Energy Changes in Chemical Reactions Chemistry TEKS 11. Learning Objective. TLW understand energy and its forms – kinetic, potential, chemical, and thermal (TEKS 11.A) TLW understand the law of conservation of energy and processes of heat transfer (TEKS 11.B)
Chemistry TEKS 11
A. Three ways energy can transfer
1. Conduction – transfer of energy as heat between particles that collide
a. two objects that are in contact with each other at unequal temperatures
b. particles within an object
c. objects must be direct contact
d. Ex. In case of a wire in a campfire, the rapidly moving air molecules close to the flame collide with the atoms at the end of the wire.
The energy transferred to the atoms in the wire causes them to vibrate rapidly.
2. Convection – transfer of energy by the movement of fluids
a. fluids are either a liquid or a gas; Ex. water & air
these particles are free to move around
b. particles in a solid are not free to move, so convection can’t take place
1) heated air rises,
then cools and falls back down
2) when something heats up – it expands
3) when something cools down – it contracts
d. This cycle of a warm fluid
that rises and then cools and falls is called a convectioncurrent
While roasting marshmallows, you may notice that tiny glowing embers from the fire rise and begin to swirl.
They are following the movement of air away from the fire.
The air close to the fire becomes hot and expands
so that there is more space between the air particles
As a result the air becomes less dense and moves upward,
carrying its energy with it
The rising warm air is replaced by cooler, denser air.
Eventually, the rising hot air cools and contracts
becomes denser, and sinks.
3. Radiation – the transfer of energy by electromagnetic waves
a. does not involve the movement of matter
b. only method of energy transfer that can take place in a vacuum
c. includes infrared radiation, visible light and ultraviolet rays
d. a hot object radiates more energy than a cooler object
e. much of the energy we receive from the sun is transferred by radiation
f. objects donot have to touch to transfer heat by radiation
g. Ex. Warmth from the fire without standing IN the fire
As the molecules in your skin
absorb the energy from the fire,
the average kinetic energy of these molecules and the temperature of your skin – increases.
A. Heat is measured in units called calories (cal)
1. a calorie is the amount of heat needed to raise 1gram of
water 1 oC
2. Note the two important factors:
a) It's 1 gram of a substance
b) and it moves 1°C
3. Ex. To raise the temperature of 1 gram of water from 4oC to 5oC, 1 calorie of heat is needed
4. The amount of heat needed to change the To depends on the mass
1. The ability of a substance to absorb heat energy is called specificheat
2. Different substances absorb different amounts of heat
3. Water has a high specific heat
4. Water has a specific heat of 1
this is one of the highest specific heats of any substance
Wood is 0.42
Aluminum is 0.22
Mercury is 0.03
5. Specific heat can be used to calculate the amount of heat gained or lost
= Mass X ∆To X Specific heat
1. ∆To = change in Temperature
(To final – To initial)
2. Raise in temperature would be positive
∆To, lower temperature is negative (- ∆To)
3. Heat gained would be positive, heat lost is
4. Specific heat is an absolute number
B. Ex. How much heat is needed to raise the To of 4 grams of aluminum 5o C?
Heat gained = 4 g X 5 oC X 0.22 cal/goC
Heat gained = 4.4 cal
C. Ex. Calculate the heat lost by 10 g of copper if it is cooled from 35 oC to 21 oC.
Heat lost =
10 g X (21o - 35o) X .09 cal/goC
= - 12.6
Heat lost 12.6 calories
1. A 500 g piece of iron changes 7 ° C when heat is added. How much heat energy produced this change in temperature?
Mass X ∆To X Specific heat
500 g X 7 oC X 0.11 cal/goC
Heat gained = 385 cal
2. When 300 cal of energy is lost from a 125 g object, the temperature decreases from 45 °C to 40 °C. What is the specific heat of this object?
Specific heat = cal/ (mass X ΔT)
125 g X (40 oC – 45 oC)
specific heat = .48 cal/goC
3. A piece of food is burned in a calorimeter that contains 200 g of water. If the temperature of the water rose from 20 °C to 45 °C, how much heat energy was contained in the food?
Heat gained = Mass X ∆T X Specific Heat
200g X (45 oC – 20 oC) X 1 cal/goC