1. Vanessa Prasad-Permaul Valencia College CHM 1045
CH 6: Thermochemistry
Vanessa Prasad-Permaul
Valencia College
CHM 1045

2. INTRODUCTION TO THERMODYNAMICS KEY DEFINITIONS
THERMOCHEMISTRY ENERGY
INTRODUCTION TO THERMODYNAMICS
KEY DEFINITIONS
FIRST LAW OF THERMODYNAMICS
HEAT, WORK & INTERNAL ENERGY
ENTHALPY
THERMODYNAMIC SYSTEM
HESS’ LAW
STANDARD ENTHALPIES OF FORMATION
ENERGY AND THE ENVIRONMENT

3. THERMOCHEMISTRY ENERGY
Energy is the capacity to do work, or supply heat.
Energy = Work + Heat
Kinetic Energy is the energy of motion.
Ek = 1/2 mv Ek = kinetic energy
m = mass (kg)
v = velocity (m/s)
(1 Joule = 1 kgm2/s2) (1 calorie = J)
Potential Energy is stored energy.

4. THERMOCHEMISTRY ENERGY
EXAMPLE 6.1
A regulation baseball weighing 143 grams travels 75 miles per hour. What is the kinetic energy of this baseball in Joules? Convert to calories.
Ek = ½ mv2
m = 143g x 1kg = kg
1000g
v = 75miles x 1hr x 1min x m = m/s
1 hour min 60sec mile
Ek = ½ x 0.143kg x (33.527m/s)2 = 80.3kg.m2/s2 = 80.3J
80.3J x 1 cal = 19.2cal
4.184J

5. THERMOCHEMISTRY ENERGY
EXERCISE 6.1
An electron whose mass is 9.11 x kg is accelerated by a positive charge to a speed of 5.0 x 106 m/s. What is the kinetic energy of the electron in Joules? In calories?
Ek = ½ mv2

6. DE = Efinal – Einitial = E2 – E1 = q + w
THERMOCHEMISTRY HEAT OF REACTION
THE FIRST LAW OF THERMODYNAMICS:
The law of the conservation of energy: Energy cannot be created or destroyed.
The energy of an isolated system must be constant.
The energy change in a system equals the work done on the system + the heat added.
DE = Efinal – Einitial = E2 – E1 = q + w
q = heat, w = work

7. Thermal Energy is the kinetic energy of molecular motion
THERMOCHEMISTRY HEAT OF REACTION
Thermal Energy is the kinetic energy of molecular motion
Thermal energy is proportional to the absolute temperature. Ethermal  T(K)
Heat is the amount of thermal energy transferred between two objects at different temperatures.

8. Enthalpies of Physical Change:
THERMOCHEMISTRY ENTHALPY
Enthalpies of Physical Change:
Enthalpy is a state function, the enthalpy change from solid to vapor does not depend on the path taken between the two states.
Hsubl = Hfusion + Hvap

9. ILLUSTRATION OF A THERMODYNAMIC SYSTEM
THERMOCHEMISTRY HEAT OF REACTION
ILLUSTRATION OF A THERMODYNAMIC SYSTEM

10. THERMOCHEMISTRY HEAT OF REACTION
In an experiment: Reactants and products are the system; everything else is the surroundings.
Energy flow from the system to the surroundings has a negative sign (loss of energy). (-E or - H)
Energy flow from the surroundings to the system has a positive sign (gain of energy). (+E or +H)

11. THERMOCHEMISTRY HEAT OF REACTION
Enthalpies of Chemical Change: Often called heats of reaction (DHreaction).
Endothermic: Heat flows into the system from the surroundings, heat is absorbed.
DH has a positive sign.
Energy added
q is (+)
Exothermic: Heat flows out of the system into the surroundings, heat is evolved.
DH has a negative sign.
Energy subtracted
q is (-)

12. SIGN CONVENTIONS FOR HEAT, WORK AND INTERNAL ENERGY
THERMOCHEMISTRY ENERGY
SIGN CONVENTIONS FOR HEAT, WORK AND INTERNAL ENERGY
q (heat) +
System gains thermal energy -
System loses thermal energy
w (work)
Work done on a system
Work done by the system
DE (change in internal energy)
Energy flows into the system
Energy flows out of the system

13. EXAMPLE 6.2 Barium hydroxide octahydrate reacts with ammonium nitrate:
THERMOCHEMISTRY HEAT OF REACTION
EXAMPLE 6.2
Barium hydroxide octahydrate reacts with ammonium nitrate:
Ba(OH)2.8H2O(s) + 2NH4NO3(s) NH3(g) + 10H2O(l) + Ba(NO3)2(aq)
When 1mol of barium hydroxide octahydrate reacts with 2 mol of ammonium nitrate, the reaction mixture absorbs 170.8kJ of heat. Is this reaction exothermic or endothermic? What is the heat of reaction (q)?
Energy is absorbed so this reaction is endothermic.
q is kJ 13

14. 4NH3(g) + 5O2(g) 4NO(g) + 6H2O(l)
THERMOCHEMISTRY HEAT OF REACTION
EXERCISE 6.2
Ammonia burns in the presence of a platinum catalyst to
give nitric oxide
4NH3(g) + 5O2(g) NO(g) + 6H2O(l)
4 mol of ammonia is burned and 1170kJ of heat is evolved. Is the reaction endothermic or exothermic? What is the value of q? Pt

15. THERMOCHEMISTRY ENTHALPY
The amount of heat exchanged between the system and the surroundings is given the symbol q.
q = DE + PDV
At constant volume (DV = 0): qv = DE
At constant pressure: qp = DE + PDV = DH
Enthalpy of Reaction: DH = Hproducts – Hreactants

16. PRESSURE-VOLUME WORK THERMOCHEMISTRY ENTHALPY
Work = -atmospheric pressure * area of piston * distance piston moves w = -PDV

17. THERMOCHEMISTRY HEAT OF REACTION
EXAMPLE 6.2A
If a balloon is inflated from 0.100L to 1.85L against an external pressure of 1.00atm, how much work is done (J)?
w = -PDV
= -1.00atm (1.85L – 0.100L)
= L.atm
Conversion 1L.atm = 101.3J
-1.75 L.atm x J
1L.atm
-177J 17

18. THERMOCHEMISTRY ENTHALPY
EXERCISE 6.2A
A cylinder equipped with a piston expands against an external pressure of 1.58 atm. If the initial volume is 0.485L and the final volume is 1.245L, how much work is done (in J)?

19. EXAMPLE 6.3 Aqueous sodium hydrogen carbonate reacts with
THERMOCHEMISTRY THERMOCHEMICAL EQUATIONS
EXAMPLE 6.3
Aqueous sodium hydrogen carbonate reacts with
hydrochloric acid to produce aqueous sodium chloride,
water and carbon dioxide gas. The reaction absorbs 12.7kJ of heat at constant pressure for each mole of NaHCO3. Write the thermochemical equation.
NaHCO3(aq) + HCl NaCl(aq) + H2O(l) + CO2(g)
DH = kJ

20. THERMOCHEMISTRY THERMOCHEMICAL EQUATIONS
EXERCISE 6.3
A propellant for rockets is obtained by mixing the liquids hydrazine (N2H4) and dinitrogen tetroxide. These compounds react to give gaseous nitrogen and water vapor and 1049kJ of heat is evolved (at constant pressure when 1 mol reacts). Write the thermochemical equation for this reaction 20

21. Reversing a reaction changes the sign of DH for a reaction.
THERMOCHEMISTRY THERMOCHEMICAL EQUATIONS
Reversing a reaction changes the sign of DH for a reaction.
C3H8(g) + 5 O2(g)  3 CO2(g) + 4 H2O(l) DH = –2219 kJ
3 CO2(g) + 4 H2O(l)  C3H8(g) + 5 O2(g) DH = kJ
Multiplying a reaction increases DH by the same factor.
3 [C3H8(g) + 15 O2(g)  9 CO2(g) + 12 H2O(l)]: DH = 3(-2219) kJ
DH = kJ

22. EXAMPLE 6.4 2H2(g) + O2(g) 2H2O(l) ; DH = -572kJ 2
THERMOCHEMISTRY THERMOCHEMICAL EQUATIONS
EXAMPLE 6.4
When 2 mol H2(g) and 1mol O2 react to give liquid water, 572kJ of heat evolves:
2H2(g) + O2(g) H2O(l) ; DH = -572kJ
Write this equation for 1 mol of liquid water. Give the reverse equation, in which 1 mol of liquid water dissociates into hydrogen and oxygen. 2
1H2(g) + ½ O2(g) H2O(l) ; DH = -286kJ
Reversing the equation:
1H2O(l) H2(g) + ½ O2(g) ; DH = +286kJ

23. THERMOCHEMISTRY THERMOCHEMICAL EQUATIONS
EXERCISE 6.4
Write the thermochemical equation for the reaction described in Exercise 6.3 for the case involving 1 mol N2H4. Write the reverse reaction.

24. Applying Stoichiometry to Heats of Reaction
THERMOCHEMISTRY HEATS OF REACTION
Applying Stoichiometry to Heats of Reaction
Grams of A (reactant or product)
Conversion Factor: grams of A to mols of A (using molar mass)
Conversion Factor: mols of A to kJ (DH) x x
kiloJoules of Heat =

25. N2(g) + 3H2(g) 2NH3(g); DH = -91.8kJ
THERMOCHEMISTRY HEATS OF REACTION
EXAMPLE 6.5
How much heat is involved when 9.007x105g of ammonia is produced according to the following reaction (assuming the reaction is at constant pressure)?
N2(g) + 3H2(g) NH3(g); DH = kJ
9.07 x 105g x 1 mol NH3 x kJ = x 106 kJ
17.0g NH mol NH3

26. THERMOCHEMISTRY HEATS OF REACTION
EXERCISE 6.5
How much heat evolves when 10.0g of hydrazine reacts according to the reaction described in EXERCISE 6.3?

27. Specific Heat: The amount of heat required to raise
THERMOCHEMISTRY HEAT CAPACITY AND SPECIFIC HEAT
Heat capacity (C) is the amount of heat required to raise the temperature of an object or substance a given amount.
qcal = Ccal x DT
Specific Heat: The amount of heat required to raise
the temperature of 1.00 g of substance by 1.00°C at
constant pressure.
q = s x m x t
q = heat required (energy)
s = specific heat
m = mass in grams
t = Tf - Ti

28. SPECIFIC HEATS AND MOLAR CAPACITIES FOR SOME COMMON SUBSTANCES @ 25oC
THERMOCHEMISTRY HEATS OF REACTION
SPECIFIC HEATS AND MOLAR CAPACITIES FOR SOME COMMON 25oC

29. THERMOCHEMISTRY CALORIMETRY & HEAT CAPACITY
Molar Heat: The amount of heat required to raise the temperature of 1.00 mole of substance by 1.00°C.
q = MH x n x t
q = heat required (energy)
MH = molar heat
n = moles
t = Tf - Ti

30. THERMOCHEMISTRY HEAT CAPACITY AND SPECIFIC HEAT
EXAMPLE 6.6
Calculate the heat absorbed by 15.0g of water to raise the temperature from 20.0oC to 50.0oC (at constant pressure). The specific heat of water is 4.18 J/g.oC
Dt = 50.0oC – 20.0oC = 30.0oC
q = s x m x Dt
q = 4.18J x 15.0g x 30.0oC = x 103 J
g.oC

31. THERMOCHEMISTRY HEAT CAPACITY AND SPECIFIC HEAT
EXERCISE 6.6
Iron metal has a specific heat of J/(g.oC). How much heat is transferred to a 5.00 g piece of iron, initially at 20.0oC when placed in a pot of boiling water? (Assume that the temperature of the water is 100.0oC and the water remains at this temperature, which is the final temperature of the iron).

32. THERMOCHEMISTRY CALORIMETRY
Calorimetry is the science of measuring heat changes (q) for chemical reactions. There are two types of calorimeters:
Bomb Calorimetry: A bomb calorimeter measures the heat change at constant volume such that q = DE.
Constant Pressure Calorimetry: A constant pressure calorimeter measures the heat change at constant pressure such that q = DH.

33. THERMOCHEMISTRY CALORIMETRY
Constant Pressure
Bomb

34. C(graphite) + O2(g) CO2(g)
THERMOCHEMISTRY CALORIMETRY
EXAMPLE 6.7
Suppose 0.562g of graphite is placed in a calorimeter with an excess of oxygen at 25.00oC and 1atm pressure. Excess O2 ensures that all carbon burns to form CO2. The graphite is ignited, and it burns according to the following equation
C(graphite) + O2(g) CO2(g)
On reaction, the calorimeter temperature rises from 25.00oC to 25.89oC. The heat capacity of the calorimeter and it’s contents was determined in a separate experiment to be 20.7 kJ/oC. What is the heat of reaction at 25.00oC and 1 atm pressure? Express in a thermochemical equation.

35. C(graphite) + O2(g) CO2(g) ; DH = -3.9 x 102 kJ
THERMOCHEMISTRY CALORIMETRY
EXAMPLE 6.7 continued…
qrxn = -CcalDt = kJ/oC x (25.89oC – 25.00oC)
= kJ/oC x 0.89oC
= kJ
0.562g x 1 mol = mol
12 g C
-18.4kJ = x 102 kJ/mol
0.0468mol
C(graphite) + O2(g) CO2(g) ; DH = x 102 kJ

36. HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l)
THERMOCHEMISTRY CALORIMETRY
EXERCISE 6.7
Suppose 33mL of 0.120M HCl is added to 42mL of a solution containing excess sodium hydroxide in a coffee cup calorimeter. The solution temperature, originally at 25oC, rises to 31.8oC. Give the enthalpy change DH for the reaction:
HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l)
Assume that the heat capacity and the density of the final solution in the cup are those of water.
s = 4.184kJ/g.oC
d = 1.000g/mL
Express the answer as a thermochemical equation.

37. THERMOCHEMISTRY HESS’S LAW
Hess’s Law: The overall enthalpy change for a reaction is equal to the sum of the enthalpy changes for the individual steps in the reaction.(not a physical change, chemical change)
3 H2(g) + N2(g)  2 NH3(g) DH° = –92.2 kJ

38. THERMOCHEMISTRY HESS’S LAW
Reactants and products in individual steps can be added and subtracted to determine the overall equation.
(1) 2 H2(g) + N2(g)  N2H4(g) DH°1 = ?
(2) N2H4(g) + H2(g)  2 NH3(g) DH°2 = –187.6 kJ
(3) 3 H2(g) + N2(g)  NH3(g) DH°3 = –92.2 kJ
DH°1 + DH°2 = DH°reaction
Then DH° = DH°reaction - DH°2
DH°1 = DH°3 – DH°2 = (–92.2 kJ) – (–187.6 kJ) = kJ

39. THERMOCHEMISTRY HESS’S LAW
EXAMPLE 6.8
What is the enthalpy of reaction, DH, for the formation of tungsten carbide (WC) from the elements?
W(s) + C(graphite) WC(s)
2W(s) + 3O2(g) WO3(s) ; DH = kJ
C(graphite) + O2(g) CO2(g) ; DH = kJ
2WC(s) + 5O2(g) WO3(s) + 2CO2(g) ; DH = kJ
LAST EQUATION NEEDS TO BE REVERSED
2CO2(g) + 2WO3(s) WC(s) + 5O2(g) ; DH = kJ

40. THERMOCHEMISTRY HESS’S LAW
EXAMPLE 6.8 cont…
2W(s) + 3O2(g) WO3(s) ; DH = kJ
C(graphite) + O2(g) CO2(g) ; DH = kJ
2CO2(g) + 2WO3(s) WC(s) + 5O2(g) ; DH = kJ
W(s) + C(graphite) WC(s)

41. EXAMPLE 6.8 cont… W(s) + C(graphite) WC(s)
THERMOCHEMISTRY HESS’S LAW
EXAMPLE 6.8 cont…
W(s) + 3/2 O2(g) WO3(s) ; DH = kJ
C(graphite) + O2(g) CO2(g) ; DH = kJ
CO2(g) + WO3(s) WC(s) + 5/2 O2(g) ; DH = kJ
W(s) + C(graphite) WC(s)
W(s) + C(graphite) WC(s) DH = -40.5kJ

42. THERMOCHEMISTRY HESS’S LAW
EXERCISE 6.8
Manganese metal can be obtained by the reaction of manganese dioxide and aluminum
4Al(s) + 3MnO2(s)  2Al2O3(s) + 3Mn(s)
What is the DH for this reaction? Use the following data:
2Al(s) + 3/2O2(g)  Al2O3(s) ; H = −1676 kJ
Mn(s) + O2(g)  MnO2(s) ; H = −520 kJ

43. THERMOCHEMISTRY STANDARD ENTHALPIES OF FORMATION
Standard Heats of Formation (DH°f): The enthalpy change for the formation of 1 mole of substance in its standard state from its constituent elements in their standard states.
The standard heat of formation for any element in its standard state is defined as being ZERO.
DH°f = 0 for an element in its standard state

44. THERMOCHEMISTRY STANDARD ENTHALPIES OF FORMATION
Some Heats of Formation, Hf° (kJ/mol)
-1131
Na2CO3(s) 49
C6H6(l)
-92
HCl(g)
-127
AgCl(s)
-235
C2H5OH(g)
95.4
N2H4(g)
-167
Cl-(aq)
-201
CH3OH(g)
-46
NH3(g)
-207
NO3-(aq)
-85
C2H6(g)
-286
H2O(l)
-240
Na+(aq) 52
C2H4(g)
-394
CO2(g)
106
Ag+(aq)
227
C2H2(g)
-111
CO(g)

45. THERMOCHEMISTRY STANDARD ENTHALPIES OF FORMATION
Thermodynamic Standard State: Most stable form of a substance at 1 atm pressure and 25°C; 1 M concentration for all substances in solution.
These are indicated by a superscript ° to the symbol of the quantity reported.
Standard enthalpy change is indicated by the symbol DH°.

46. THERMOCHEMISTRY STANDARD ENTHALPIES OF FORMATION
Calculating DH° for a reaction:
DH° = DH°f (Products) – DH°f (Reactants)
For a balanced equation, each heat of formation must be multiplied by the stoichiometric coefficient.
aA + bB  cC + dD
DH° = [cDH°f (C) + dDH°f (D)] – [aDH°f (A) + bDH°f (B)]

47. THERMOCHEMISTRY STANDARD ENTHALPIES OF FORMATION
EXAMPLE 6.9
Use the values of DHof to calculate the heat of vaporization DHovap of carbon disulfide at 25oC. The vaporization process is:
CS2(l) CS2(g)
DHof = 89.7kJ/mol
DHof = 116.9kJ/mol
DHovap = Sn DHof(products) – Sm DHof (reactants)
DHof[CS2(g)] - DHof [CS2(l)]
116.9kJ kJ = 27.2kJ

48. THERMOCHEMISTRY STANDARD ENTHALPIES OF FORMATION
EXERCISE 6.9
Calculate the heat of vaporization, DHovap of water using standard enthalpies of formation

49. THERMOCHEMISTRY STANDARD ENTHALPIES OF FORMATION
EXAMPLE 6.10
Large quantities of ammonia are used to prepared nitric acid. The first consists of the catalytic oxidation of ammonia to nitric oxide
4NH3(g) + 5O2(g) NO(g) + 6H2O(g)
45.9kJ kJ kJ kJ
What is the standard enthalpy change for this reaction?
DHovap = Sn DHof(products) – SmDHof (reactants)
= [4(90.3) + 6(-241.8)]kJ - [4(-45.9) + 5(0)]kJ
= -906kJ

50. THERMOCHEMISTRY STANDARD ENTHALPIES OF FORMATION
EXERCISE 6.10
Calculate the enthalpy change for the following reaction:
3NO2(g) + H2O(l) HNO3(aq) + NO(g)

51. THERMOCHEMISTRY STANDARD ENTHALPIES OF FORMATION
EXERCISE 6.11
Calculate the standard enthalpy change for the reaction of an aqueous solution of barium hydroxide with an aqueous solution of ammonium nitrate at 25oC.

52. THERMOCHEMISTRY FUELS
FUELS is any substance that is burned or react to
provide heat and other forms of energy.
FOODS AS FUELS
FOSSIL FUELS
COAL GASIFICATION AND LIQUEFICATION
ROCKET FUELS

53. THERMOCHEMISTRY FUELS
Sources of energy consumed in the United States.

54. Example 2: Work
How much work is done (in kilojoules) and in which direction, as a result of the following reaction?

55. 2 C7H5N3O6(s)  12 CO(g) + 5 H2(g) + 3 N2(g) + 2 C(s)
Example 3: Work
The explosion of 2.00 mol of solid TNT with a volume of approximately L produces gases with a volume of 489 L at room temperature. How much PV (in kilojoules) work is done during the explosion? Assume P = 1 atm, T = 25°C.
2 C7H5N3O6(s)  12 CO(g) + 5 H2(g) + 3 N2(g) + 2 C(s)

56. Example 5:
Is an endothermic reaction a favorable process thermodynamically speaking?
Yes No

57. CH4(g) + 2 Cl2(g) CH2Cl2(g) + 2 HCl(g)
Example 6: Hess’s Law
The industrial degreasing solvent methylene chloride (CH2Cl2, dichloromethane) is prepared from methane by reaction with chlorine:
CH4(g) + 2 Cl2(g) CH2Cl2(g) + 2 HCl(g)
Use the following data to calculate DH° (in kilojoules) for the above reaction:
CH4(g) + Cl2(g)  CH3Cl(g) + HCl(g)
DH° = –98.3 kJ
CH3Cl(g) + Cl2(g)  CH2Cl2(g) + HCl(g)
DH° = –104 kJ

58. Example 7: Standard heat of formation
Calculate DH° (in kilojoules) for the reaction of ammonia with O2 to yield nitric oxide (NO) and H2O(g), a step in the Ostwald process for the commercial production of nitric acid.

59. Example 8: Standard heat of formation
Calculate DH° (in kilojoules) for the photosynthesis of glucose and O2 from CO2 and liquid water, a reaction carried out by all green plants.

60. Which of the following would indicate an endothermic reaction? Why?
Example 9:
Which of the following would indicate an endothermic reaction? Why?
-H
+ H

61. Example 10: Specific Heat
What is the specific heat of lead if it takes 96 J to raise the temperature of a 75 g block by 10.0°C?

62. Example 11: Specific Heat
How much energy (in J) does it take to increase the temperature of 12.8 g of Gold from 56C to 85C?

63. Example 12: Molar Heat
How much energy (in J) does it take to increase the temperature of 1.45 x104 moles of water from 69C to 94C?

64. Ba(OH)2·8 H2O(s) + 2 NH4Cl(s)  BaCl2(aq) + 2 NH3(aq) + 10 H2O(l)
THERMOCHEMISTRY HEAT OF REACTION
How much heat (in kilojoules) is evolved or absorbed in each of the following reactions?
a) Burning of 15.5 g of propane: C3H8(g) + 5 O2(g)  3 CO2(g) + 4 H2O(l)
DH = –2219 kJ/mole
b) Reaction of 4.88 g of barium hydroxide octahydrate with ammonium chloride:
Ba(OH)2·8 H2O(s) + 2 NH4Cl(s)  BaCl2(aq) + 2 NH3(aq) + 10 H2O(l)
DH = kJ/mole

65. Heat of Phase Transitions from Hf
Calculate the heat of vaporization, Hvap of water, using standard enthalpies of formation Hf H2O(g) kJ/mol H2O(l) kJ/mol