MYP Chemistry 5.3 Balancing Equations Notes

Why Must Chemical Equations Be Balanced?

1. When chemists describe a chemical reaction, they write a chemical equation.
2. This is like a recipe: it shows which substances react (reactants) and what new substances are formed (products).
3. For the equation to accurately represent what happens, it must be balanced.

Link to the Law of Conservation of Mass

From the previous article, we know about the law of conservation of mass:

Mass is neither created nor destroyed during a chemical reaction. The total mass of the reactants equals the total mass of the products.

1. Because atoms are the building blocks of mass:
   1. The same number of each type of atom must appear before and after the reaction.
   2. In other words, atoms are rearranged, not created or destroyed.
2. A balanced equation is one where the number of each kind of atom on the left (reactants) = the number of that same atom on the right (products).

What Does a Balanced Equation Reveal?

1. Equality of atoms:
   1. Each element has the same number of atoms on both sides.
2. Predicting quantities:
   1. The coefficients (big numbers in front of formulas) tell us the ratios in which substances react and are produced.

How Do We Systematically Balance Chemical Equations?

Balancing equations is a step-by-step process. Here’s a reliable method you can use.

Step 1: List the Atoms on Each Side

Write the unbalanced equation, then list how many atoms of each element appear on the reactant side and on the product side.

Step 2: Start with the Most Complex Substance

Begin with the formula that:

1. Has the most different elements, or
2. Appears least often in the equation.

This is usually a compound, not a simple element like O₂ or H₂.

Step 3: Use Coefficients, Not Subscripts

1. Coefficients are the big numbers in front of formulas (e.g. 2H₂O).
2. Subscripts are the small numbers inside formulas (e.g. H₂O).

Step 4: Balance Polyatomic Ions as Units (When Possible)

If a polyatomic ion (like SO₄²⁻, NO₃⁻, CO₃²⁻) appears unchanged on both sides, you can balance it as a whole unitinstead of balancing each atom separately.

Step 5: Double-Check Your Work

• Make sure every element is balanced.
• Confirm that coefficients have no common factor (simplify if needed).
• Check that you have not changed any subscripts.

What Do Coefficients Tell Us About Amounts of Reactants and Products?

The coefficients in a balanced equation do more than just balance atoms. They tell us the relative amounts of substances that react and form.

Particles and Moles

1. In the equation: $$2 \mathrm{H}_2(\mathrm{~g})+\mathrm{O}_2(\mathrm{~g}) \rightarrow 2 \mathrm{H}_2 \mathrm{O}(\mathrm{l})$$
2. The coefficients mean:
   1. Particle view: 2 molecules of H₂ react with 1 molecule of O₂ to form 2 molecules of H₂O.
   2. Mole view: 2 moles of H₂ react with 1 mole of O₂ to form 2 moles of H₂O.
3. Because 1 mole = $6.02×10^{23}$ particles (Avogadro’s number), the same ratio applies at the particle level and at the macroscopic (mole) level.

From Moles to Masses

Using the molar mass (mass of 1 mole), we can convert between moles and grams.

Steps:

1. Use the balanced equation to get the mole ratio.
2. Convert the given mass to moles.
3. Use the mole ratio to find the moles of another substance.
4. Convert back to mass if needed.

Scaling Reactions

1. Balanced equations can be scaled up or down:
2. If the equation says: $$\mathrm{CH}_4+2 \mathrm{O}_2 \rightarrow \mathrm{CO}_2+2 \mathrm{H}_2 \mathrm{O}$$ then doubling everything: $$2 \mathrm{CH}_4+4 \mathrm{O}_2 \rightarrow 2 \mathrm{CO}_2+4 \mathrm{H}_2 \mathrm{O}$$ describes burning twice as much methane while keeping the same ratios.
3. This is essential in industry, where reactions must be scaled to produce kilograms or tonnes of product.