Heat in Chemical Reactions Ch. 16

Energy in Chemical Reactions Every reaction has an energy change associated with it Energy is stored in bonds between atoms Making bonds releases energy Breaking bonds absorbs energy

Exothermic Reactions In exothermic reactions, the products are lower in energy than the reactants Reaction releases energy Reaction often releases heat (surroundings get hot)

Example 2H 2 + O 2 2H 2 O Reaction releases kJ/mol

Endothermic Reactions In endothermic reactions, the products are higher in energy than the reactants Reaction absorbs energy Reaction absorbs heat (surroundings get cooler)

Example CaCO 3 CaO + CO 2 Reaction absorbs 176 kJ/mol

Thermochemical Equations Equations that include energy Energy is treated as a product or a reactant Example: CH 4 + 2O 2 CO 2 + 2H 2 O kJ Energy is a product in this reaction 1 mole of CH 4 makes kJ of energy

Enthalpy ( H) Enthalpy describes the heat released or absorbed in a chemical reaction Represented by the letter H ΔH is the change in enthalpy Enthalpy difference between reactants and products equals the heat absorbed or released during the reaction when the pressure remains constant

ΔHΔH ΔH = (H products ) – (H reactants ) Always positive for an endothermic reaction (products have higher enthalpy than reactants) Always negative for exothermic reactions (products have lower enthalpy than reactants)

Endothermic or Exothermic (hint – is it releasing heat or absorbing heat)? C 6 H 12 O 6 (s) + 6O 2 (g)→ 6CO 2 (g) + 6H 2 O (l) kJ 2H 2 O 2 (l) → 2H 2 O (l) + O 2 (g) kJ 2SO 3 (g) kJ → 2SO 2 (g) + O 2 (g)

Enthalpy of Formation Enthalpy change that occurs when one mole of a compound is formed from its elements in their standard state at STP. Indicated by the following symbol: ΔH f °

Enthalpy of Combustion The enthalpy change that occurs during the complete combustion of one mole of a substance. Symbol is ΔH c

Rewrite these equations with the enthalpy change included in the reaction and indicate if endothermic or exothermic. A. H 2 + O 2 → H 2 O ΔH° = kJ B.2Mg + O 2 → 2MgO ΔH° = kJ C.I 2 (s) → I 2 (g) ΔH° = kJ D.3CO (g) + Fe 2 O 3 (s) → 2Fe (s) + 3CO 2 (g) ΔH° = kJ