1. Unit 8: More fun with stoichiometry
Energy and stoichiometry

2. 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?

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

4. How can we represent the energy?
Write the chem eqn with the nrg term.
CH O2  CO H2O
Which side should we put E on? kJ
We can now quantify energy in a rxn!

5. Ex: What mass of oxygen is consumed when 35 kJ are released by combustion of methane?
CH O  CO H2O kJ
mO2=?
g-MM=32.0 g
E=35 kJ
What number do I start with?

6. How can we represent the energy?
CH O  CO H2O kJ
Energy graph: H t R P
Energy bar chart: Q
CH4+O2

7. 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
CH O2  CO H2O ∆H° = –890 kJ
Why negative?

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

9. 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