UNIT 5 THERMOCHEMISTRY 1 A study of the energy produced by Chemical Reactions Chemical Reactions

HEAT Energy: The ability to do work Energy: The ability to do work HEAT: form of energy transferred from a body at high temperature to one at lower temperature 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 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) JOULES (J) or (kJ): unit of measure of heat (or any form of energy) PHASE: state of matter and depends on temperature and pressure PHASE: state of matter and depends on temperature and pressure

Heat System: part of the universe on which attention is focused System: part of the universe on which attention is focused Surrounding- exchanges energy with the system, make up the rest of the universe Surrounding- exchanges energy with the system, make up the rest of the universe Example: a beaker with water sitting on a hot plate warming. Example: a beaker with water sitting on a hot plate warming. System: water System: water Surroundings: beaker, hot plate, air surrounding the beaker Surroundings: beaker, hot plate, air surrounding the beaker

HEAT is represented by q ENDOTHERMIC: heat flows from surroundings into system; heat absorbed; +q ENDOTHERMIC: heat flows from surroundings into system; heat absorbed; +q 50 g of water is being warmed on the hot plate 50 g of water is being warmed on the hot plate EXOTHERMIC: heat flows from system into surroundings; heat escapes; -q 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 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 1 o C; C = J/  o C (will sometimes be given in KJ HEAT CAPACITY: amount of heat to raise the temperature 1 o C; C = J/  o C (will sometimes be given in KJ SPECIFIC HEAT: amount of heat to raise 1 g of a substance 1 o C; c = J/g  o C SPECIFIC HEAT: amount of heat to raise 1 g of a substance 1 o C; c = J/g  o C Specific heat is an intensive property. 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. A device to measure heat flow. Walls are insulated do no exchange with outside air. Two-types Two-types Coffee-cup calorimeter Coffee-cup calorimeter Bomb calorimeter (used for gases) Bomb calorimeter (used for gases) q reaction = - q calorimeter

Calorimeter q = m c  t (q in J, m in g, t in Celsius) q = m c  t (q in J, m in g, t in Celsius) q = -C  t q = -C  t Exothermic when q reaction is < 0, q calorimeter is + Exothermic when q reaction is < 0, q calorimeter is + Endothermic 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 25 o C to o C. 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? When 1 gram of calcium chloride is added to 50 grams of water in a coffee-cup calorimeter, it dissolves. The temperature rises from 25 o C to o C. 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 20 o C to o C. Taking the heat capacity of the calorimeter to be 9.33 kJ/ o C, calculate the amount of heat evolved in the reaction. 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 20 o C to o C. Taking the heat capacity of the calorimeter to be 9.33 kJ/ o C, calculate the amount of heat evolved in the reaction.

Enthalpy The is the measure of heat from at constant pressure between reactants and products. The is the measure of heat from at constant pressure between reactants and products.  H rxn =  Hproducts -  H reactants  H rxn =  Hproducts -  H reactants q=  H rxn q=  H rxn

Exothermic vs Endothermic Energy Diagram

Thermochemical equations An equation will be given and it will tell you the overall  H rxn An equation will be given and it will tell you the overall  H rxn The sign of  H rxn indicates whether the reaction is endothermic and exothermic. The sign of  H rxn indicates whether the reaction is endothermic and exothermic. The coefficients represent the number of moles The coefficients represent the number of moles The phase symbols must be used The phase symbols must be used Again, this is at constant pressure and at 25 o C Again, this is at constant pressure and at 25 o C

Rules of Thermochemistry 1. the magnitude of  H is directly proportional to the amount of reactant and product Heat of fusion: solid to liquid Heat of fusion: solid to liquid Heat of vaporization: liquid to gas Heat of vaporization: liquid to gas If going from in opposite direction (liquid to solid) Heat of fusion is same but opposite If going from in opposite direction (liquid to solid) Heat of fusion is same but opposite

Calorimetry A g sample of quinone, C 6 H 4 O 2, is burned in a bomb calorimeter whose heat capacity is kJ/ o  C. The temperature increases from o C to o C. A g sample of quinone, C 6 H 4 O 2, is burned in a bomb calorimeter whose heat capacity is kJ/ o  C. The temperature increases from o C to o C. What is the heat of combustion per gram? What is the heat of combustion per gram? Per mole? Per mole?

Calorimetry The heat of combustion of glucose, C 6 H 12 O 6, is kJ/g. A g sample burned in a bomb calorimeter raises the temperature from o C to o C. The heat of combustion of glucose, C 6 H 12 O 6, is kJ/g. A g sample burned in a bomb calorimeter raises the temperature from o C to o C. What is the heat capacity of the calorimeter? What is the heat capacity of the calorimeter?

Calorimetry A g sample of benzoic acid, HC 7 H 5 O 2, is burned in a calorimeter with a heat capacity of kJ/ o  C. When the calorimeter contains kg of water, the temperature rises from o C to o C. A g sample of benzoic acid, HC 7 H 5 O 2, is burned in a calorimeter with a heat capacity of kJ/ o  C. When the calorimeter contains kg of water, the temperature rises from o C to o C. What is the heat of combustion of benzoic acid in kJ/mol? 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 of a material changes based on temperature and pressure. The phase is dependent on the motion of the molecules of the substance. 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. When a substance melts(freezes) or boils(condenses), there is no temperature change, but there is a change of heat involved.

Phase Diagrams

PHASE DIAGRAM Normal melting (freezing) and boiling (condensing) points at standard pressure Normal melting (freezing) and boiling (condensing) points at standard pressure Triple point: 3 phases exist at same T&P Triple point: 3 phases exist at same T&P Critical temperature: above this T, only exists as a gas Critical temperature: above this T, only exists as a gas Critical pressure: the pressure to cause condensation at critical temperature Critical pressure: the pressure to cause condensation at critical temperature Sublimation: from solid to vapor directly Sublimation: from solid to vapor directly

Example How much heat is needed to melt 25 g of ice? How much heat is needed to melt 25 g of ice?

Example How much heat is evolved when one mole of water cools from 100 o C to 5 o C? How much heat is evolved when one mole of water cools from 100 o C to 5 o C?

Rules of Thermochemistry  H for a reaction is equal in magnitude by opposite in sign to  H for the reverse reaction. 3. The value of  H for a reaction is the same whether it occurs in one step or in a series AKA: Hess’s Law AKA: Hess’s Law  Htotal =  H1 +  H2, etc..  Htotal =  H1 +  H2, etc..

How much heat? Calculate  H for the decomposition of liquid sulfuric acid to steam, oxygen and sulfur dioxide gas. Calculate  H 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? How much heat will be generated when 25 g of sulfuric acid decomposes?

What is the H f ? 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. 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 H f for chlorine trifluoride. Find H f for chlorine trifluoride.

Enthalpy Change and H f Acetylene, C 2 H 2, and benzene, C 6 H 6, have the same empirical formula. Benzene can be made from acetylene: 3C 2 H 2  C 6 H 6. Acetylene, C 2 H 2, and benzene, C 6 H 6, have the same empirical formula. Benzene can be made from acetylene: 3C 2 H 2  C 6 H 6. The heats of combustion for C 2 H 2 and C 6 H 6 are kJ/mol and kJ/mol, respectively. The heats of combustion for C 2 H 2 and C 6 H 6 are kJ/mol and kJ/mol, respectively. Calculate the heat of formation for each and the heat of reaction of the formation of benzene from acetylene. Calculate the heat of formation for each and the heat of reaction of the formation of benzene from acetylene.