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Balance Equations

Worksheet Overview

QUESTION 1 of 10

Balanced equations contain lots of information about chemical reactions. In this activity you will learn how to balance equations and how to read state symbols.

When a chemical reaction happens, we take something, pull its atoms apart, then put them back together in a different pattern. The atoms at the start and at the end are the same; they're just joined together in a different way. This idea gives us something called the Law of Conservation of Mass:

No atoms are made or destroyed during a chemical reaction. The total mass of all the reactants is the same as the total mass of all the products.

(Reactants are the chemicals we have at the start of a chemical reaction; products are the chemicals we produce by the end of the chemical reaction.)

The Law of Conservation of Mass means we have to be careful how we write equations for chemical reactions. We write equations for chemical reactions like this:

reactants → products

(it's a good idea to read the → as "makes" or "becomes", so you have the sense of before and after).

For the Law of Conservation of Mass to work, we have to have the same number of each type of atom on the reactants side of the arrow as on the products side. For example, sodium and chlorine react to make sodium chloride. 

2 Na + Cl2 → 2 NaCl (the large 2's in front of Na and NaCl mean that there are two copies of that chemical in the reaction).

To check that the reaction is balanced, count the number of atoms of each type on each side of the reaction.

Na: before the reaction happens, there are 2 Na, and after ther reaction happens, there are 2 Na.

Cl: before the reaction happens, there are 2 Cl, and after the reaction happens, there are 2 Cl.

No atoms have been made or destroyed, so the reaction is balanced.

If we need to balance an equation, we can't change the formulas of the chemicals we have, but we can change how many units of that chemical we use. So we were allowed to change NaCl to 2 NaCl, but not to Na2Cl. We can change the large numbers, but not the small ones.

Often, we add state symbols to each chemical, to tell us what the state the chemical is in. There are four symbols you need to know:

Symbol Meaning
(s) solid
(l) liquid
(g) gas
(aq) aqueous (dissolved in water)

Let's put all that together in an example. When we burn methane gas, we react it with oxygen to make carbon dioxide and water. In symbols, the first guess at the equation is

CH4 + O2 →CO2 + H2O but that equation isn't balanced. The carbons are balanced (one in CH4 on the reactants side, one in CO2 on the products side)

Look at the hydrogens. There are four on the reactants side, in CH4. On the products side, there are only two, in H2O. We can't change H2O to H4O, but we can have two water molecules in the reaction. Now it looks like

CH4 + O2 →CO2 + 2 H2O but that equation still isn't balanced.

Look at the oxygens. There are two on the reactants side in O2, and four on the products side (two in CO2 and two in the water molecules- one in each H2O). We can't change O2 to O4, but we can have two molecules of O2. So the balanced equation is

CH4 + 2 O2 →CO2 + 2 H2O.

Methane, oxygen and carbon dioxide are all gases. Water is a liquid. So when we add the state symbols, we end up with

CH4 (g) + 2 O2 (g) →CO2 (g) + 2 H2O (l).

If you're checking whether a reaction is balanced, count the atoms before and after for each element in turn. The Law of Conservation of Mass says that the numbers for each element have to be the same. To balance an equation, check each element in turn; you can change the large numbers in front of the formulas, but not the small numbers in the formulas.

 

Complete the gaps in the Law of Conservation of Mass.

Think about this reaction

2 CuO + C → CO2 + 2 Cu. Type numbers into the gaps in the paragraph to describe this reaction. For the final gap, choose "balanced" or "unbalanced".

This equation isn't balanced yet...

Na + F2 → NaF

Which of these changes would balance the reaction correctly?

Change "Na" to "2 Na" and "NaF" to "2 NaF"

Change NaF to NaF2

Change "F2" to "F"

Fill in the blank number to make this reaction balance.

Change "Na" to "2 Na" and "NaF" to "2 NaF"

Change NaF to NaF2

Change "F2" to "F"

Fill in the blank numbers to make this reaction balance.

Change "Na" to "2 Na" and "NaF" to "2 NaF"

Change NaF to NaF2

Change "F2" to "F"

Fill in the blank numbers to make this reaction balance.

Change "Na" to "2 Na" and "NaF" to "2 NaF"

Change NaF to NaF2

Change "F2" to "F"

Match the state symbols with their meanings

Column A

Column B

(s)
aqueous
(l)
solid
(g)
gas
(aq)
liquid

What would be the symbol for "sodium chloride dissolved in water"?

Column A

Column B

(s)
aqueous
(l)
solid
(g)
gas
(aq)
liquid

Solid aluminium reacts with oxygen gas to make solid aluminium oxide. Pick the right symbols and state symbols for solid aluminium and oxygen gas.

Column A

Column B

(s)
aqueous
(l)
solid
(g)
gas
(aq)
liquid

Add numbers and state symbols to balance the equation for the reaction copper + oxygen → copper oxide.

Column A

Column B

(s)
aqueous
(l)
solid
(g)
gas
(aq)
liquid
  • Question 1

Complete the gaps in the Law of Conservation of Mass.

CORRECT ANSWER
EDDIE SAYS
This is a key phrase to memorise. It explains a lot of how equation balancing needs to work, and you might be asked to quote it.
  • Question 2

Think about this reaction

2 CuO + C → CO2 + 2 Cu. Type numbers into the gaps in the paragraph to describe this reaction. For the final gap, choose "balanced" or "unbalanced".

CORRECT ANSWER
EDDIE SAYS
2 CuO means "2 units of CuO", so it includes 2 Cu ions and 2 O ions. If you\'re not sure about the difference (and similarity) between atoms and ions, have a look at the activities about ionic bonding.
  • Question 3

This equation isn't balanced yet...

Na + F2 → NaF

Which of these changes would balance the reaction correctly?

CORRECT ANSWER
Change "Na" to "2 Na" and "NaF" to "2 NaF"
EDDIE SAYS
The important thing is that we can't change the formula of any of the chemicals, so changing NaF to NaF2 isn't allowed. To make both Na and F balance, we need two units of NaF and Na for each F2.
  • Question 4

Fill in the blank number to make this reaction balance.

CORRECT ANSWER
EDDIE SAYS
There are four Cl atoms on the products side, so we need two units of Cl2 on the reactants side.
  • Question 5

Fill in the blank numbers to make this reaction balance.

CORRECT ANSWER
EDDIE SAYS
Each Fe2O3 needs 3 oxygen ions, so 3 O2 molecules make 2 units of Fe2O3. Making 2 units of Fe2O3 needs 4 oxygen atoms, so 2 O2 molecules.
  • Question 6

Fill in the blank numbers to make this reaction balance.

CORRECT ANSWER
EDDIE SAYS
There are four hydrogen atoms on the reactants side, so we make 2 molecules of H2O to use them all up. There are two carbon atoms in C2H4, so we make two molecules of CO2. That means there are six oxygen atoms on the products side, so we need 3 O2 molecules to provide those atoms on the reactants side.
  • Question 7

Match the state symbols with their meanings

CORRECT ANSWER

Column A

Column B

(s)
solid
(l)
liquid
(g)
gas
(aq)
aqueous
EDDIE SAYS
Most of these symbols are simple to remember. "Aqueous" is a bit less obvious. "Aqua" means water- think of words like aquarium.
  • Question 8

What would be the symbol for "sodium chloride dissolved in water"?

CORRECT ANSWER
EDDIE SAYS
"(d)" isn't a state symbol. "(l)" would be liquid (molten) sodium chloride; "(aq)" is dissolved in water.
  • Question 9

Solid aluminium reacts with oxygen gas to make solid aluminium oxide. Pick the right symbols and state symbols for solid aluminium and oxygen gas.

CORRECT ANSWER
EDDIE SAYS
You need to be careful about chemical symbols; aluminium is Al, not A, and oxygen gas comes in molecules of O2, not single O atoms.
  • Question 10

Add numbers and state symbols to balance the equation for the reaction copper + oxygen → copper oxide.

CORRECT ANSWER
EDDIE SAYS
To use all the oxygen atoms, we need to make two CuO units. To make two CuO units, we need two Cu atoms.
---- OR ----

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