Balancing Equations and Reaction Energy

Objective: Balance simple equations by counting atoms.

A balanced chemical equation has equal counts of each element on both sides because ordinary reactions rearrange atoms without creating or destroying them. Adjust coefficients before complete formulas, not subscripts within formulas. For H₂ + O₂ → H₂O, balanced coefficients are 2, 1, 2: 2H₂ + O₂ → 2H₂O. Count atoms in polyatomic groups carefully and check every element. Coefficients express relative numbers of particles or amount of substance, not necessarily absolute numbers in one sample. State physical conditions when needed; a balanced equation does not by itself prove that the reaction proceeds or predict its rate.

Objective: Interpret coefficients as ratios and identify a simple limiting case.

Equation coefficients give stoichiometric ratios of reacting particles and, at bulk scale, moles. For 2H₂ + O₂ → 2H₂O, two moles of H₂ require one mole of O₂ and can yield two moles of H₂O under complete conversion. Starting with 4 moles H₂ and 1 mole O₂, oxygen limits the reaction to 2 moles H₂O while 2 moles H₂ remain, in the ideal model. A limiting reactant is consumed first according to the ratio, not necessarily the reactant with the smallest numerical mass. Actual yield may be less than theoretical due to incomplete reaction, competing processes, or losses. State whether numbers are particles, moles, or masses.

Objective: Distinguish net energy change from reaction rate and barriers.

An exothermic reaction transfers energy to surroundings under the stated conditions, while an endothermic process absorbs energy. An energy diagram can show reactant and product energy levels and a higher transition-state barrier; the barrier height relates to activation energy, not the net difference. Catalysts lower activation energy for a pathway and can increase reaction rate without changing the equilibrium energy difference between starting and ending states. Temperature, concentration, surface area, and catalysts can change rates in context. A faster reaction is not necessarily more exothermic. Energy conservation includes reacting system plus surroundings, and a temperature change alone needs a controlled context for interpretation.