Download specific heat of water = 4.18 J/g•°C heat of vaporization of water

WLHS / Chem / Monson
Name
UNIT 7 STUDY GUIDE: THERMOCHEMISTRY (CH 17)
Key Terms:
• exothermic vs. endothermic reactions
• measuring heat flow
• calorimetry
(coffee cup calorimeters; bomb calorimeters)
• specific heat capacity; heat capacity
• enthalpy
• heat in balanced chemical equations
• calculating H (Hess’ Law)
• enthalpies of formation
• heat transfer in solids, liquids, and gases
• heat transfer in phase changes
(melting, freezing, vaporization, condensation)
Equations:
q = mct
q = (Ccal) (t)
H = Σ(Hproducts) – Σ(HReactants)
q = (mass) (heat of fusion)
q = (mass) (heat of vaporization)
Constants:
specific heat of ice = 2.09 J/g•°C
specific heat of water = 4.18 J/g•°C
specific heat of steam = 2.03 J/g•°C
heat of fusion of water = 334 J/g
heat of vaporization of water = 2260 J/g
Problems: PLUS review all previous problem sets for additional practice / review!!
1) Draw (& label!) a rough curve (x-axis time, y-axis temperature)
for the cooling of water vapor at 125.0oC to ice at -5.0oC.
2) On a sunny day, why does the concrete deck around an outdoor swimming pool become hot, while the water
stays cold?
3) A piece of metal is heated to 80oC. It is then brought into contact with a piece of metal at 200oC. What will
happen to each piece of metal in the first seconds of contact? Predict the temperature of each metal block after
one hour if left out on a lab table.
4) There are two identical containers filled with equal amounts of water and alcohol, both starting at 20oC. Both
containers are heated with two identical hotplates. After two minutes the temperature of the alcohol is 30 oC. After
4 minutes the temperature of the water is 30oC. Why does this happen? Does one liquid receive more heat?
5) A silver bracelet (cAg = 0.24 J/(g oC)) receives 525.0 J of heat from the sun, causing a temperature change of
25oC. What is the mass of the bracelet?
6) An underwater explosion caused the temperature of a pond to change from 76.0oC to 78.5oC. If the pond has a
volume of 10,500 L, how many kilojoules of heat was released by the explosion? What law allows one to assume
all heat lost from the explosion was absorbed by the water? Hint: D water = 1.00 g/mL, cwater = 4.18 J/(g oC)
7) A copper pot with a mass of 772 grams absorbs 22.7 kJ of heat. It’s final temperature is 137.0°C. What was its
initial temperature? (the specific heat of copper is 0.385 J/g•°C)
8) Classify each of these reactions as exothermic or endothermic:
ENDO / EXO?
A) CaO(s) + H2O(l)  Ca(OH)2(s) + 65.2 kJ
B) 2 NaHCO3(s)  Na2CO3(s) + H2O(g) + CO2(g)
∆H = +129kJ
C) steam condensing
D) C(s) + H2O(g)
 CO(g) + H2(g)
∆H = +131.3 kJ
E) H2O(l) + 40.7 kJ  H2O(g)
F) CH4(g) + 2O2(g)  CO2(g) + 2H2O(g)
ΔH = -890 kJ
G) H2(g) + F2(g)  2HF(g) + 536 kJ
9) Methane gas (CH4) reacts with oxygen according to the following equation: CH4 + 2 O2  CO2 + 2 H2O + 890 kJ
How much heat is released from the combustion of 11.55 g of CH4?
10) Use the following balanced thermochemical equation to determine the mass of ethanol (C2H5OH) needed to
produce 5250 kJ of heat.
C2H5OH + 3 O2  2 CO2 + 3 H2O
∆H = -1235 kJ
11) Nitrogen monoxide (NO) has been found to react with oxygen gas (O 2) to produce the brown gas nitrogen
dioxide (NO2). When one mole of NO reacts with oxygen, 57.0 kJ of heat is released.
A) Write the balanced thermochemical equation for this reaction.
B) Is the reaction exothermic or endothermic?
C) What is H when 5.00 g of nitrogen monoxide reacts?
D) How many grams of nitrogen monoxide must react with an excess of oxygen to produce 127.2 kJ of heat?
12) Strontium metal (Sr) combines with graphite (C) and oxygen gas (O 2) to produce strontium carbonate (SrCO3).
The formation of one mole of SrCO3 releases 1220.0 kJ of heat.
A) Write a balanced thermochemical equation for this reaction.
B) Is the reaction exothermic or endothermic?
C) What is H when 45.85 g of strontium reacts with excess graphite and oxygen?
D) What mass of SrCO3 forms when 2355 kJ of heat are released?
Use the following table to answer the questions below:
m.p.
b.p.
c solid
oC
oC
J/goC
water
0.0
100.0
2.06
ethanol
-117
78
cesium
28
678
0.246
c liquid
J/goC
4.18
2.46
0.252
c gas
J/goC
2.02
0.954
0.156
∆Hfus
J/g
333
109
15.7
∆Hvap
J/g
2260
855
514
13) A 10.0 gram sample of cesium and a 10.0 gram sample of ice, both at 0.0oC, are melted. Which process takes
more energy? Prove it mathetmatically!
14) Determine the heat needed to take 16.0 g of liquid ethanol from 22.0oC to 88oC (CAREFUL! notice the boiling
point of ethanol!).
15) A sample of cesium was heated from 45.0oC to 425.0oC upon addition of 5701 J of energy. What was the mass
of the cesium sample?
16) If 42.5 kJ of heat is added to 5.00 g of water at a temperature of 5oC. What is the final temperature?
17) How much heat is absorbed by a 15.5 g piece of gold as it is heated from 4.56°C to 177.4°C? (the specific heat
of gold is 0.129 J/g•°C)
18) Naphthalene, C10H8, is the compound present in moth balls. The heat of combustion of naphthalene is 5150.
kJ/mol. A sample of naphthalene burns in a calorimeter filled with 2350 g of water and increases the temperature of
the water in the calorimeter from 25.1°C to 28.4°C.
A) What quantity of heat (in kJ) is absorbed by the water in the calorimeter?
B) What quantity of heat (in kJ) is given off by the burning naphthalene?
C) Based on the information given, how many grams of naphthalene burned? (HINT: use the heat of combustion of
naphthalene!)
19) How much energy is needed to heat a 164.5 g sample of ice at –27.6°C to liquid water at 72.3°C?
20) How much energy is released when 52.3 g of steam at 136.5°C cools and condenses to form water at 93.2°C?
21) Calculate the enthalpy change (ΔH) in kJ for the following reaction: 2Al(s) + Fe2O3 (s)  2Fe(s) + Al2O3 (s)
Use Hess’s Law and the enthalpy changes for the combustion of aluminum and iron:
2Al(s) + 1½ O2(g)  Al2O3 (s)
ΔH = -1668.8 kJ
2Fe(s) + 1½ O2 (g)  Fe2O3 (s) ΔH = -824.2 kJ
22) Find the enthalpy change for the formation of phosphorus pentachloride from its elements.
2 P(s) + 5 Cl2(g)  2 PCl5(s)
Use the following thermochemical equations:
PCl5(s)  PCl3(g) + Cl2(g)
2 P(s) + 3 Cl2(g)  2 PCl3(g)
ΔH = 87.9 kJ
ΔH = -574 kJ
23) Given: 2CuO(s)  2Cu(s) + O2(g)
H = 314.6 kJ
Determine the heat of formation (ΔHf°) of CuO.
24) Limestone, CaCO3, when subjected to a temperature of 900°C in a kiln, decomposes to calcium oxide and
carbon dioxide.
A) Write a balanced chemical equation for this reaction.
B) Given the heats of formation below, determine H for the reaction. (add this heat term to the above chemical
equation)
ΔHf° for CaCO3(s) = -1207.0 kJ/mol
ΔHf° for CaO(s) = -635.1 kJ/mol
ΔHf° for CO2(g) = -393.5 kJ/mol
C) How much heat is absorbed or released when 1.00 gram of limestone decomposes? (specify gain or loss of
heat)