Balancing Chemical Equations

Purpose of Balancing Equations

Balancing chemical equations is essential because it follows the law of conservation of mass. This law states that during a chemical reaction, no atoms are lost or gained; they are simply rearranged. Therefore, the total mass of the reactants must equal the total mass of the products.

Balancing ensures the same number of atoms of each element appear on both sides of the equation. This accurately represents the chemical reaction, showing the correct proportions of substances involved.

For example, in the reaction of hydrogen and oxygen to form water:

Unbalanced: $\mathrm{H_2 + O_2 \rightarrow H_2O}$

This equation does not show the same number of oxygen atoms on both sides, so it is unbalanced.

For instance, balancing the reaction of hydrogen and chlorine to form hydrogen chloride:

Unbalanced: $\mathrm{H_2 + Cl_2 \rightarrow HCl}$

Balance chlorine by placing 2 before HCl:

$\mathrm{H_2 + Cl_2 \rightarrow 2HCl}$

Now count atoms:

• Left: H = 2, Cl = 2
• Right: H = 2, Cl = 2

The equation is balanced.

Steps to Balance Equations

Balancing chemical equations involves a clear, step-by-step approach:

• Write the correct formulas for all reactants and products using chemical symbols and formulae.
• Count the atoms of each element on both sides of the equation.
• Adjust coefficients (the numbers in front of formulas) to balance the number of atoms for each element. Never change the subscripts in a formula as this changes the substance itself.
• Check the final balance to ensure the same number of atoms of each element are on both sides.

For instance, balancing the water formation reaction:

Start with unbalanced: $\mathrm{H_2 + O_2 \rightarrow H_2O}$

Count atoms:

• Left: H = 2, O = 2
• Right: H = 2, O = 1

Balance oxygen by placing a coefficient 2 before water:

$\mathrm{H_2 + O_2 \rightarrow 2H_2O}$

Now count atoms:

• Left: H = 2, O = 2
• Right: H = 4, O = 2

Balance hydrogen by placing a coefficient 2 before hydrogen:

$\mathrm{2H_2 + O_2 \rightarrow 2H_2O}$

Final count:

• Left: H = 4, O = 2
• Right: H = 4, O = 2

The equation is now balanced.

Common Mistakes

When balancing equations, watch out for these common errors:

• Changing subscripts: Never alter the small numbers in chemical formulas (subscripts). Changing subscripts changes the identity of the compound.
• Ignoring polyatomic ions: If a group of atoms (polyatomic ion) appears unchanged on both sides, balance it as a whole rather than balancing each element separately. For example, sulfate $\mathrm{SO_4^{2-}}$ often stays together.
• Not recounting atoms after changes: Always recount atoms after adjusting coefficients to ensure the equation is balanced correctly.

Symbols in Equations

Chemical equations use specific symbols to provide extra information:

• State symbols: Indicate the physical state of each substance:
  • $(s)$ = solid
  • $(l)$ = liquid
  • $(g)$ = gas
  • $(aq)$ = aqueous solution (dissolved in water)
• Plus sign (+): Separates reactants or products.
• Arrow (→): Shows the direction of the reaction, from reactants to products.
• Coefficients: Numbers placed before formulas to balance atoms. They tell how many molecules or moles of each substance are involved.

For example, the balanced reaction of magnesium with oxygen is:

$\mathrm{2Mg (s) + O_2 (g) \rightarrow 2MgO (s)}$

This means 2 atoms of magnesium react with 1 molecule of oxygen gas to produce 2 units of magnesium oxide solid.

Balancing Chemical Equations Examples

Example: Balance the equation for the reaction between aluminium and oxygen to form aluminium oxide.

Unbalanced: $\mathrm{Al + O_2 \rightarrow Al_2O_3}$

Step 1: Count atoms on each side:

• Left: Al = 1, O = 2
• Right: Al = 2, O = 3

Step 2: Balance aluminium by placing 4 before Al:

$\mathrm{4Al + O_2 \rightarrow Al_2O_3}$

Step 3: Balance oxygen. Oxygen atoms are 2 on left and 3 on right. The lowest common multiple of 2 and 3 is 6.

Put 3 in front of $\mathrm{O_2}$ and 2 in front of $\mathrm{Al_2O_3}$:

$\mathrm{4Al + 3O_2 \rightarrow 2Al_2O_3}$

Final count:

• Al: 4 on both sides
• O: 6 on both sides

The equation is balanced.