Unit 8: More fun with stoichiometry Energy and stoichiometry

In your groups On whiteboards: 1. Write a balanced equation for the combustion of candle wax, C25H52 2. Add the energy term to the appropriate side of the equation 3. Is the reaction endothermic or exothermic? 1 38 25 26

Energy in chem rxns Consider the reaction in which methane burns in air to produce carbon dioxide and water. CH4 + 2 O2  CO2 + 2 H2O What type of rxn? Experiments show that when one mole of CH4 is burned, 890 kJ are released Exothermic or endothermic?

How can we represent the energy? Write the chem eqn with the nrg term. CH4 + 2 O2  CO2 + 2 H2O Which side should we put E on? + 890 kJ We can now quantify energy in a rxn!

Ex: What mass of oxygen is consumed when 35 kJ are released by combustion of methane? CH4 + 2 O2  CO2 + 2 H2O + 890 kJ mO2=? g-MM=32.0 g E=35 kJ What number do I start with?

How can we represent the energy? CH4 + 2 O2  CO2 + 2 H2O + 890 kJ Energy graph: H t R P Energy bar chart: Q CH4+O2

How can we represent the energy? Enthalpy (∆H°) notation Enthalpy: a measure of the internal nrg in a system H = Eth + Eint + Echem + … ∆H = H2 - H1 CH4 + 2 O2  CO2 + 2 H2O ∆H° = –890 kJ Why negative?

H2 + Br2  2 HBr 1. 2. 3. H2 + Br2 Q Requires 72.8 kJ to proceed

Assignment: Lab 9.2 due Monday Finish PhET,EF, 1, 2, 3 WS You should be done! Unit 9 WS # 4 Got Objectives? Empirical Formula/Ion Quiz next Tuesday Unit 9 test next week