Thermochemistry Study Guide
1) Describe the difference between potential energy and kinetic energy.
PE: stored energy; KE: energy of motion/vibration/reaction
2) What is the difference between heat and temperature?
Temperature is a measure of the kinetic energy of molecules in a substance. Heat is a measurement of the total energy
in a substance (potential and kinetic).
3) Describe what we mean by conservation of energy. Give an example.
Conservation of energy means that energy cannot be created or destroyed, only converted from one form to
another. For example, in a chemical reaction, potential energy stored in chemical bonds may be converted to
heat, light, or sound energy as a result of a chemical reaction.
4) Draw a picture showing the direction of heat flow in an endothermic reaction versus an exothermic reaction.
Define the system and the surroundings in each case.
(See figure 17.2 on page 506)
5) Explain why boiling water is an endothermic process. (Hint: Think about what is happening to the
attractive forces between water molecules.)
Boiling water is endothermic because heat must be absorbed (go into) the system in order to break the
intermolecular forces that hold molecules together.
6) Hydrogen gas and oxygen gas release 482.6 kJ of heat when they combine to form steam. Is this reaction
endothermic or exothermic? In which direction does heat transfer (between system and the surroundings) for
this reaction? Is ∆H for this reaction positive or negative?
Exothermic (release is the key word). Heat transfers from the system to the surroundings. ∆H is negative.
7) 2 H2 (g) + O2 (g) 2 H2O (g)
∆H = -482.6 kJ
a. Interpret this thermochemical equation (i.e., how much heat is given off per amount of each substance?).
For every 2 moles of H2, 482.6 kJ of energy are given off (482.6 kJ/2 mol H2). We can also write this in terms
of oxygen or steam: 482.6 kJ/1 mol O2; 482.6 kJ/2 mol H2O
b. How much heat is released if we begin with 2.0087 g of O2 gas?
2.0087 g O2
mol O2
heat released: 30.29 kJ of heat given off
c. How much heat is released if we begin with 1.5021 g of H2 gas?
1.5021 g H2
mol H2
heat released: 179.8 kJ of heat given off
8) Which substance in each pair below has a higher specific heat? Circle your answer.
water
a)
aluminum foil
b)
wood
metal
c)
ethanol (C = 2460 J/kg·oC) gold (C = 129 J/kg·oC)
d)
mercury
copper
9) How much heat is lost when a 640 g piece of copper cools from 375oC to 26oC? (The specific heat of
copper is 0.385 J/g·oC)
Q=mc∆T
(640g)(0.385J/g·ºC)(26ºC-375ºC)
86 kJ of heat lost or -86kJ
10) The specific heat of iron is 0.4494 J/g·oC. How much heat is transferred when a 24.7 kg iron bar is cooled
from 880oC to 13oC?
Q=mc∆T
24.7kg · 1000g/1kg = 24700g
(24000g)(0.4494J/g·ºC)(13ºC - 880ºC)= -9623856.06J
9.6 x 103 kJ of heat are transferred from the iron to air or water (or whatever cooler substance it was placed in)
11) 8750 J of heat are applied to a 170 g sample of metal, causing a 56oC increase in its temperature. What is
the specific heat of the metal? Which metal is it?
Q=mc∆T 8750J=(170g)(c)(56ºC)
c = 8750J/(170g·56ºC)
c = 0.9191 = 0.92J/g·ºC Aluminum
12) Use the following enthalpies of reaction to determine the enthalpy for the reaction of ethylene with
fluorine.
C2H4 (g ) + 6F2 (g) 2CF4 (g) + 4HF (g) ∆H = ?
∆H = (sum of ∆H for products) – (sum of ∆H for reactants)
HF (g)
∆H = -268.5 kJ/mol
[(2 · -680kJ/mol + 4 · -680kJ/mol) – (1 · 52.3kJ/mol + 6 · 0kJ/mol)]
CF4 (g)
∆H = -680 kJ/mol
C2H4 (g)
∆H = 52.3 kJ/mol
-2486.3 kJ = -2486kJ
13) Calculate the ∆Ho of reaction for:
BaO (s) + SO3 (g) BaSO4 (s)
The values of ∆Hof are as follows: BaO (s) = -548 kJ/mol; SO3 (g) = -395.7 kJ/mol; BaSO4 (s) = -1473 kJ/mol.
∆H = (sum of ∆H for products) – (sum of ∆H for reactants)
[(1 · -1473kJ/mol) – (1 · -548kJ/mol + 1 · -395.7kJ/mol)]
[-1473kJ/mol + 943.7kJ/mol]
-529.3kJ = -593kJ
14) Calculate the ∆Ho of reaction for:
C3H8 (g) + 5O2 (g) 3CO2 (g) + 4H2O (l)
The values of ∆Hof are as follows: C3H8 (g) = -103.95 kJ/mol; CO2 (g) = -393.5 kJ/mol; H2O (l) = -285.8 kJ/mol
∆H = (sum of ∆H for products) – (sum of ∆H for reactants)
[(3 · -393.5kJ/mol + 4 · -285.8kJ/mol) – (1 · -103.95kJ/mol + 5 · 0kJ/mol)]
[-2323.7kJ/mol + 103.95 kJ/mol]
-2219.75kJ = -2219.8kJ
15) How much heat will be released when 6.44 g of Sulfur reacts with excess O2 according to the following
equation?
2S + 3O2 → 2SO3
6.44gS 1molS
1
32.07g
∆H° = -791.4kJ
-791.4kJ
2molS
= -79.4608 = -79.5kJ
16) How much heat will be released when 4.72g of Carbon reacts with excess O2 according to the following
equation?
C + O2
CO2
∆H°= -393.5kJ
4.72gC 1molC
-393.5kJ = -154.6478kJ = -155kJ
1
12.01gC 1molC
17) Calculate ∆H when 13.7g of Nitrogen reacts with excess O2 according to the following equation?
N2 + O2 2NO
∆H°= -180.0kJ
13.7gN2 1molN2
-180.0kJ = -88.0086kJ = -88.0kJ
1
28.02gN2 1molN2
I forgot to change the ∆H so this is the
answer you should have gotten
-192kJ
18) How many kilojoules of heat energy are required to heat all the aluminum (c= 0.902J/g•°C) in a roll of
aluminum foil (500.0g) from room temperature (25.0°C) to the temperature of a hot oven (250.0°C)?
Q=mc∆T (500.0g)(0.902J/g·ºC)(250.0ºC – 25.0ºC) = 101.475kJ = 101kJ
1000
19) The specific heat of silver is 0.24J/g•°C. How much heat in joules must be added to a silver block with a
mass of 86.0g to raise its temperature by 9.00°C?
Q=mc∆T (86.0g)(0.24J/g·ºC)(9.00ºC) = 185.76J = 185J
20) A 15.75-g piece of iron absorbs 1086.75 joules of heat energy, and its temperature changes from 25°C to
175°C. Calculate the specific heat capacity of iron.
Q=mc∆T
21) Explain why ice melts at 0°C without an increase of temperature, even though heat flows from the
surroundings to the system(ice).
22) Calculate the quantity of heat gained or lost when 3.50mol of water freezes at 0°C.
23) Calculate the quanity of heat gained or lost when 0.44mol of steam condenses at 100.0°C.
24) Calculate the heat of solution when 1.25mol NaOH(s) ( ∆Hsol =-425.9kJ/mol) dissolves in water.
25)Sodium acetate dissolves readily in water according to the following equation.
NaC2H3O2(s)
NaC2H3O2(aq)
∆Hsol =-17.3kJ/mol
Would this process increase or decrease the temperature of the water? Why or Why not?
26) How much energy is required or released to …
a. Melt 15 g of ice at 0°C and heat the water to 22°C?
b. Condense 15 g of steam at 100°C and cool the water to 22°C?
c. Change 100.0 g of ice at -10.0°C to steam at 118°C?
27)What is happening to the average kinetic energy of the molecules in the sample during section 2?
28)
As a substance goes through section (2), what happens to the distance between the particles?
29)
What is the name of the process happening during section (4)?
30)
What would be the name of the process happening during section (4) if time were going the other way?
31)
What is the melting point of this substance?
32)
At what temperature would this sample finish boiling?
33) Answer the following questions based on the potential energy diagram shown here:
a. Does the graph represent an endothermic or exothermic reaction?
b. Label the postion of the reactants, products, and activated complex.
c. Determine the heat of reaction, ∆H, (enthalpy change) for this reaction.
d. Determine the activation energy, Ea for this reaction.
e. How much energy is released or absorbed during the reaction?