Heat & Enthalpy Changes in Chemical Reactions
Name________________________________
Period________Date____________________
Matching: Write the letter of the definition that best matches the term.
________ 1. heat
A. reactions that release heat
________ 2. thermochemistry
B. the SI unit of energy and heat
________ 3. exothermic reactions
C. the energy that is transferred from one object to another
________ 4. endothermic reactions D. the study of the changes in heat in chemical reactions
________ 5. joule
E. example of an exothermic reaction
________ 6. combustion
F. reactions that absorb heat
Energy Diagram (thermochemical vs. H)
Problems: Solve each problem and then indicate whether it is an endothermic or exothermic reaction.
7. Calculate the amount of heat released by the combustion of 1.75 mol of benzene (C6H6).
2C6H6
+
15O2
→
12 CO2
+
6H2O
∆Hº = -98.0 kJ
8. How much heat is transferred when 100.0 g of calcium oxide reacts with carbon?
CaO +
3C
→
CaC2 +
CO
∆Hº = +464.8 kJ
9. Ammonium dichromate decomposes in a vigorous reaction when it is heated. Calculate the heat
transferred for the decomposition of 53.0 g of ammonium dichromate.
(NH4)2Cr2O7
→ N2
+ 4H2O
+
Cr2O3
∆Hº = -315 kJ
Stoichiometry of Energy Changes
10) 2S + 3O2  2SO3
∆Ho = -791.4 kJ
a. How much heat will be released when 6.44 g of sulfur reacts?
b. Is this reaction endothermic or exothermic?
11) C + O2  CO2
∆Ho = -393.5 kJ
a. How many grams of carbon would be required to liberate 284 kJ of thermal energy?
b. Draw and energy diagram for this reaction.
12) H2 + Br2 + 72.80 kJ  2HBr
a. How much heat will be absorbed when 38.2 g of bromine reacts?
b. Is this reaction endothermic or exothermic?
13) 2P + 5Cl2  2PCl5
∆Ho = -886 kJ
a. Determine the amount of heat produced if 1.98 L of chlorine gas are reacted at STP.
b. Write the thermochemical equation for this reaction.
14) C2H5OH + 3O2  2CO2 + 3H2O
∆Ho = -1366.7 kJ
a. If this reaction gives off 951 kJ of heat, how many molecules of water will be produced?
b. Will this reaction cause the surroundings to increase or decrease in temperature?
15) N2 + O2 + 180 kJ  2NO
a. How much heat is required to react 13.7 g of nitrogen.
b. Draw an energy diagram for this reaction
16) 3Fe + 2O2  Fe3O4 + 1120.48 kJ
a. How many grams of iron would be required for this reaction to liberate 7985 kJ of thermal
energy?
b. Write this equation in ∆H notation.
17) 4NH3 + 5O2  4NO + 6H2O ∆Ho =-1170.0 kJ
a. How much heat is transferred when 14.9 g of ammonia react?
b. Rewrite this equation for 1 mole of ammonia using ∆H notation.
Energy and Change of Enthalpy
18. Indicate the following as either exothermic or endothermic.
a) CH4 + 74.4 kJ → C + 2H2
a) _________
b) 1/8 S8 + O2 →
SO2
b) _________
C3H8 + 104 kJ
c) _________
→
d) _________
c) 3 C + 4 H2
→
d) butane + oxygen
e) CO2
→
CO + ½ O2
carbon dioxide + water
ΔH = +238kJ
e) _________
19. The energy term of an exothermic reaction is added to the (right / left) side of chemical equation
because energy is (absorbed / released).
20. The ∆H of an endothermic reaction is indicated as a (+ / -) value because the enthalpy (potential
energy) of the reactants is ( > / < ) than the enthalpy of the products.
21. All chemical fuels produce an (endothermic / exothermic) reaction upon ordinary combustion.
22. Baking a potato is an example of an (exothermic / endothermic ) reaction.
23. Draw an energy diagram for 4 C + S8 + 3511 kJ
→ 4CS2. Be sure to label all parts.
(progress of the reaction, enthalpy or energy, ∆H, reactants, and products)
24.
CaO
+
3C
→
CaC2
+
CO
H = +464.8 k
Is this reaction endo or exothermic? _______ Draw a simple energy diagram for this reaction below:
25. 2CO (g)
+
O2(g)
→
2CO2(g)
+
566.0 kJ
Is this reaction endo or exothermic? _______ Draw a simple energy diagram for this reaction below:
Hess’s Law
Determining ∆H Using Hess’s Law: Each manipulated equation must be rewritten.
2P + 3Cl2 → 2PCl3
2P + 5Cl2 → 2PCl5
Calculate ΔH for:
PCl3 + Cl2 → PCl5
ΔH = −640. kJ
ΔH = −886 kJ
2HF → H2 + F2
2H2 + O2 → 2H2O
Calculate ΔH for:
2F2 + 2H2O → 4HF + O2
ΔH = +542.2 kJ
ΔH = −571.6 kJ
26) Given:
27) Given:
4NH3 + 5O2 → 4NO + 6H2O
4NH3 + 3O2 → 2N2 + 6H2O
Calculate ΔH for:
N2 + O2 → 2NO
ΔH = −1170 kJ
ΔH = −1530 kJ
4Al + 3O2 → 2Al2O3
4Fe + 3O2 → 2Fe2O3
Calculate ΔH for:
2Al + Fe2O3 → 2Fe + Al2O3
ΔH = −3351.4 kJ
ΔH = −1648.4 kJ
C + O2 → CO2
H2 + ½ O2 → H2O
2C2H2 + 5O2 → 4CO2 + 2H2O
Calculate ΔH for:
2C + H2 → C2H2
ΔH = −393.5 kJ
ΔH = −286 kJ
ΔH = −2598.8 kJ
28) Given:
29) Given:
30) Given:
30A. Use the following thermochemical equations to determine the enthalpy change for the reaction
NO(g) + O(g) → NO2(g) ΔHrxn = ?
NO(g) + O3(g) → NO2(g) + O2(g)
O3(g) → 3/2 O2(g)
O2(g) → 2 O(g)
ΔH = −198.9 kJ
ΔH = −142.3 kJ
ΔH = +495.0 kJ
31) Use Hess’s Law to find ΔH for the following reaction:
N2O(g) + NO2(g) →
N2(g) + O2(g) → 2 NO(g)
ΔH = +180.7 kJ
2 NO(g) + O2(g) → 2 NO2(g)
ΔH = −113.1 kJ
2 N2O(g) → 2 N2(g) + O2(g)
ΔH = −163.2 kJ
3 NO(g)
32) Calculate ΔH for: 2 F2(g) + 2 H2O(l) → 4 HF(g)
Given: H2(g) + F2(g) → 2 HF(g)
2 H2(g) + O2(g) → 2 H2O(l)
+ O2(g)
ΔH = −537 kJ
ΔH = −572 kJ
33) Apply Hess’s Law to the given equations to determine the enthalpy change for
C2H4(g) + 6 F2(g) → 2 CF4(g) + 4 HF(g)
H2(g) + F2(g) → 2 HF(g)
ΔH= −537 kJ
C(s) + 2 F2(g) → CF4(g)
ΔH= −680 kJ
2 C(s) + 2 H2(g) → C2H4(g)
ΔH= +52.3 kJ
Calorimetry
Part I: Complete each sentence with the correct term.
water
calorimetry
heat capacity
temperature
specific heat
calorimeter
heat
34) The study of heat flow and heat measurement is called
__________________________________.
35) The words calorimeter, calorimetry and calorie are all derived from the Latin word calor, which
means _________________.
36) The amount of heat needed to raise an object’s temperature depends on its
__________________.
37) Every substance has a _________________________, which tells you how much heat is
necessary to raise the temperature of 1 gram of the substance by 1 degree Celcius.
38) __________________ has one of the highest specific heats of any common substance.
39) A transfer of heat is detected by measuring a ___________________________ change.
40) A _______________________ is a well-insulated container used to measure temperature
changes.
Part II: Define the following symbols:
41) Ti
_____________________________________________________________________
42) Tf
_____________________________________________________________________
43) qrxn _____________________________________________________________________
44) qsur _____________________________________________________________________
45) m
_____________________________________________________________________
46) Cp _____________________________________________________________________
47) How much heat is released when 432 g of water cools from 71.0 ºC to 18.0 ºC?
48) How many grams of a sample of calcium carbide, CaC2, will be required if as it cools from
74.2 ºC to 11.5 ºC it releases 2680 J of heat? The Cp of calcium carbide is 1.00 J/gº
50. How much heat energy is required to raise the temperature of 789 grams of acetic acid,
CH3COOH, from 25.0 ºC to 82.7 ºC? The Cp of acetic acid is 2.05 J/g ºC.
51. What is the specific heat of aluminum if the temperature of a 28.4 g sample of aluminum is increased
by 8.1 ºC when 207 J of heat is added?
52. 15.3 g of NaNO3 were dissolved in 100.0 g of water in a calorimeter. The temperature of the water
dropped from 25.00 ºC to 21.56 ºC. Calculate ∆H for this process.
53. What is the specific heat of silicon if the temperature of a 4.11 g sample of silicon is increased by 3.8
ºC when 11.1 J of heat is added?
54. In order to make 4 cups of tea, 1.00 kg of water is heated from 22.0 ºC to 99.0 ºC. How much heat
energy was required to do this?
55. A 1.0 g sample of octane (C8H18) is burned in a calorimeter containing 1200 g of water. The
temperature of the water rises from 25.0 ºC to 33.2 ºC. Calculate ∆H for this process.
56. A metal has a mass of 23.4 g. If it takes 6.18 J of heat to raise the temperature 1.00ºC, what is the
metal’s specific heat?
57. How much heat is required to raise the temperature of 20.0 g of iron from 26ºC to 72.30ºC. The
specific heat of iron is 0.447 J/gºC.
58. What is the specific heat of aluminum if the temperature of a 28.4 g sample of aluminum is increased
by 8.1 ˚C when 207 J of heat is added?
59. How much heat is required to raise the temperature of 3.90 g of water 45 ˚C?
60. How many grams of silver (specific heat = 0.235 J/gºC) can be heated from 49.3 ºC to 60.5 ºC when
40.5 J are added?
61. What mass of water will undergo an increase of temperature of 74.2 ºC when 2905 J of heat is added?
62. What is the specific heat of titanium if the temperature of a 36.7g sample of titanium is increased by
4.8 ˚C when 91.6 J of heat is added?
Heat Review

63. Provide the following information from the above diagram:
a. Does the reaction absorb or release heat? __________________
b. endothermic or exothermic _______________________
c. The value (number and sign) of H: _____________
d. The thermochemical equation:
e. Will the temperature increase or decrease during this reaction?___________________
f. The value (number and sign) of H for the reverse reaction: ___________
64. TiO2 + CCl4
→ TiCl4 + CO2 + 155KJ
a. Rewrite the equation in H notation.
b. How many kilojoules of heat are produced if 37.8 g of CCl4 is used?
c. Draw an enthalpy diagram for the reaction.
65. If 159 J of heat are added to an unknown metal of mass 135 g, the temperature of the metal goes from
25.2ºC to 27.3ºC. Calculate the specific heat of the metal.
Reaction 1:
2MnO(s)  2Mn(s)
Reaction 2:
2MnO(s) +
+
O2(g) H = +770.4 KJ/mol
O2(g)  2MnO2(s) H = -269.7 KJ/mol
66. Apply Hess’s Law to reactions 1 and 2 and determine the enthalpy change for the following reaction:
Reaction 3:
Mn(s) + O2(g)  MnO2(s)
67. Does reaction 3 release or absorb heat?
68. Is reaction 3 exothermic or endothermic?
69. Will reaction 3 exhibit an increase or decrease in temperature?
__________________
__________________
__________________
70. Write a thermochemical equation for reaction 3.
71. Write the equation for reaction 3 in H notation.
72. Draw and fully label an energy diagram for reaction 3.
73. Calculate the number of kilojoules of thermal energy involved if 16.9 g of MnO2 are produced
according to reaction 3.
74. Calculate the number of liters of oxygen at STP consumed upon the transfer of 2.97 KJ of heat.