KEY for AP Continuous Learning Assignment 5 Thermochemistry 2 Worksheet
1.
The combustion of ethane, C2H4, is an exothermic reaction.
C2H4(g)
+
3 O2(g)  2 CO2(g)
+
2 H2O(l) ∆H = -1390J/mol
Calculate the amount of heat given off when 4.79 g of C2H4 reacts with excess oxygen.
4.79 g C2H4 x 1 mol C2H4
x
1.39 x 103J x
1 kJ
= 0.238 kJ
28.0 g C2H4
1 mol C2H4
1000 J
2.
Given the following reaction: C(s)
+
2 S(s) +
89.3 kJ

CS2(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 (23.0oC) into 40.0 g of steam at 145.0oC?
specific heat(C)
Cliquid= 4.18 J/goC
C steam= 1.67 J/goC
ΔHvap=40.7 kJ/mol
Starting with the T = 23.0oC the water is liquid need to use the Cliquid = 4.18 J/goC,
to change it to liquid water at 100.0oC (Δ T = 77.0oC).
q = (40.0 g) • (4.18 J) • (77.0oC) x ( 1 kJ ) = 12.9 kJ
g oC
1000 J
Change the liquid water to steam (gas), use the Hvap number
q = m Hvap
q = (40.0 g) • (40.7 kJ) x ( 1 mol ) =
90.4 kJ
mol
18.0 g
Now gas at a T = 100.0oC and need to use the Csteam = 1.67 J/goC, to change it to
steam at 145oC (Δ T = 45.0oC).
q= mCΔT
q = (40.0 g) • (1.67 J) • (45.0oC) x ( 1 kJ ) = 3.01 kJ
g oC
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
q= mCΔT
6.
If 15.0 grams of O2 reacts with excess methane in a combustion reaction how much
energy is produced?
ΔH = -357 kJ/mol
15.0 g O2 x
1 mol O2
x
32.0 g O2
357 kJ
=
2 mol O2
83.7 kJ
7.
Ammonia burns in the presence of a platinum catalyst to produce nitric oxide, NO.
4 NH3 (g) +
5 O2 (g)

4 NO (g)
+
6 H2O (g) H = ?
What is the heat of reaction at constant pressure? Use the following thermochemical
equations: N2 (g) +
O2 (g)

2 NO (g)
H = +180.6 kJ Double it
N2 (g)
3 H2 (g)

2 NH3 (g)
H = -191.8 kJ Flip & double it
2 H2 (g)
O2 (g)

2 H2O (g)
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
4
6
4
N2 (g) + 2 O2(g)  4 NO (g)
NH3 (g)
 2 N2 (g) + 6 H2 (g)
H2 (g) + 3 O2 (g)  6 H2O (g)
NH3 + 5 O2  4 NO + 6 H2O
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?
N2 (g)
3 H2 (g)
 2 NH3 (g)
68.0 g NH3 x 1 mol NH3 x -191.8 kJ = -384 kJ
17.0 g NH3
2 mol NH3
H = -191.8 kJ