks together). A lot of heat is given out by the second reaction (it is exothermic) – if you’ve ever mixed cement you will know it gets hot. This is why.
Lime is also used to ‘sweeten’ clay soils to make them easier to work and more productive.
Slaked lime is dissolved in water to make lime water, which you use to test for carbon dioxide. The milkiness is due to the particles of insoluble calcium carbonate formed when calcium hydroxide reacts with carbon dioxide.
Ca(OH)2 + CO2 → CaCO3 + H2O