1. Chapter 10- Causes of Change
Heat and Chemical change
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2. Chapter 6.1 The flow of energy -heat:
Energy and heat:
Thermochemistry- concerned w/ the _____ _____ that occur during _______ ___.
Energy- capacity for _____ ___ or ________ _____
Heat ( )- energy that transfers from one object to another due to temperature difference
only changes in heat can be detected!
flows from warmer  cooler object until objects have the same temperature
this energy that transfers is thermal energy
Joule (J) the SI unit of heat and energy
heat
change
chemical rxn
doing work
supplying
heat q
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3. Exothermic and Endothermic Processes
Essentially all chemical reactions, and changes in physical state, involve either:
release of heat (exothermic), or
absorption of heat (endothermic)
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4. Ex. water freezing, raining, & Ex. ice melting, sweating, &
Exothermic - heat flowing out of a system into its surroundings:
defined as negative
q has a negative value
system loses heat as the surroundings heat up
Ex. water freezing, raining, &
CH4 + 2O2 ® CO2 + 2H2O + Heat
Endothermic - heat flowing into a system from its surroundings:
defined as positive
q has a positive value
system gains heat as the surroundings cool down
Ex. ice melting, sweating, &
CaCO3 + heat ® CaO + CO2
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5. Heat Capacity Ans:Fe Factor 1: Chemical Structure Ans: Cup A
Heat Capacity: the amount of heat needed to increase the temperature of an object exactly 1 oC
Heat Capacity of a substance depends on two factors:
1) A 20 g iron bar and 20 g of water (both at room temp) are set outside on a sunny day for 20 minutes. After the 20 minutes elapse which will have a higher temperature?
2) Two cups of room temp water are set outside on a sunny day for 1 hour.
Cup A: 20 g of water
Cup B: 200 g of water
Which will be warmer after the one hour?
Ans:Fe
Factor 1: Chemical Structure
Ans: Cup A
Factor 2: Mass
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6. Heat Capacity and Specific Heat
A calorie is defined as the quantity of heat needed to raise the temperature of 1 g of pure water 1 oC
a Calorie, written with a capital C, always refers to the energy in food
1 Calorie = 1 kilocalorie = 1000 cal.
1 cal = J
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7. also called “Specific Heat” cal/g°C J/g°C (or J/gK) water 1.00 4.18
specific heat capacity (Cp )- the amount of heat it takes to raise the temperature of 1 gram of the substance by 1 oC at constant pressure
also called “Specific Heat”
cal/g°C J/g°C (or J/gK)
water
aluminum
copper
silver
gold
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8. Heat Capacity and Specific Heat
For water, C = 4.18 J/(g oC), and
also C = 1.00 cal/(g oC)
Thus, for water:
it takes a long time to heat up, and
it takes a long time to cool off!
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9. Chapter 10
Visual Concepts
Specific Heat
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10. To solve for heat capacity (q)…
q = mCp T
To calculate, use the formula:
“q” is heat, unit- J or cal
“m” is mass, unit- g
“Cp”or “C”= Specific Heat Capacity
units: J/(g oC) or cal/(g oC)
J/(gK) or cal/(gK)
“T” = change in temperature, (T = Tf -Ti ) unit- oC or K
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11. Same eq’n, different look!
q = m · Cp · T
Cp =
T = (T = Tf -Ti )
m = q
m ·T q
m · Cp q
Cp·T
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12. Example Problems 0.14 cal/(g oC)
It takes 24.3 calories to heat 15.4 g of a metal from 22 ºC to 33ºC. What is the specific heat of the metal?
given: q = 24.3 cal
T = Tf -Ti =33 – 22 = 11ºC
m = 15.4 g
Cp = ??
q = m Cp T
Cp = = = q
m T
(24.3 cal)
(15.4 g)(11 ºC)
0.14 cal/(g oC)
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13. Example Problems q m Cp T = T = 28 ºC
2. Iron has a specific heat of 0.39 J/gºC. How much will the temperature change when 520 J of energy is absorbed by g of Fe?
given: q= 520 J T =?? m = 48.3 g Cp = 0.39 J/gºC
q = mCp T
T = q
m Cp
(520 J)
(48.3 g)(0.39 J/gºC)
T =
T =
28 ºC
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14. Practice Problem
ex. When 435 J of heat is added to 3.4 g of olive oil at 21 C, the temperature increases to 85 C. What is the specific heat of olive oil?
2.0 J/g·°C
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15. Enthalpy Enthalpy (H) Is the total energy content of a sample
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16. Chapter 10
Visual Concepts
Enthalpy Change
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17. 10.3 Measuring and Expressing Heat changes:
-         Calorimetry- accurate and precise measurement of heat change for chemical and physical processes
enthalpy ( )-
-         enthalpy change ( ):
∆H = Hproducts – Hreactants
H is negative for exothermic rxns
H is positive for endothermic rxns
H heat content at constant P
∆ H change in enthalpy
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18. Calorimeter
Foam cups are excellent heat insulators, and are commonly used as simple calorimeters
The heat absorbed by the water is equal to the heat released by the substance
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19. Calorimeter Chapter 10 Sample is ignited in high pressured O2
Visual Concepts
Calorimeter
Sample is ignited in high pressured O2
Energy from combustion is absorbed by water bath
Csp of water is known
With temperature increase, q can be measured
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20. Change in enthalpy of a reaction (Using Q to determine H)
B. Units of ____:
C. Exothermic reactions: ___ is ________.
D. Endothermic reactions: ___ is _______.
E. Heat transfer between reaction and water:
Q released by reaction = Q absorbed by water in calorimeter
q = mCpT
J/mol; cal/mol; H H
negative
Temp. of H20 ↑ H
positive
Temp. of H20 ↓
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21. q = mcT q = (500.0 g)(4.184J/goC)(20.3oC) q = 42467.6 J = 42500 J
1. When 5.00 g of liquid toluene, C7H8 is burned, it causes mL (500g) of water at 20.0C to rise to 40.3C. What is the heat of combustion (H c) of liquid toluene in kJ/mol?
mwater = 500ml = 500g
T = 40.3 – 20.0 = 20.3 oC
Cp water =4.184 J/(goC)
q = ?
q = mcT
q = (500.0 g)(4.184J/goC)(20.3oC)
q = J
= J
= 42.5 kJ
5.00 g C7H8
1 mol C7H8
= mol C7H8
92.15 g C7H8
= kJ/mol of C7H8
exothermic
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22. + Q = mcT Q = (1000.0 g)(4.184J/goC)(7.50 oC) Q = 31380 J = 31.4 kJ
2. What is the H of a reaction that lowers the temperature of 1.00 L (1000g) of water by 7.50C when 5.00 g of the substance (molar mass = g/mol) is reacted. Give your answer in kJ/mol
mwater = 1.00L = 1000g
T = 7.50 oC
Cp water =4.184 J/(goC)
q = ?
Q = mcT
Q = ( g)(4.184J/goC)(7.50 oC)
Q = J
= 31.4 kJ
5.00 g
1 mol
= mol
120.0 g
= kJ/mol +
endothermic
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23. Chemistry Happens in MOLES
An equation that includes energy is called a thermochemical equation
CH4 + 2O2 ® CO2 + 2H2O kJ
1 mole of CH4 releases kJ of energy.
When you make kJ you also make 2 moles of water
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24. Thermochemical Equations
A heat of reaction (H°rxn) is the heat change for the equation, exactly as written
The physical state of reactants and products must also be given.
Standard conditions for the reaction is 1 atm and 25 oC
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25. 1 mol CH4 802.2 kJ 10. 3 g CH4 16.05 g CH4 1 mol CH4 = 515 kJ
CH4 + 2 O2 ® CO2 + 2 H2O kJ
If grams of CH4 are burned completely, how much heat will be produced? (just like stoichiometry)
1 mol CH4
802.2 kJ
10. 3 g CH4
16.05 g CH4
1 mol CH4
= 515 kJ
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26. How many grams of water would be produced with 506 kJ of heat?
CH4 + 2 O2 ® CO2 + 2 H2O kJ
How many liters of O2 at STP would be required to produce 23 kJ of heat?(22.4L/mol)
23 kJ 2 mol oxygen 22.4 L oxygen kJ 1 mol oxygen = 1.3 L oxygen
How many grams of water would be produced with 506 kJ of heat?
506 kJ 2 mol water g water kJ 1 mol water = 22.7 g water
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27. ***Note states of matter***
IV. Change in enthalpy of a reaction (Using Heats of Formation) – 2nd Technique
released
A. Standard heat of formation (Hf)= The energy ________ or __________ when one mole of a substance is formed from its _________ at standard conditions (______and ______). ***Note difference from STP***
Example:C(s) + O2(g)  CO2(g) Hf = kJ/mol of CO2
B. Standard heats of formation are given in Appendix ___ in your book and attached tables.
C. Hf = ___ for elements at standard state. It takes ____ energy to go from an element to an element.
Example:
  Br2(l) = _________ Br2(g) = __________
***Note states of matter***
@ 25C, bromine is a ________.
absorbed
elements
1 atm
25 oC
A-11 no
Heat +
Br2(l)
Br2(g)
0 kJ/mol
30.71 kJ/mol
liquid
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28. Horxn = Hof products - Hof reactants
sum of
D. Calculation of Hrxn  (sigma) = _________
Units of H:
Hf (Heat of formation) = ________________
Hc (Heat of combustion) = ________________
Hrxn (Heat of reaction) = ________________
Horxn = Hof products - Hof reactants
kJ/mol
(A compound is formed)
kJ/mol
(Lit on fire; reacts w/ O2)
kJ per rxn
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29. F. Sample problems: CH4(g) + O2(g) 2 CO2(g) + H2O(l) 2
1. Calculate the heat of combustion of methane gas.
(a) Balanced equation (include states):
(b) Sum products and reactants and subtract.
(c) Exothermic or endothermic reaction? ____________
CH4(g)
+ O2(g) 2
CO2(g)
+ H2O(l) 2
Horxn = Hof products - Hof reactants
Horxn = [(CO2) + 2(H2O)] – [(CH4) + 2 (O2)]
Horxn = [1 mol( kJ/mol) + 2 mol( kJ/mol)] – [1 mol(-74.8 kJ/mol) + 2 mol(0 kJ/mol)]
Horxn = kJ
Exothermic
= –890.5 kJ/mol CH4
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30. Horxn = Hof products - Hof reactants
2. Find Hrxn:
Cl2(g) HBr(g)  2 HCl(g) + Br2(g)
Horxn = Hof products - Hof reactants
Horxn = [2(HCl) + (Br2)] – [(Cl2) + 2(HBr)]
Horxn = [2 mol(-92.3 kJ/mol) + 1 mol(30.9 kJ/mol)] – [1 mol( 0 kJ/mol) + 2 mol(-36.2 kJ/mol)]
Horxn = kJ
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31. Horxn = Hof products - Hof reactants
3. Find Hrxn and Hc of acetylene gas (C2H2).
(a) Balanced equation:
(b) Calculation: 2
C2H2(g)
+ O2(g) 5 4
CO2(g)
H2O(l) 2
Horxn = Hof products - Hof reactants
Horxn = [4(CO2) + 2(H2O)] – [2(C2H2) + 5(O2)]
Horxn = [4 mol( kJ/mol) + 2 mol( kJ/mol)] – [2 mol(226.6 kJ/mol) + 5 mol(0)]
a) Horxn = kJ
= kJ/mol of C2H2
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32. Horxn = Hof products - Hof reactants
4. Find Hrxn and Hc of propane gas (C3H8).
(a) Balanced equation:
(b) Calculation:
C3H8(g)
+ O2(g) 5 3
CO2(g)
+ H2O(l) 4
Horxn = Hof products - Hof reactants
Horxn = [3(CO2) + 4(H2O)] – [(C3H8) + 5(O2)]
Horxn = [3 (-393.5) + 4 ( )] – [( ) + 5 (0)]
a) Horxn = kJ
= kJ/mol of C3H8
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