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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)
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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
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.
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
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
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.
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
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
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
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
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
Heat gained = 4 g X 5 oC X 0.22 cal/goC
Heat gained = 4.4 cal
Heat lost =
10 g X (21o - 35o) X .09 cal/goC
= - 12.6
Heat lost 12.6 calories
Mass X ∆To X Specific heat
500 g X 7 oC X 0.11 cal/goC
Heat gained = 385 cal
Specific heat = cal/ (mass X ΔT)
125 g X (40 oC – 45 oC)
specific heat = .48 cal/goC
Heat gained = Mass X ∆T X Specific Heat
200g X (45 oC – 20 oC) X 1 cal/goC