Chemistry: Principles and Reactions
Masterton/Hurley
H Chemistry
NPHS
Chapter 8 p. 241
16) Calcium carbide, CaC
2
, is the raw material for the production of acetylene (used in welding torches). Calcium carbide is produced by reacting calcium oxide with carbon, producing carbon monoxide as a by-product. When one mole of calcium carbide is formed, 464.8 kJ is absorbed. a) Write the thermochemical equation for this reaction.
CaO + 3C  CaC
2
+ CO  H = +464.8 kJ b) Is the reaction exothermic or endothermic?
Endothermic (  H = positive) c) Draw and energy diagram showing the path of this reaction. (see ex – Fig. 8.4) enthalpy reactants rxn path products
 H > 0 d) What is  H when 1.00 g of CaC
2
(g) is formed?
464 mol
.
8 kJ
CaC
2

1 mol
64 .
0 g

1 .
00 g
 7 .
263 kJ e) How many grams of carbon are used up when 20.00 kJ of heat is absorbed?
20 .
0 kJ

3 mol
464 .
8
C kJ

1
12 .
0 mol g
C
 1 .
549 g
17) Calcium chloride is a compound frequently found in first aid hot packs. It gives off heat when dissolved in water. The following reaction takes place.
CaCl
2
(s)  Ca 2+
(aq) + 2Cl -
(aq)  H = -81.4 kJ a) Calculate  H when one mole of calcium chloride precipitates from solution.
+81.4 kJ b) What is  H when 10.0 g of calcium chloride precipitates?
81 .
4 kJ mol

1 mol
111 .
CaCl
2
1 g

10 .
0 g
 7 .
33 kJ
18) Consider the dissociation of water into ions.
H
2
O (l)  H +
(aq) + OH -
(aq)  H = +55.8 kJ a) Calculate  H when one mole of calcium chloride precipitates from solution
-55.8 kJ b) What is  H when 10.0 g of calcium chloride precipitates?
 55 .
8 mol kJ

1 mol
18 .
H
0
2 g
O

1 .
0 g
  3 .
10 kJ

Chemistry: Principles and Reactions
Masterton/Hurley
H Chemistry
NPHS
19) Nitroglycerine, C
3
H
5
(NO
3
)
3
(l) , is an explosive most often used in mine or quarry blasting.
It is a powerful explosive because four gases (N
2
, O
2
, CO
2
, and steam) are formed when nitroglycerine is detonated. In addition, 6.26 kJ of heat is given off per gram of nitroglycerine detonated. a) Write a balanced thermochemical equation for the reaction.
ΔH 
 6 .
28 g kJ

227.0
mol g

4 mol
  5684 .
1 kJ  5680 kJ
4C
3
H
5
(NO
3
)
3
 6N
2
+ O
2
+ 12CO
2
+ 10H
2
O b) What is  H when 4.65 moles of products are formed?

29
5684 mol
.
1 kJ product

4 .
65 mol x product
 H = -5680 kJ x   911 kJ
20) Strontium metal is responsible for the red color in fireworks. Fireworks manufacturers use strontium carbonate, which can be produced by combining strontium metal, graphite
(C), and oxygen gas. The formation of one mole of SrCO
3
releases 1.220 x 10 3 kJ of energy. a) Write a balanced thermochemical equation for the reaction.
Sr + C + 3/2 O
2
 SrCO3  H = -1.22 x 10 3 kJ b) What is  H when 10.00 L of oxygen at 25 ºC and 1.00 atm are used by the reaction?
PV = nRT
(1 atm)(10.0 L) = (n)(0.0821 L·atm/mol·K)(298 K) n = 0.4087 mol
 1 .
1 .
22
5
 10 mol O
3
2 kJ

0 .
x
4087 mol x   332 kJ
21) A typical fat in the body is glyceryl trioleate C body, it combines with oxygen to produce carbon dioxide, water, and 3.022 x 10 heat per mole of fat.
57
H
104
O
6
. When it is metabolized in the
4 kJ of a) Write a balanced thermochemical equation for the metabolism of fat.
C
57
H
104
O
6
(s) + 80O
2
 57CO
2
+ 52H
2
O  H = 3.022 x 10 4 kJ b) How many kJ of energy must be evolved in the form of heat if you want to get rid of five pounds of this fat by combustion?
3 .
022 mol C

57
10 4
H
104 kJ
O
6

1 mol
884 .
0 g

453
1 lb
.
6 g

5 lb
  7 .
753  10 4 kJ c) How many nutritional calories is this (1 nutritional calorie = 1 x 10 3 calories)?
3 .
022  10 4 kJ

1 kcal
4 .
184 kJ
  18 , 530 kJ

Chemistry: Principles and Reactions
Masterton/Hurley
H Chemistry
NPHS
27) To produce silicon, used in semiconductors, from sand (SiO
2
), a reaction is used that can be broken down into three steps:
SiO
2
(s) + 2C (s)  Si (s) + 2CO (g)
Si (s) + 2Cl
2
(g)  SiCl
4
(g)
SiCl
4
(g) + 2Mg (s)  2MgCl
2
(s) + Si (s)


H = +689.9 kJ
H = -657.0 kJ
 H = -625.6 kJ a) Write the thermochemical equation for the overall reaction for the formation of silicon from silicon dioxide; CO and MgCl
2
are by-products.
2C + 2Cl
2
+ 2Mg + SiO
2
 Si + 2CO + 2MgCl
2
 H = -592.7 kJ b) What is the  H for the formation of one mole of silicon?
 H = 689.9 kJ -657.0 kJ – 625.6 kJ = -592.7 kJ c) Is the overall reaction exothermic? exothermic