Chemical Reactions and Conservation of Mass
Core concepts
A chemical reaction rearranges atoms into new substances. Evidence includes color change, gas production, precipitate formation, and temperature change. Reactants become products, and equations must be balanced because matter is conserved: atoms are neither created nor destroyed. Know the main reaction types: synthesis, decomposition, single replacement, double replacement, and combustion. Exothermic reactions release energy; endothermic reactions absorb it. Reaction rates increase with temperature, concentration, surface area, and catalysts.
Traps to watch
- Balancing by changing subscripts instead of coefficients.
- Forgetting diatomic elements (H2, N2, O2, F2, Cl2, Br2, I2) when writing formulas.
- Assuming mass "disappears" when a gas escapes; the system was open.
- Confusing a physical change (state, shape) with a chemical change.
- Treating a catalyst as a reactant that is consumed.
Worked example
Balance: CH4 + O2 → CO2 + H2O. Carbon and hydrogen balance first, then oxygen. CH4 + 2O2 → CO2 + 2H2O. Check: C 1=1, H 4=4, O 4=4. If 16 g of methane reacts fully with 64 g of oxygen, the products total 80 g, consistent with conservation of mass.
Rule of thumb
Balance one element at a time, save hydrogen and oxygen for last, and verify each atom count on both sides.