Chemical Reactions and Balancing Equations
Balance equations without changing substances, interpret coefficients correctly, and recognize reaction evidence and patterns.
Chemical Reactions and Balancing Equations
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Preserve every formula; adjust only the number of particles
A chemical equation represents atoms being rearranged. It must show the same number of atoms of every element before and after the reaction.
Use coefficients before formulas to change how many particles or moles participate. Never change a subscript merely to balance an equation: H₂O and H₂O₂ are different substances.
Coefficients also provide particle and mole ratios. They do not state direct gram ratios. Reaction patterns—synthesis, decomposition, single replacement, double replacement, and combustion—help predict and interpret products.
Balance one element at a time
Count atoms on both sides, adjust a coefficient, and recount. Leave elements appearing in several compounds—often H and O—until later when practical.
Evidence needs interpretation
Gas, precipitate, color, light, or temperature change may support a reaction, but observations must be distinguished from physical processes such as boiling or dissolving.
C–H–O is a fast order for hydrocarbon combustion
Problem: Balance C₂H₆ + O₂ → CO₂ + H₂O.
Carbon: Put 2 before CO₂.
Hydrogen: Put 3 before H₂O.
Oxygen: Products now contain 7 O atoms, which would require 7/2 O₂.
Clear the fraction: Multiply every coefficient by 2.
2C₂H₆ + 7O₂ → 4CO₂ + 6H₂OWhy it works
Balancing carbon and hydrogen first fixes the oxygen demand. Multiplying the entire equation clears a fractional coefficient without changing the reaction ratio.
Five forms you should recognize
Problem: Mg + O₂ → MgO
Oxygen arrives in pairs, so place 2 before MgO, then 2 before Mg.
2Mg + O₂ → 2MgOProblem: KClO₃ → KCl + O₂
Use 2KClO₃ to give six oxygen atoms; these form 3O₂. Then use 2KCl.
2KClO₃ → 2KCl + 3O₂Equation: N₂ + 3H₂ → 2NH₃
One molecule or mole of N₂ reacts with three of H₂ to form two of NH₃. It does not mean 1 gram reacts with 3 grams.
Problem: Two clear ionic solutions form an insoluble solid.
The ions exchanged partners, and one new combination could not remain dissolved. This is a double-replacement precipitation reaction.
Problem: In a sealed vessel, 10.0 g of A forms 26.5 g of product after combining with B.
mass of B = 26.5 g − 10.0 g = 16.5 gCheck before you commit
- Changing subscripts instead of coefficients
- Balancing molecules but failing to recount every atom
- Reading coefficients as gram ratios
- Forgetting that a coefficient multiplies the entire formula
- Calling every bubble chemical evidence without ruling out boiling
- Reducing some coefficients but not the entire coefficient set
Do you need the lesson-or just practice?
One original question in each form recommends your next step. It does not yet verify mastery.
Work at the level you need.
Foundations
Build the core procedure with immediate explanations.
Core Practice
Use mixed forms with less scaffolding.
UPCAT-Style Transfer
Apply the competency in unfamiliar representations.
Ready to verify this competency?
A score of 5/5 verifies mastery. An unsuccessful attempt loads a different five-form bank.
Chemical Reactions and Balancing Equations FAQ
May I use a fractional coefficient?
It can be useful during balancing, but final school-level equations are normally expressed using the smallest whole-number coefficients.
Does a balanced equation prove that a reaction will occur?
No. It shows conservation if the reaction occurs; feasibility depends on chemical conditions and energetics.
Why can measured mass fall in an open container?
A gaseous product may leave the measured system even though total matter remains conserved.
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