UNIT 5THERMOCHEMISTRY 1 A study of the energy produced by Chemical Reactions
HEAT • Energy: The ability to do work • HEAT: form of energy transferred from a body at high temperature to one at lower temperature • TEMPERATURE: average kinetic energy of the molecules of a substance; measures heat flow • JOULES (J) or (kJ): unit of measure of heat (or any form of energy) • PHASE: state of matter and depends on temperature and pressure
Heat • System: part of the universe on which attention is focused • Surrounding- exchanges energy with the system, make up the rest of the universe • Example: a beaker with water sitting on a hot plate warming. • System: water • Surroundings: beaker, hot plate, air surrounding the beaker
HEAT is represented by q • ENDOTHERMIC: heat flows from surroundings into system; heat absorbed; +q • 50 g of water is being warmed on the hot plate • EXOTHERMIC: heat flows from system into surroundings; heat escapes; -q • The hot plate is turned off and the water starts heating its surroundings. The temp of water decreases and the temp of the surroundings increase
Measuring Heat • HEAT CAPACITY: amount of heat to raise the temperature 1oC; C = J/ DoC (will sometimes be given in KJ • SPECIFIC HEAT: amount of heat to raise 1 g of a substance 1oC; c = J/g DoC • Specific heat is an intensive property. ***Note that Heat Capacity is a Big C and Specific Heat is a little c
Calorimeter • A device to measure heat flow. Walls are insulated do no exchange with outside air. • Two-types • Coffee-cup calorimeter • Bomb calorimeter (used for gases) q reaction = - q calorimeter
Calorimeter • q = m cDt (q in J, m in g, t in Celsius) • q = -CDt • Exothermic when q reaction is < 0, q calorimeter is + • Endothermic when q reaction is > 0, q calorimeter is -
Example: Coffee-Cup Calorimeter • When 1 gram of calcium chloride is added to 50 grams of water in a coffee-cup calorimeter, it dissolves. The temperature rises from 25oC to 28.51oC. Assuming that all the heat given off by the reaction is transferred to the water, calculate q for the reaction system. Is this exothermic or endothermic?
Example: Bomb Calorimeter • The reaction between hydrogen and chlorine can be studied in a bomb calorimeter. It is found that when a 1.00 gram of hydrogen completely reacts, the temperature rises from 20oC to 29.82oC. Taking the heat capacity of the calorimeter to be 9.33 kJ/oC, calculate the amount of heat evolved in the reaction.
Enthalpy • The is the measure of heat from at constant pressure between reactants and products. • DH rxn = DHproducts - DH reactants • q= DH rxn
Thermochemical equations • An equation will be given and it will tell you the overall DH rxn • The sign of DH rxn indicates whether the reaction is endothermic and exothermic. • The coefficients represent the number of moles • The phase symbols must be used • Again, this is at constant pressure and at 25oC
Rules of Thermochemistry • the magnitude of DH is directly proportional to the amount of reactant and product • Heat of fusion: solid to liquid • Heat of vaporization: liquid to gas • If going from in opposite direction (liquid to solid) Heat of fusion is same but opposite
Calorimetry • A 2.200 g sample of quinone, C6H4O2, is burned in a bomb calorimeter whose heat capacity is 7.854 kJ/oDC. The temperature increases from 23.44oC to 30.57oC. • What is the heat of combustion per gram? • Per mole?
Calorimetry • The heat of combustion of glucose, C6H12O6, is 15.57 kJ/g. A 2.500 g sample burned in a bomb calorimeter raises the temperature from 20.55oC to 23.25oC. • What is the heat capacity of the calorimeter?
Calorimetry • A 1.200 g sample of benzoic acid, HC7H5O2, is burned in a calorimeter with a heat capacity of 2.423 kJ/oDC. When the calorimeter contains 1.500 kg of water, the temperature rises from 22.45oC to 26.10oC. • What is the heat of combustion of benzoic acid in kJ/mol?
Phase Changes • The phase of a material changes based on temperature and pressure. • The phase is dependent on the motion of the molecules of the substance. • When a substance melts(freezes) or boils(condenses), there is no temperature change, but there is a change of heat involved.
PHASE DIAGRAM • Normal melting (freezing) and boiling (condensing) points at standard pressure • Triple point: 3 phases exist at same T&P • Critical temperature: above this T, only exists as a gas • Critical pressure: the pressure to cause condensation at critical temperature • Sublimation: from solid to vapor directly
Example • How much heat is needed to melt 25 g of ice?
Example • How much heat is evolved when one mole of water cools from 100oCto 5oC?
Rules of Thermochemistry 2. DH for a reaction is equal in magnitude by opposite in sign to DH for the reverse reaction. 3. The value of DH for a reaction is the same whether it occurs in one step or in a series • AKA: Hess’s Law • DHtotal = DH1 +DH2, etc..
How much heat? • Calculate DH for the decomposition of liquid sulfuric acid to steam, oxygen and sulfur dioxide gas. • How much heat will be generated when 25 g of sulfuric acid decomposes?
What is the Hf? • Chlorine trifluoride reacts with ammonia to form nitrogen, chlorine and hydrogen fluoride gases. When two moles of chlorine trifluoride reacts, 1196 kJ of heat is evolved. • Find Hf for chlorine trifluoride.
Enthalpy Change and Hf • Acetylene, C2H2, and benzene, C6H6, have the same empirical formula. Benzene can be made from acetylene: 3C2H2C6H6. • The heats of combustion for C2H2 and C6H6 are -1299.4 kJ/mol and -3267.4 kJ/mol, respectively. • Calculate the heat of formation for each and the heat of reaction of the formation of benzene from acetylene.