1. Intro to Energy Changes and Rates of Reaction – Section 5.1, 5.3 Thermodynamics – the study of energy and energy changes Thermochemistry – the study of the energy involved in chemical rxns. Law of Conservation of Energy – the total energy of the universe is constant. – It is neither created or destroyed ∆E universe = 0

2. Energy can be transferred from one object to another Energy can be transformed from one form to another When looking at energy changes in chemical reactions we define: Chemical System – as the reactants and products (what is studied) Surroundings – as the rest of the universe Universe = system + surroundings ∆E universe = ∆E system + ∆E surroundings

3. Therefore, any change that happens to a system results in an equal but opposite change to the surroundings. ∆E system = -∆E surroundings Heat and Temperature Heat (Q) – the transfer of kinetic energy - measured in Joules(J) - transferred spontaneously from warm  cool Temperature (T) – the average kinetic energy of the particles of a substance or system (°C or K)

4. Enthalpy and Enthalpy Change Enthalpy (H) – the total internal energy of a substance at constant pressure. Enthalpy Change (∆H) – the energy change in a system at constant pressure. In chemistry, we use study ∆H for a particular system or reaction. ∆H = heat change at constant pressure

5. Enthalpy Changes in Chemical Rxns In chemical rxns, enthalpy changes result from bonds being broken and formed. – Breaking bonds  requires energy – Forming bonds  releases energy Ex. N 2(g) + O 2(g)  2NO 2(g) – 1 mol of N-N triple bonds and 1 mol of O-O double bonds are broken to form 2 mol of NO bonds – More energy is absorbed to break bonds than is released to create bonds

6. Endothermic vs. Exothermic Rxns Endothermic rxns – more energy is absorbed to break bonds than is released by forming new bonds. Therefore, energy is absorbed by the system. Energy is lost by the surroundings. Exothermic rxns – more energy is released forming bonds than is absorbed breaking them. Therefore, energy is released. Energy is gained by the surroundings. Is the following reaction: N 2(g) + O 2(g)  2NO 2(g) endothermic or exothermic?

7. Endothermic vs. Exothermic Rxns

8. Showing Enthalpy Changes Enthalpy Change of Reaction (∆H rxn ) – the enthalpy change of a reaction – ∆H rxn is dependent on temperature and pressure – Sometimes referred to as heat of reaction Standard Enthalpy of Reaction (∆H° rxn often given as ∆H°) – the enthalpy change of a reaction that occurs at STAP (standard temperature and atmospheric pressure, 25°C and 100 kPa)

9. Representing Exothermic Reactions 3 ways 1.Thermochemical equation – balanced chemical equation that includes the amount of heat absorbed or released by the reaction H 2(g) + ½O 2  H 2 O (l) + 285.8 kJ Heat is shown with products because it is produced by the reaction

10. Representing Exothermic Reactions 2. By a separate expression beside the equation H 2(g) + ½O 2  H 2 O (l) ∆H° rxn = -285.8 kJ In exothermic rxns ∆H° rxn is always negative as energy is lost by the system to the surroundings

11. Representing Exothermic Reactions 3. Using an enthalpy diagram

12. Endothermic Reactions Can be represented in the same 3 ways 1.Thermochemical Equation 117.3 kJ + MgCO 3(s)  MgO (s) + CO 2(g) 2.By a separate expression beside the equation MgCO 3(s)  MgO (s) + CO 2(g) ∆H° rxn = 117.3 kJ 3.By an enthalpy diagram