KEY for AP Continuous Learning Assignment 5 Thermochemistry 2 Worksheet
1. The combustion of ethane, C
C
2
H
4(g)
2
H
4
, is an exothermic reaction.
+ 3 O
2(g)
 2 CO
2(g)
+ 2 H
2
O
(l)
∆H = -1390J/mol
Calculate the amount of heat given off when 4.79 g of C
2
H
4
reacts with excess oxygen.
4.79 g C
2
H
4
x 1 mol C
2
H
4
x 1.39 x 10 3 J x 1 kJ = 0.238 kJ
28.0 g C
2
H
4
1 mol C
2
H
4
1000 J
2. Given the following reaction: C
(s)
+ 2 S
(s)
+ 89.3 kJ  CS
2(l)
How many grams of Carbon can be burned if 520.0 kJ of energy are available?
520.0 kJ x 1 mol C x 12.0 g C = 69.9 g Carbon
89.3 kJ 1 mol C
3.
Describe the changes in energy that occur and the particle arrangement as a substance that is liquid changes to a gas. Be sure to use terms like endothermic or exothermic, and kinetic or potential energy, when writing out an explanation.
As a liquid changes to gas, the particles are absorbing energy so it is an endothermic process. The particles are in a more organized arrangement as liquid so as heat is added, they move faster and faster because kinetic energy is being added (endo) therefore, the temperature increases until it reaches the boiling point.
But as the particles change from the liquid arrangement to the gaseous arrangement the temperature does not change because the particles are changing position. At the boiling point the added energy is used to change the position, so the particles do not move/vibrate more they simply move apart from each other. This change in position is a change in potential energy. The energy required to move the particles apart counters the inter-molecular forces between the molecules.
4.
If substance X has a higher specific heat value, how will the temperature of substance X compare to substance A with a low specific heat if both substances gain the same amount of energy and have the same initial temperature?
Substance X will end at a lower temperature than substance A.

5.
What amount of energy is required to change 40.0 grams of liquid water at room temperature (23.0
o C) into 40.0 g of steam at 145.0
o C?
specific heat(C) C liquid
= 4.18 J/g o C C
steam
= 1.67 J/g o C
ΔH vap
=40.7 kJ/mol
Starting with the T = 23.0
o C the water is liquid need to use the C liquid
= 4.18 J/g o C, to change it to liquid water at 100.0
o C (Δ T = 77.0
o C). q = m C Δ T q = (40.0 g) • ( 4.18 J) • (77.0
o C) x ( 1 kJ ) = 12.9 kJ
g o C
Change the liquid water to steam (gas), use the H vap
number
1000 J q = m H vap
q = (40.0 g) • (40.7 kJ) x ( 1 mol ) = 90.4 kJ
Now gas at a T = 100.0
o steam at 145 o C (Δ T = 45.0
o C).
mol
C and need to use the C steam
18.0 g
= 1.67 J/g o q = m C Δ T q = (40.0 g) • ( 1.67 J) • (45.0
o
C, to change it to
C) x ( 1 kJ ) = 3.01 kJ
g o C 1000 J
To determine the total energy the values need to be added together:
12.9 kJ + 90.4 kJ + 3.01 kJ = 106.3 kJ need correct sig figs and unit
6. If 15.0 grams of O energy is produced?
2
reacts with excess methane in a combustion reaction how much
15.0 g O
2
x 1 mol O
2
x 357 kJ = 83.7 kJ
32.0 g O
2
2 mol O
2
7. Ammonia burns in the presence of a platinum catalyst to produce nitric oxide, NO.
4 NH
3 (g)
+ 5 O
2 (g)
 4 NO
(g)
+ 6 H
2
O
(g)
 H = ?
What is the heat of reaction at constant pressure? Use the following thermochemical equations: N
2 (g)
+ O
2 (g)
 2 NO
(g)
 H = +180.6 kJ Double it
N
2 (g)
2 H
3 H
2 (g)
2 (g)
O
 2 NH
2 (g)
 2 H
2
3 (g)
O
(g)
 H = -191.8 kJ Flip & double it
 H = -483.7 kJ Triple it
Think of the three equations above as puzzle pieces that have to be arranged to get to the goal equation.
2 N
2 (g)
+ 2 O
2(g)
6 H
2 (g)
+ 3 O
 4 NO
(g)
2 (g)
 6 H
2
O
(g)
4 NH
3
+ 5 O
2
 4 NO + 6 H
2
O
4 NH
3 (g)
 2 N
2 (g)
+ 6 H
2 (g)
 H = 180.6 kJ (2) = + 361.2 kJ
 H = - 191.8 kJ ( -2) = + 383.6 kJ
 H = -483.7 kJ (3) = - 1451.1 kJ
- 706.3 kJ

8. How much energy is involved when 68.0 grams of Ammonia is formed from nitrogen and hydrogen?
N
2 (g)
3 H
2 (g)

2 NH
3 (g)
68.0 g NH
3
x 1 mol NH
3
x -191.8 kJ = -384 kJ
17.0 g NH
3
2 mol NH
3

H = -191.8 kJ