Chemical Reactions and Balancing Equations
Key vocabulary, rules, and reaction types for writing and balancing chemical equations.
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Questions Covered in This Set
11 cards to master
Reactants vs. products
Reactants are the starting substances (left of the arrow); products are the new substances formed (right of the arrow). The arrow means 'yields'.
Law of conservation of mass
Matter is neither created nor destroyed in a chemical reaction (Lavoisier, 1780s) — atoms are only rearranged, so each element must have equal atom counts on both sides.
Why can you change coefficients but never subscripts?
A coefficient just says how many of a molecule you have; changing a subscript changes the substance itself (H₂O → H₂O₂ turns water into hydrogen peroxide).
What do the state symbols (s), (l), (g), and (aq) mean?
(s) solid, (l) liquid, (g) gas, (aq) aqueous — dissolved in water.
How many atoms of each element are in 3H₂SO₄?
6 H, 3 S, and 12 O — the coefficient multiplies everything in the formula.
Balance: CH₄ + O₂ → CO₂ + H₂O
CH₄ + 2O₂ → CO₂ + 2H₂O (C 1=1, H 4=4, O 4=4).
Balance: Fe + O₂ → Fe₂O₃
4Fe + 3O₂ → 2Fe₂O₃
What should you balance first, and what last?
Balance the element that appears in the fewest places first; save hydrogen, oxygen, and lone elements (like O₂) for last.
What do you do if balancing gives a fraction like 5/2 O₂?
Multiply the entire equation by 2 (or whatever denominator) to clear the fraction into whole-number coefficients.
What makes a double replacement reaction actually 'go'?
Something must leave the solution: a precipitate (insoluble solid), a gas, or water (as in neutralization).
Signs that a chemical reaction has occurred
Color change, gas bubbles (not from boiling), a precipitate forming, temperature change, or light produced.
