1. Thermochemistry: Part 2
SCH4U

2. Heat and Physical Changes
ΔHvap= - ΔHcond ΔHmelt = - Δhfre

3. Common Molar Enthalpies

4. Methods of Measuring ΔH
Calorimetry (experimental)
Hess’s Law: using Standard Enthalpy of Reaction (ΔHrxn) of a series of reaction steps (indirect method).
Standard Enthalpy of Formation (ΔHf ) used with Hess’s Law (direct method)
Bond Energies used with Hess’s Law

5. Specific Heat Capacity
the amount of heat (Q) required to raise the temperature of one gram (1g) of the substance by one degree Celsius. ( 1C)

6. Heat (Q) absorbed or released
Q=mcΔT
Q= amount of heat transferred (Joules, J)
m= mass of a substance (g)
c= specific heat capacity (J/g• 0C)
ΔT = Tfinal – Tinitial (change in temperature) ( 0C)

7. Heat Examples
When a 1.25kg sample of water was heated in a kettle, its temperature increased from C to 98.90C. How much heat did the water absorb?

8. Heat Examples
When a 1.25kg sample of water was heated in a kettle, its temperature increased from C to 98.90C. How much heat did the water absorb?
Q= 1.25x103g x 4.18 J/g0C x (+82.50C )
= J
= 4.32x105 J

9. Heat Examples
2) How much heat must be added to g of steam at C to increase its temperature to C?
A solid substance has a mass of 250.0g. It is cooled by C and loses 4.937kJ of heat. What is the specific heat capacity of the substance?

10. Heat Examples
2) How much heat must be added to g of steam at C to increase its temperature to C?
A solid substance has a mass of 250.0g. It is cooled by C and loses 4.937kJ of heat. What is the specific heat capacity of the substance?
Q = gx 2.02J/g0Cx ( ) 0C
= 1.65x104 J
c= Q/m(∆T)
c= x103J/(250.0 g ( C))
c= J/g0C

11. Additional Examples
How much heat is released when the temperature of 789g of liquid ammonia decreases from 82.70C to 25.00C?
2) On a warm day, how much solar energy does a kg piece of concrete absorb as heat if its temperature increases from C to C

12. Additional Examples
1) How much heat is released when the temperature of 789g of liquid ammonia decreases from 82.70C to 25.00C? Q= mc∆T = 789 g x 4.70 J/g0C x ( ) = J change to x105 J 2) On a warm day, how much solar energy does a 3.982kg piece of concrete absorb as heat if its temperature increases from C to C? Q= 3982 g x 0.88 J/g0C x ( ) = J change to 3.2 x103J

13. Measuring Q experimentally
Calorimeter  a device that is used to measure enthalpy changes for chemical and physical reactions.
Qreactions = - Q insulated system

14. Using Flame Calorimetry To Determine the Enthalpy of Combustion
UNIT 3
Chapter 5: Energy Changes
Section 5.2
Using Flame Calorimetry To Determine the Enthalpy of Combustion
Flame calorimeters are flame-resistant and often made of metals cans
A flame calorimeter
is used for determining ΔHcomb
absorbs a great deal of energy, which must be included in energy calculations
is used for burning impure materials like food; ΔHcomb is reported in kJ/g
Image source: MHR, Chemistry 12 © ISBN ; page 306

15. Using Bomb Calorimetry To Measure Enthalpy Changes during Combustion
UNIT 3
Chapter 5: Energy Changes
Section 5.2
Using Bomb Calorimetry To Measure Enthalpy Changes during Combustion
A bomb calorimeter
is used for more accurately determining ΔHcomb
determines ΔHcomb at constant volume
has a particular heat capacity, C
Image source: MHR, Chemistry 12 © ISBN ; page 307
Q = CΔT is used for bomb calorimetry calculations
Bomb calorimeters are much more sophisticated than flame calorimeters or simple calorimeters.

16. Using a Simple Calorimeter
UNIT 3
Chapter 5: Energy Changes
Section 5.2
Using a Simple Calorimeter
Constant-Pressure Calorimeter
No boundary between system and surroundings
Reactants and products are system, and water in which they dissolve in, is part of the surroundings
the change in temperature of the water is measured; the solution absorbs or releases energy
the solution is dilute enough so that the specific heat capacity of water is used
Thermometer
Styrofoam
cover
cups
Stirrer
Image source: MHR, Chemistry 12 © ISBN ; page 301

17. Practice #1

18. Practice #2
240 g of water (initially at 20oC) are mixed with an unknown mass of iron (initially at 500oC). When thermal equilibrium is reached, the system has a temperature of 42oC. Find the mass of the iron.

19. Find the mass of the iron.
240 g of water (initially at 20oC) are mixed with an unknown mass of iron (initially at 500oC).
When thermal equilibrium is reached, the system has a temperature of 42oC.
Find the mass of the iron. Fe
T = 500oC
mass = ? grams
T = 20oC
mass = 240 g -
LOSE heat = GAIN heat
- [(Cp,Fe) (mass) (DT)] = (Cp,H2O) (mass) (DT)
- [( J/goC) (X g) (42oC - 500oC)] = (4.184 J/goC) (240 g) (42oC - 20oC)]
Drop Units:
- [(0.4495) (X) (-458)] = (4.184) (240 g) (22)
205.9 X =
X = g Fe
Calorimetry Problems 2
question #5

20. Practice #3
A 97 g sample of gold at 785oC is dropped into 323 g of water, which has an initial temperature of 15oC. If gold has a specific heat of J/goC, what is the final temperature of the mixture? Assume that the gold experiences no change in state of matter.

21. A 97 g sample of gold at 785oC is dropped into 323 g of water, which has an initial
temperature of 15oC. If gold has a specific heat of J/goC, what is the final
temperature of the mixture? Assume that the gold experiences no change in state
of matter. Au
T = 785oC
mass = 97 g
T = 15oC
mass = 323 g
LOSE heat = GAIN heat -
- [(Cp,Au) (mass) (DT)] = (Cp,H2O) (mass) (DT)
Drop Units:
- [(0.129 J/goC) (97 g) (Tf - 785oC)] = (4.184 J/goC) (323 g) (Tf - 15oC)]
- [(12.5) (Tf - 785oC)] = (1.35 x 103) (Tf - 15oC)]
-12.5 Tf x = x 103 Tf x 104
3 x 104 = x 103 Tf
Tf = oC
Calorimetry Problems 2
question #8

22. Practice #4
If 59 g of water at 13oC are mixed with 87 g of water at 72oC, find the final temperature of the system.

23. If 59 g of water at 13oC are mixed with 87 g of water at 72oC, find the final temperature
of the system.
T = 13oC
mass = 59 g
T = 72oC
mass = 87 g
LOSE heat = GAIN heat -
- [(Cp,H2O) (mass) (DT)] = (Cp,H2O) (mass) (DT)
Drop Units:
- [(4.184 J/goC) (87 g) (Tf - 72oC)] = (4.184 J/goC) (59 g) (Tf - 13oC)
- [(364.0) (Tf - 72oC)] = (246.8) (Tf - 13oC)
-364 Tf = Tf
= Tf
Tf = oC
Calorimetry Problems 2
question #9

24. Practice #5
A 322 g sample of lead (specific heat = J/goC) is placed into 264 g of water at 25oC. If the system's final temperature is 46oC, what was the initial temperature of the lead?

25. A 322 g sample of lead (specific heat = 0
A 322 g sample of lead (specific heat = J/goC) is placed into 264 g of water at 25oC.
If the system's final temperature is 46oC, what was the initial temperature of the lead?
T = ? oC Pb
mass = 322 g
Ti = 25oC
mass = 264 g Pb
Tf = 46oC -
LOSE heat = GAIN heat
- [(Cp,Pb) (mass) (DT)] = (Cp,H2O) (mass) (DT)
Drop Units:
- [(0.138 J/goC) (322 g) (46oC - Ti)] = (4.184 J/goC) (264 g) (46oC- 25oC)]
- [(44.44) (46oC - Ti)] = (1104.6) (21oC)]
Ti =
44.44 Ti =
Ti = 568oC
Calorimetry Problem

26. Molar Enthalpy Change ΔHn:
Molar Enthalpy Change (ΔHn): enthalpy change associated with any change (physical, chemical or nuclear) in one mole of a substance; J/mol or kJ/mol
ΔHrxn = nΔHn

27. Example:
Ethanol is used to disinfect skin before receiving a flu shot. How does this feel on your skin? The enthalpy of vaporization (ΔHvap)of ethanol is 38.6 kJ/mol. What is the enthalpy change (ΔHrxn) of this reaction if 1.0g of ethanol is rubbed on your skin?