Hydrocarbons and Heat
• Most hydrocarbons are used as fuels.
• Knowing how much energy a fuel provides,
can tell us if it is useful for a certain application.
• For example, the amount of energy a food
releases when burned, can tell us about it’s
caloric content (fats release lots of energy).
• Heat energy released during combustion can
be measured with a calorimeter.
• A “bomb calorimeter” is shown. It
includes water in a heavily
insulated container, a stirrer,
valve, bomb chamber, ignition
wires, & a thermometer (pg 577).
Exothermic and Endothermic changes
• An alternative to the bomb calorimeter is a
“coffee cup” calorimeter, where two nested
polystyrene cups take the place of the container
• In either case, the change in heat of the water
tells us about the reaction of the chemicals.
• An increase in water temperature indicates that
the chemicals released energy when they
reacted. This is called an “exothermic” reaction.
• In an “endothermic” reaction, water temperature
decreases as the chemicals absorb energy.
• We will see that heat is measured in Joules (J)
or kiloJoules (kJ). Before we do any heat
calculations, you should know several terms …
Specific heat capacity
balloon demo
The heat needed to  the temperature of 1 g of
a substance by 1 C. Symbol: c, units: J/(gC).
Heat capacity
The heat needed to  the temperature of an
object by 1 C. Symbol: C (=c x m), units: J/C
Heat of reaction
The heat released during a chemical reaction.
Symbol: none, units: J.
Specific heat (of reaction)
The heat released during a chemical reaction
per gram of reactant. Symbol: h, units: J/g.
Molar heat of reaction
The heat released during a chemical reaction
per mole of reactant. Symbol: H, units: J/mol.
Heat Calculations
• To determine the amount of heat a substance
produces or absorbs we often use q = cmT
• q: heat in J, c: specific heat capacity in J/(gC),
m: mass in g, T: temperature change in C,
• This equation makes sense if you consider units
J
J=
x g x C
gC
For a list of c values see page 568 (table 3)
Sample problem: (must know water = 4.18 J/gC)
When 12 g of a food was burned in a calorimeter,
the 100 mL of water in the calorimeter changed
from 20C to 33C. Calculate the heat released.
q=cmT = 4.18 J/(gC) x 100 g x 13C = 5.4 kJ
More practice
1. 5 g of copper was heated from 20C to 80C.
How much energy was used to heat the Cu?
q=cmT = 0.38 J/(gC) x 5 g x 60 C = 114 J
2. If a 3.1 g ring is heated using 10.0 J, it’s temp.
rises by 17.9C. Calculate the specific heat
capacity of the ring. Is the ring pure gold?
q=cmT
10.0 J
c = q/mT=
= 0.18 J/(g°C)
3.1 g x 17.9C
The ring is not pure. Gold is 0.13 J/(gC) -pg. 568
3. Do questions 5, 6 on pg. 596
Do questions 7, 8 on pg. 570
5. q=cmT = 4.18 J/(gC) x 2570 g x 92 C
= 988319 J = 988 kJ = 0.988 MJ
6. q=cmT
1 750 000 J
T = q/cm =
= 33.5°C
4.18 J/(gC) x 12500 g
Since the temperature started at 5.0°C, the
final temperature is 38.5°C.
7. q=cmT = 0.86 J/(gC) x 2500 g x 335C
= 720 kJ or 0.72 MJ
8. Total heat = water heat + pot heat
= 4.18 J/(gC) x 1200 g x 53.0C = 265 848 J
= 0.510 J/(gC) x 450 g x 53.0C = 12163.5 J
= 278 kJ
Heat Capacity Calculations
• Recall that heat capacity (J/C) is different from
specific heat capacity (J/gC).
• Heat capacity is sometimes a more useful value
• For example, because a calorimeter includes
wires, the stirrer, thermometer, etc. some heat
will be transferred to these other materials.
• Rather than having to calculate q for each
material (like question 8) a J/C value is used.
Sample problem:
q=cmT
A calorimeter has a heat capacity of 2.05 kJ/C.
How much heat is released if the temperature
change in the calorimeter is 11.6C?
q = cm T q= 2.05 kJ/C x 11.6 C = 23.8 kJ
Thermochemical Equations
Thermochemical equations are chemical
equations with an added heat term.
• KBrO3(s) + 42 kJ  KBrO3(aq)
This is endothermic (heat is absorbed/used)
• 2 Mg(s) + O2(g)  2 MgO(s) + 1200 kJ
This is exothermic (heat is produced/released)
Read 12.3 (pages 582 – 585). Do 2-6 (pg. 585).
Sample problem:
3.00 g of octane was burned in a calorimeter
with excess oxygen, the 1000 mL of water in
the calorimeter rose from 23.0C to 57.6C.
Write the thermochemical equation for octane,
representing the molar heat of combustion.
Page 585
2. Specific refers to mass in grams.
3. Specific heat of reaction: J/g or kJ/g, etc.
Molar heat of reaction: J/mol or kJ/mol, etc.
4. J/mol = J/g x g/mol (molar mass is used)
5. Exothermic.
E.g. CH4 + 2O2  CO2 + 2H2O + x kJ
Other examples include propane, octane, Mg
6.
49.90 kJ 26.04 g
= 1299 kJ/mol
x
g
mol
C2H2 + 2.5O2  2CO2 + H2O + 1299 kJ
or 2C2H2 + 5O2  4CO2 + 2H2O + 2598 kJ
Sample: First, calculate the heat released by the
combustion reaction via q=cmT …
= 4.18 J/(gC) x 1000 g x 34.6 C = 144 628 J
Octane (C8H18) has a molar mass of 114.26 g/mol
We can determine the molar heat of reaction
1) via the specific heat of reaction or 2) directly
h = J = 144 628 J H = 48.2 kJ x 114.26 g
g
3.00 g
g
mol
= 48209 J/g = 48.2 kJ/g
= 5508 kJ/mol
or
J
144 628 J
= 5508 kJ/mol
H =
=
mol
0.0263 mol
C8H18 + 12.5O2  8CO2 + 9H2O + 5508 kJ
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