6.4-6.6 Quantifying Heat and Work, and Enthalpy

Heat The difference between temperature and heat: Temperature: a measure of the thermal energy within a sample of matter. Heat: the transfer of thermal energy. When does heat transfer stop? When the system and the surrounding reach a thermal equilibrium.

Temperature Change (ΔT) and Heat Capacity (C) Heat capacity (C): the quantity of heat required to change the temperature of a system by 1oC. Units are J/oC. Heat capacity is an extensive property, meaning that it depends on the amount of matter being heated. Specific heat capacity (Cs): the amount of heat required to raise the temperature of one gram of a substance 1oC. Units are J/goC q = m X Cs X ΔT (C in your reference table is lowercase) Molar heat capacity: the amount of heat required to raise the temperature of 1 mol of a substance by 1oC. Both specific heat capacity and molar heat capacity are intrinsic properties; That is that they depend on the type of matter, not the amount.

Practice Problem Which has a higher specific heat capacity, water or iron? Water has a higher specific heat, meaning that it takes longer to raise the temperature of 1.00 g water than it does to raise the temperature of 1.00 g of iron. Don’t believe me? What would you rather touch after a hot summers day, a puddle of water or a street sign?

Let’s Try Another To determine whether a shiny gold-colored rock is actually gold, a chemistry student decides to measure its heat capacity. She first weighs the rock and finds it has a mass of 4.7 g. She then finds that upon absorption of 57.2 J of heat, the temperature of the rock rises from 25oC to 57oC. Find the specific heat capacity of the substance composing the rock and determine whether the value is consistent with the rock being pure gold. Specific heat of gold: 0.128 J/goC q = m Cs ΔT q 57.2 J Cs = --------- = ---------------------------- = 0.380 J/goC m ΔT (4.7 g) (57oC – 25oC) This gold colored rock is not actually gold

Thermal Energy Transfer Remember that energy is neither created nor destroyed, so the energy lost by one substance equals the amount of energy gained by the other. qsys = -qsurr Msys X Cs(sys) X ΔTsys = -(Msurr X Cs(surr) X ΔTsurr)

Let’s Try a Practice Problem A block of copper of unknown mass has an initial temperature of 65.4oC. The copper is immersed in a beaker containing 95.7 g of water at 22.7oC. When the two substances reach thermal equilibrium, the final temperature is 24.2oC. What is the mass of the copper block? Specific heat of copper: 0.385 J/goC Specific heat of water: 4.18 J/goC mCu X 0.385 J/goC X (24.2oC – 65.4oC) = -(95.7 g H2O X 4.18 J/ goC) X (24.2oC – 22.7oC)) mcu = 37.8 g Cu

Measuring ΔE for Chemical Reactions ΔE: a measure of all the energy (heat and work) exchanged with the surroundings. Calorimetry: a method to measure the thermal energy exchange between the reaction (the system) and the surroundings, by observing the change in temperature of the surroundings. qcal = Ccal X ΔT = -qrxn -qrxn ΔErxn = ------------------ units: kJ/mol mol system

Let’s Try a Practice Problem When 1.550 g of liquid hexane (C6H14) undergo combustion in a bomb calorimeter, the temperature rises from 25.87oC to 38.13oC. Find ΔErxn for the reaction in kJ/mol hexane. The heat capacity of the bomb calorimeter, determined in a separate experiment, is 5.73 kJ/oC. qcal = Ccal ΔT = -qrxn -qrxn ΔErxn =------------------ mol hexane ΔT = (38.13 – 25.87) = 12.26oC qcal = (5.73 kJ/oC)(12.26oC) = 70.2 kJ = -qrxn = -70.2 kJ -qrxn -70.2 kJ ΔErxn =------------------ = -------------------------------------------- = -3.90X103 kJ/mol mol hexane (1.550 g X (1.00 mol / 86.172 g)

Enthalpy Enthalpy (H): the sum of the systems internal energy and the product of its pressure and volume. H = E + PV Endothermic reaction: a chemical reaction with a positive ΔH, which means heat is absorbed from its surroundings. Exothermic reaction: a chemical reaction with a negative ΔH, which means heat is released to the surroundings.

Stoichiometry Involving ΔH ΔHrxn, is also called the enthalpy of the reaction or heat of reaction and it is an extensive property, one that depends on the amount of material undergoing the reaction. Balanced chemical reactions with enthalpy changes can be used as conversion factors to calculate heat emitted or absorbed with given different quantities of reactants. Two examples of how to write a thermochemical equation: C3H8(g) + 5O2 (g)  3CO2(g) + 4H2O(g) ΔH = -2044 kJ C3H8(g) + 5O2 (g)  3CO2(g) + 4H2O(g) + 2044 kJ

Let’s Try a Practice Problem Ammonia reacts with oxygen according to the equation: 4NH3(g) + 5O2(g)  4NO(g) + 6H2O(g) ΔH = -906 kJ Calculate the heat (kJ) associated with the complete reaction of 155 g of NH3. 1.00 mol NH3 -906 kJ 155 g NH3 X -------------------- X ------------------ = -2.06x103 kJ 17.034 g NH3 4.00 mol NH3

6.4-6.6 pg. 288 #’s 48, 50, 60, 62, & 66 (a’s only) Read 6.7-6.9 pgs. 269-278