DO NOW: On back of Notes! How much heat (in kJ) is given out when 85.0g of lead cools from 200.0C to 10.0C? (c=.129 J/gC)

Unit: Thermochemistry Notes Unit: Thermochemistry Thermochemical Equations

After today you will be able to… Define what a thermochemical equation is and use its information to calculate the energy used or produced in a chemical reaction Explain what the ΔH of a reaction is and how it can be found

Thermochemical equation: A chemical equation which includes the energy used or produced in a chemical reaction. (Every reaction has an energy change associated!)

Enthalpy: (H) Heat content of a substance at constant pressure. “Heat of a reaction” - heat that is absorbed or released during a chemical reaction ΔH= change in enthalpy

Exothermic Reactions can be shown two ways: CH4(g) + 2O2(g)  CO2(g) + 2H2O(g) ΔH= -890.4kJ CH4(g) + 2O2(g)  CO2(g) + 2H2O(g) + 890.4kJ Negative sign means the reaction is exothermic! Energy is a product in an exothermic reaction!

Endothermic Reactions can be shown two ways: C(s) + 2S(s)  CS2(l) ΔH= +89.3kJ C(s) + 2S(s) + 89.3kJ  CS2(l) Positive sign means the reaction is endothermic! Energy is a reactant in an endothermic reaction!

NEW conversion factor: The heat of a reaction (ΔH) is the amount of energy used or produced when the coefficient equals the number of moles in a reaction. Coefficient of any substance = ΔH NEW conversion factor:

Examples How much energy is needed when 1.27 moles of sulfur reacts with excess carbon? C(s) + 2S(s)  CS2(l) ΔH= +89.3kJ 1.27 mol S x __________ = K: 1.27 mol S U: ? kJ + 89.3 kJ +56.7kJ 2 mol S

Examples How many moles of water would be produced if this reaction gave off 347.1kJ of energy? CH4(g) + 2O2(g)  CO2(g) + 2H2O(g) + 890.4kJ -347.1 kJ x __________ = K: -347.1kJ U: ? mol H2O 2 mol H2O 0.7796 mol H2O -890.4 kJ

C2H5OH(l) + 3O2(g)  2CO2(l) + 3H2O(g) + 1235kJ How much energy is produced when 75.0g of oxygen reacts with excess ethanol (C2H5OH)? C2H5OH(l) + 3O2(g)  2CO2(l) + 3H2O(g) + 1235kJ 75.0gO2 x _________ x _________= K: 75.0 gO2 U: ? kJ 1 mol O2 -1235kJ -965kJ 32.00gO2 3 mol O2

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