Chemistry Thermochemistry Enthalpy of reactions

1. ChemistryThermochemistryEnthalpy of reactions Calculators Required Periodic Tables Optional

2. Heat in Chemical reactions • Thermochemistry • the study of changes of heat in chemical reactions • Enthalpy • The heat change in a chemical reaction at constant pressure (Δ H)

3. Energy (enthalpy) Change • Exothermic – energy (heat) given off in a reaction (- Δ H) produced • Endothermic – energy (heat) absorbed in a reaction (+ Δ H) reacts

4. Energy Diagrams • Exothermic Reaction • gives off more energy than it requires • energy is lost as heat (energy is a ‘product’) • Potential energy decreases Potential Energy Time

5. Energy Diagrams • Endothermic Reaction • Reaction that requires more energy than it gives off. • Potential energy increases • Energy is a ‘reactant’ Potential Energy Time

6. Practice • Balance the reaction. • Identify if the reaction is endothermic or exothermic and sign of ΔH • Draw an energy diagram for each. Label the reactants and products, x-axis, y axis and draw the energy curve. 1. energy + H2O  H2 + O2 2. C3H6 + O2 CO2 + H2O + energy 3. CH3OH+ O2 CO2 + H2O + energy

7. Chromium (III) oxide decomposes into its elements when heat is added • Write/balance the reaction • Write the word “energy” on the appropriate side • Draw energy diagram – label each axis • Is this endo or exothermic? • What is the sign of ΔH?

8. Remember the ball… What has to occur for the ball to roll down the hill? Potential Energy Time

9. Potential Energy Time Why doesn’t paper spontaneously combust if the reaction is exothermic? Hint:Think of a match

10. 1000 J 1200J 300 J PotentialEnergy Time Energy of Activation • The energy required to start an exothermic reaction • Ea = 1200 J – 1000 J = 200 J reactants products

11. Molar Heat of Combustion • The amount of heat released (absorbed) when 1 mole of a substance is burned CH4+ O2 CO2 + H2O ∆H = -890 KJ 1mole = 890 KJ • If 56.7g of C3H6 make 1,067 KJ, what is the molar heat of combustion (KJ/mole)?