9-26-08 Blake Chemistry Heat and Chemistry Energy changes E = q + w 1. Heat Change 2. Work (another type of energy change) 3. What drives the world and economy The signs are very important. We need to know if a system is gaining or losing energy 1. E = Change in internal E system 2. E = positive system gains E, negative = loses E 3. Gains heat q=+ 4. Releases Heat q = 5. Work Done On System = + 6. Work Done By System = Heat Change and Temperature Change q = mc (delta T) m = mass, c = specific heat required to raise temp. 1 degree, change in temp. Specific Heat is how much it takes to change 1 gram of material 1 degree Celsius If you add heat to an object, its temperature will increase If 15.0 grams of water at 23 degrees Celsius gains 250 J of heat, what is its new temperature c = 4.184 J/g degrees C +250 J = (15 grams)(4.18 J/g degrees Celsius)(Tf-23 C) Change is 4 degrees Celsius New Temperature is 27 degrees Celsius Phase Changes Heat of vaporization of water: 2256 J/g of Heat fusion of water: 333 J/g Vaporization of H2O H2O(l) = H2O(g) = +2256 J/g (2256 J/g H2O) * (18 g H2O / 1 mole) = 40,608 J/mole H2O Change in H = q = - [Exothermic] Change in H = q = + [Endothermic] Stoichiometry of Heat Thermochemical Equations 1. Balanced chemical equations that include the heat changes Combustion of methane change in H = -890 kJ/mole How much heat is release when 4.0 grams of methane are burned CH4 + 2O2 = CO2 + 2H2O H = -890 kJ/mole
(4/16) = .25 moles Delta H = (0.25 moles) * (-890 kJ/mole) = -223 kJ
Hesss Law If you burn twice as much methane, you will get twice the heat If you run 2 reactions [ADD THE TWO], the overall heat change is the sum of the two individual heat changes If you reverse a reaction, you change the sign of the heat change CH4 + 1/2O2 = CH3OH Delta H = ? [TARGET REACTION]
CH4 + 2O2 = CO2 + 2H2O (l) Delta H = -802.4 kJ/mole CH3OH + 3/2O2 = CO2 + 2H2O Delta H = -676 kJ/mole -802.4 kJ/mole + 676 kJ/mole = -126.4 kJ/mole