Converting word equations to symbol equations using chemical formulae

Symbol equations build on word equations by replacing substance names with their chemical formulae. A chemical formula is a combination of symbols and numbers that shows the types and numbers of atoms in a substance. For example, water is H₂O, meaning two hydrogen atoms and one oxygen atom.

To convert a word equation to a symbol equation, replace each name with its correct formula. Keep the same structure: reactants on the left, arrow, products on the right. However, symbol equations must eventually be balanced to accurately represent the reaction.

Steps for converting word to symbol equations:

1. Write the word equation first to identify all reactants and products.
2. Look up or recall the correct chemical formula for each substance.
3. Substitute the formulae into the equation, using plus signs between multiple reactants or products.
4. Do not balance yet – that comes later; focus on getting the formulae right.

Examples of symbol equations (unbalanced):

• Magnesium + oxygen → magnesium oxide becomes Mg + O₂ → MgO
• Hydrogen + chlorine → hydrogen chloride becomes H₂ + Cl₂ → HCl
• Sodium + water → sodium hydroxide + hydrogen becomes Na + H₂O → NaOH + H₂

This conversion makes the equation more precise by showing the actual atoms involved, setting the stage for balancing.

The principle of conservation of mass requiring equal atoms on both sides

The law of conservation of mass states that matter cannot be created or destroyed in a chemical reaction – it can only change form. This means the total mass of the reactants must equal the total mass of the products.

In terms of atoms, this principle requires that the number and type of atoms on the reactant side must exactly match those on the product side. Atoms are rearranged during the reaction, but none are added or lost. If an equation does not have equal atoms on both sides, it is unbalanced and does not accurately represent the reaction.

Why equal atoms matter:

• Mass balance - Ensures the equation follows the law of conservation of mass, as atoms determine the mass
• Accurate representation - Reflects what actually happens in nature, where atoms are conserved
• Prediction tool - Allows chemists to calculate amounts of substances needed or produced

For instance, in the unbalanced equation Mg + O₂ → MgO, there are two oxygen atoms on the left but only one on the right, violating conservation. Balancing fixes this issue.

How to balance equations

Balancing a chemical equation involves making the number of each type of atom the same on both sides. This is done by adding coefficients, which are whole numbers placed in front of chemical formulae. Coefficients multiply the entire formula, indicating how many molecules or units are involved.

Steps for balancing chemical equations:

1. Write the unbalanced symbol equation with correct formulae for reactants and products.
2. Count the number of each type of atom on both sides.
3. Choose an element that appears in unequal numbers and add a coefficient to one side to equalize it (start with elements that appear in only one reactant and one product if possible).
4. Recount the atoms and adjust coefficients for other elements as needed.
5. Repeat the process, using trial and error, until all elements have equal numbers on both sides.
6. Ensure all coefficients are whole numbers; if fractions appear, multiply everything by a number to make them whole.
7. Double-check the final counts for all atoms.

The rule against changing chemical formulae during balancing

Chemical formulae represent the fixed composition of substances, with subscripts showing the exact ratio of atoms. You must never change the subscripts in the formulae themselves, as that would represent a different substance. For example, CO₂ is carbon dioxide, but CO is carbon monoxide – a completely different substance.

Instead, only coefficients (the big numbers in front) can be adjusted. This rule maintains the identity of reactants and products while allowing the equation to reflect the correct number of molecules involved in the reaction.

Worked example - Balancing a simple chemical equation

Balance the equation for the reaction of magnesium with oxygen to form magnesium oxide: Mg + O₂ → MgO.

Step 1: Count initial atoms

• Left: 1 Mg, 2 O
• Right: 1 Mg, 1 O

Step 2: Balance oxygen

Add a coefficient of 2 in front of MgO: Mg + O₂ → 2MgO

• Now left: 1 Mg, 2 O
• Right: 2 Mg, 2 O

Step 3: Balance magnesium

Add a coefficient of 2 in front of Mg: 2Mg + O₂ → 2MgO

• Now left: 2 Mg, 2 O
• Right: 2 Mg, 2 O

Step 4: Verify

All atoms are equal, and coefficients are whole numbers.

The balanced equation is 2Mg + O₂ → 2MgO.

Worked example - Balancing a more complex equation

Balance the equation for sodium reacting with water: Na + H₂O → NaOH + H₂.

Step 1: Count initial atoms

• Left: 1 Na, 2 H, 1 O
• Right: 1 Na, 3 H, 1 O (1 H from NaOH + 2 H from H₂)

Step 2: Balance hydrogen

Add a coefficient of 2 in front of NaOH and 2 in front of H₂O (trial adjustment): Na + 2H₂O → 2NaOH + H₂

• Now left: 1 Na, 4 H, 2 O
• Right: 2 Na, 4 H, 2 O (4 H from 2NaOH + H₂, but wait: 2NaOH has 2 Na, 2 O, 2 H; H₂ adds 2 H, total right: 2 Na, 4 H, 2 O)

Step 3: Balance sodium

Add a coefficient of 2 in front of Na: 2Na + 2H₂O → 2NaOH + H₂

• Now left: 2 Na, 4 H, 2 O
• Right: 2 Na, 4 H, 2 O

Step 4: Verify

All atoms match with whole-number coefficients.

The balanced equation is 2Na + 2H₂O → 2NaOH + H₂.