Hess’s Law If you add two or more thermochemical equations to give a final equation, then you can also add the heats of reaction to give the final heat of reaction. Allows you to determine the heat of reaction.

Manipulations If you reverse a reaction, change the sign of ΔH. C + O2  CO2(g) = -393.5kJ CO2(g)  C + O2 = 393.5kJ

Manipulations If you double/triple/etc. a reaction, you also must double/triple/etc. ΔH. C + O2  CO2(g) = -393.5kJ 2C +2O2  2CO2(g) = 2*-393.5kJ

4NH3(g) + 3O2(g) --> 2N2(g) + 6H2O(l) Example Work backwards… Given the following, calculate ∆H for 4NH3(g) + 3O2(g) --> 2N2(g) + 6H2O(l) 2N2O(g) --> O2(g) + 2N2(g) ∆H = -164kJ 2NH3(g) + 3N2O(g) --> 4N2(g) + 3H2O(l) ∆H = -1012kJ Answer: -1532 kJ

Standard Enthalpy of Formation Represented by ∆H°f Sometimes calorimeters can not be used to find ∆H (like if the process is very slow) Shows how much energy is required to make 1 mole of a compound from its elements with all substances in their standard state Table 6.2 in your book

Standard State Definition COMPOUND: Pressure = 1 atm Pure liquid or solid If in solution, concentration = 1M ELEMENT: Temperature = 25°C State = whatever state it exists in these conditions

DH0reaction =∑npDHf0(products)–∑nrDHf0(reactants) Calculating DH0reaction =∑npDHf0(products)–∑nrDHf0(reactants) ∑ means sum of… np is moles of product…nr is moles of reactant If dealing with an element, the DHf0 is zero

CH4(g) + 2O2(g)  CO2(g) +2H2O(l) Example Calculate the change in enthalpy: CH4(g) + 2O2(g)  CO2(g) +2H2O(l) Answer: Products: [-393.5kJ + 2(-285.8kJ)]= -965.1kJ Reactants: [-74.86kJ + 2(0.0kJ)] = -74.86kJ P-R = -965.1kJ-(-74.86kJ) = -890.24kJ

Fossil Fuels When decayed plants are burned, the energy stored in them can be used Petroleum and natural gas (hydrocarbons) Coal: creates about 23% of U.S. energy

Hydrocarbons Prefixes are the same in organic chemistry Meth- Eth- Prop- But- Pent- Hex- Hept- Oct- Suffixes represent different bonding/compounds

Environmental Impacts Greenhouse effect: CO2 created from combustion of fossil fuels absorbs infrared radiation given off by the earth and does not allow it to leave the atmosphere H2O can also absorb radiation (humidity), but earth’s H2O has not changed much…

New Energy Sources Coal gasification: creates a gaseous form of coal (syngas) that is easily transported and reacted with oxygen in a combustion reaction, releasing energy This material can be used to produce other fuels like methanol

New Energy Sources Coal Slurries: coal ground up and mixed with water and used instead of solid coal in power plants