ENERGY is the ability to do work or produce heat
2 forms
Kinetic Energy –
energy of motion
Potential Energy – energy due to the
composition or position of an object.
(Chemical Potential Energy – energy stored
in a substance because of its composition.
The potential energy results from the
arrangement of the atoms and the strength of
the bonds that join them. Stored energy is
released when bonds are broken.)
Converted into
Work
Heat – symbol is q; energy in the process of
flowing from a warm object to a cooler one
units
metric
calorie (cal)
SI
kilocalorie
called a Calorie with a capital “C”
sometimes called nutritional calories
1 Calorie = 1000 calories
joule (J)
One calorie
equals 4.184
joules
Conversions:
1. A breakfast of cereal, juice, and milk contains 230 Calories.
How much energy in joules will this breakfast supply?
230 Calories 1000 calories 4.184 J
962320 J
1 Calorie
1 calorie
962000 J
2. A fruit and oatmeal bar contains 142 Calories. Convert this
energy to calories.
142 Calories 1000 calories
= 142 000 calories
1 Calorie
3. An exothermic reaction releases 86.5 kilojoules. (1 kilojoule
= 1000 joules) How many kilocalories of energy are
released?
86.5 KJ 1000 joules
1 cal
1 KJ
4.184 J
1 Kcal
= 20.7 kcal
1000 cal
4. An endothermic process absorbs 256 J, how many
kilocalories are absorbed?
256 J
1 cal
1 kcal
= 6.37 10-6 kcal
4.184 J 1000 cal
Thermochemistry – the study of heat changes that
accompany chemical reactions and phase changes
Universe
System: The specific part
of the universe that
contains the reaction or
process you wish to study.
Surroundings:
Everything in the universe
other than the system.
Enthalpy (H) is the heat content of a system.
REMEMBER – this is not temperature!!
Chemical reactions and phase changes absorb or
release heat.
To find heat used in a phase change we use ΔHfus and
ΔHvap. There is no change in temperature during a
phase change.
q = m  ΔH
Enthalpy of fusion (ΔHf) – the energy required to melt a
specific amount of substance at its melting point.
1. How much heat is required to melt 5.67 g of iron (II)
oxide (FeO) if its enthalpy of fusion is 32.2 kJ/g?
q = m  ΔHf
q = (5.67 g FeO)  (32.2 kJ/g) = 183 kJ
Enthalpy of vaporization (ΔHv) – the energy required to
vaporize a specific amount of a substance at its boiling
point.
2. How much heat is required to vaporize 10.0 g of water
at its boiling point? ΔHv = 2260 J/g
q = m  ΔHv
q = (10.0 g)  (2260 J/g) = 22600 J or 2.26 kJ
Remember:
If we have a temperature change, we need to use:
q = m  Cp  ΔT
If we have a change of state at the boiling or melting point,
we need to use:
q = m  ΔHf or q = m  ΔHv
1. How much energy is needed to melt 25.4 g of I2. The
ΔHf of I2 is 61.7 J/g.
q = m  ΔHf
q = (25.4 g I2)  (61.7 J/g) = 1567.18 J or 1.57 kJ
2. How much energy is needed to heat 10.0 g of ice from
–10.0°C to 0.00°C? Cp of ice = 2.06 J/g°C
q = m  Cp  ΔT
q = (10.0 g)(2.06 J/g°C)(10.0°C) = 206 J or 0.206 kJ
3. How much energy is needed to melt 45.3 grams of iron
at the melting point if the ΔHf of iron is 42.3 kJ/mol?
q = m  ΔHf Notice: ΔHf is in kJ/mol
q=(
45.3 g Fe 1 mole FeO
)  (42.3 kJ/mole) = 41.2 kJ
55.8 g Fe
Specific Heat – the amount of heat required to raise the
temperature of one gram of a substance by one degree Celsius.
Each substance has its own specific heat (Cp). For liquid water
Cp = 4.184 J/g°C.
The heat absorbed or released by a substance during a change in
temperature depends on the specific heat, the mass of the
substance, and the amount the temperature changes.
q =
heat absorbed
or released
Cp  m  ΔT
specific
heat
mass
in grams
temperature
change
Examples:
1. How much heat is absorbed when a 4.68 g piece of metal
experiences a temperature change of 182°C? (Cp = .301
J/g°C)
q = mCpΔT
= (4.68 g)(.301 J/g°C)(182°C)
= 256.37 J
256 J
2. The temperature of a sample of water increases from 20.0°C
to 46.6°C as it absorbs 5650 J of heat. What is the mass of
the sample?
ΔT = 46.6°C – 20.0°C = 26.6
°C
q = mCpΔT
5650 J = m (4.184 J/g°C)(26.6°C)
5650 J = m (111.2944 J/g)
111.2944 J/g
111.2944 J/g
m = 50.766 g 50.8 g
Try:
1. How much heat is released to the surroundings
when 200 g of water at 96.0 °C cools to 25.0
°C?
Answer = 59 400 J