General Chemistry Name: Period: _______
Unit 2 – Energy and KMT Note Packet
Energy: the capacity to do work (the ability to move or change matter); energy has no mass and no volume; it’s intangible; it’s NOT matter
Heat versus temperature
heat temperature
def:
symbol =
units:
measured only when it is used to do work
calorimetry
does work by speeding up the motion of molecules and atoms
flows from ________ areas to ________
areas
heat is dependent on __________
def:
symbol =
units:
refers to the intensity of heat in an object
change in T = T = Tf – Ti
Temperature is NOT a form of energy
Temperature IS a predictor of heat flow from areas of _______ T to _______ T
objects can be the same temperature but have different amounts of _________
__________ (two words) Temperature scales
sig figs and temperature: because the Celsius temperature scale is a continuum with both positive and negative values, a temperature measurement of 0C has 1 sig fig (0.1C = 2 sig figs; 0.98C = 3 sig figs)
0 K ; all molecular motion stops (all atoms condense into one big fuzzy atom)
0 K theoretical temperature not yet obtained (within a millionth of a degree)
C K F
water melts/freezes water boils/condenses Conversions:
K = C + 273
C = K – 273
F = 9/5 C + 32
C = 5/9 ( F – 32 )
Practice temperature conversions:
C K F
1 450 K
2 98.6 F
3 -273 C
4 294 K
5 77 F
6 225 K
7 -40 C
unit 2 notes - student version 1617 - 2 -
Heating Curve for Water at 1 atm
-20 0 20 40 60 80 100 120 140
Time (sec)
Temperature (°C)
Kinetic Molecular Theory*
1.
2.
3.
*the basic principles of KMT are theoretical and begin to break down under certain circumstances KMT is better at describing matter in higher energy states (gases, for example)
states/phases of matter
state shape and volume distance btw molecules
entropy motion
HEATING AND COOLING CURVES FOR WATER
Law of Conservation of Energy
within a closed system, energy transforms from one type to another
example: electricity lights a bulb: resistance builds up in the tungsten wire, it glows and gives off light and heat;
the total energy in the heat and light = energy in the electricity
example: when heat is added to water on a hot plate, that heat energy is absorbed by the water molecules, which move faster and faster (increased kinetic energy higher temperature)
Law of Conservation of Matter
matter can also be transformed during chemical and physical changes
example: when ice melts to make water during a phase change, every molecule of H2O in the original ice crystal/cube can be accounted for in the resulting liquid H2O
example: when two chemicals are mixed, the atoms may be rearranged to form new chemical compounds, but every original atom can be accounted for in the new substances
(on our large scale, we see matter and energy as separate, but matter and energy interconvert at the subatomic level according to Einstein’s Theory of Relativity E = mc2)
A quantitative look at the heating curve for water
Water Temperature and Time Measurements* (100. g of ice made from distilled water, heat added = 10. cal/sec) Time (sec) Temp (C) Observations
0. 0.0 ice beginning to melt
400. 0.0 mixture of melting ice and water
800. 0.0 last of ice melted; only water present
1800. 100.0 water beginning to boil
4000. 100.0 boiling water (less than the original amount) and steam/vapor 7200. 100.0 last of water turned to vapor
Plot this
data on the following graph, please connect the dots (do not draw a best fit line).
-20 0 20 40 60 80 100 120 140
Temperature (°C)
Time (sec)
Cooling Curve for Water at 1 atm
unit 2 notes - student version 1617 - 4 -
Heating Curve Logic: if heat is added at 10 cal/sec for 1000 sec…how many calories are added?
Mass of water Temperature change Calories added
100 g 100˚C 1000 sec x 10 cal/sec = 10000 cal
100 g 10 ˚C
100 g 1 ˚C
1 g 1 ˚C
That means we need ______ of heat to raise the temperature of 1 g of water by 1 ˚C!
That is called the specific heat capacity = c = 1 cal/g˚C for water.
Knowing that, how many calories do we need to raise the temperature of 10 g of water from 50˚C to 75˚C?
Here is the formal equation for what you just did logically
Q = mcΔT
Q = heat energy (cal) m = mass (g) c = specific heat (cal/g˚C) ΔT = temperature change (˚C)
What about the 1st and 3rd sections of the graph? There is no temperature change…so this equation won’t work…
1st section: Lasted 800 sec, if we were adding heat at 10 cal/sec we added 8000 calories Those 8000 calories were able to melt 100 g of solid water.
So…we need _____ calories to melt 1 g of solid water.
That is called the heat of fusion = Hf = 80.0 cal/g for water
Q = mH
fQ = heat (cal) m = mass (g) Hf = heat of fusion (cal/g)
3rd section: Lasted 5400 sec! If we added heat at 10 cal/sec we added 54000 calories Those 54000 calories were able to vaporize (boil) _______ g of liquid water.
So…we need _______ calories to vaporize 1 g of liquid water (turn it all into gas).
That is called the heat of vaporization = Hv = 540 cal/g for water
Q = mH
vQ = heat (cal) m = mass (g) Hv = heat of vaporization (cal/g) What if we had a sample of water vapor, and cooled it off?
How much heat energy do you think would need to be removed to condense one gram of water vapor to one gram of liquid water? Why do you think this?
(hint: How much heat energy had to be added to turn it into a vapor? Therefore, how much heat energy would have to be removed to reverse the process?)
How much heat energy do you think would need to be removed to solidify one gram of liquid water to one gram of solid ice? Why do you think this?
(hint: How much heat energy had to be added to melt it? Therefore, how much heat energy would have to be removed to reverse the process?)
This leads us to two more equations:
Q = mHc (used when a gas condenses to become a liquid) Q = mHs (used when a liquid freezes to become a solid)
unit 2 notes - student version 1617 - 6 -
Calorie Problems
Theoretical values for energy changes during the heating or cooling of a substance, or during a phase change, can be calculated using five basic equations.
Q = mcT
(used when the temperature is changing and the phase stays the same)
Q = heat energy
c = specific heat for liquid water = 1 cal/g ºC m = mass of sample
T = change in temperature of sample in ºC
Q = mHv (used when a liquid vaporizes to become a gas) Q = mHc (used when a gas condenses to become a liquid)
m = mass of sample
Hv = heat of vaporization (for water = 539.4 cal/g) Hc = heat of condensation (for water = 539.4 cal/g)
Q = mHf (used when a solid melts to become a liquid) Q = mHs (used when a liquid freezes to become a solid)
m = mass of sample
Hf = heat of fusion (for water = 79.72 cal/g) Hs = heat of solidification (for water = 79.72 cal/g)
The heat energy (Q) can be calculated in terms of calories (cal), kilocalories (Cal or kcal), or joules. (1 calorie = 4.184 cal) A calorie is defined as the amount of energy required to raise 1.0 g of water exactly 1.0 ºC.
The value of Q for any substance can be calculated, but note that each substance has unique values for specific heat capacity (c), heat of fusion (
H
f), and heat of vaporization (H
v).high specific heat capacity (c) = a large amount of energy must be added in order to increase the temperature low specific heat capacity (c) = a small amount of energy must be added in order to increase the temperature Write a definition for specific heat capacity:
Write a definition for heat of fusion:
Explain the difference between heat of fusion and heat of solidification:
Write a definition for heat of vaporization:
Explain the difference between heat of vaporization and heat of condensation:
EXAMPLE CALORIE PROBLEMS:
For any calorie problem, you can find the requested information as long as you are given all the other information you need. Some problems require you to use algebra to rearrange the equation to solve it.
Q = mcT
How much heat is required to raise the temperature of 10.0 g of water from 5.0degC to 25.0degC?
What will be the temperature change if 100. cal of heat are added to 25 g of water?
Q = mHf
How much heat is needed to melt 5.0 g of ice (frozen water)?
Q = mHv
How many grams of water can be vaporized by 750. calories of heat energy?
Q = mHs
How much heat is released when 6.2 g of liquid water freezes to become solid?
Q = mHc
How many grams of water will condense if 375 calories of heat energy are removed from it?
Calorimetry
Physical changes are often accompanied by the transfer of energy
To understand the transfer of energy, you must consider both the substance in question and its surroundings
o Energy transfers that result in a temperature change for the substance and its surroundings
Examples:
1.
2.
3.
4.
Substance and surroundings
unit 2 notes - student version 1617 - 8 -
o Energy transfers that result in a temperature change for the surroundings (but not the substance)
Example:
1.
Energy changes in a laboratory setting are measured using a .
If heat is produced during the change, then the process/change/reaction is and the temperature of the surroundings will increase.
Examples: 1. 2.
If heat is consumed during the change, then the process/change/reaction is and the temperature of the surroundings will decrease.
Examples: 1. 2.
Practice Situations:
A: A beaker of cold water is placed on a hot plate that is turned on.
1. What will happen to the temperature of the water? _______________________________________
2. Will the water gain or lose energy over time? If it gained energy, where did that energy come from? If it lost energy, where did that energy go? __________________________________________
After about 15 minutes, you turn the hot plate off, and place the beaker of water on the lab bench.
3. Over time what will happen to the temperature of the water? ________________________________
4. Will the water gain or lose energy over time? If it gained energy, where did that energy come from? If it lost energy, where did that energy go? ____________________________________________________
5. If you could measure it, what do you expect would happen to the temperature of the air right around the beaker as the water cooled off? ______________________________________________________
B: A piece of hot metal is placed in a beaker of cold water.
1. What will happen to the temperature of the metal over time? _________________________________
2. Will the metal gain or lose energy? If it gained energy, where did that energy come from? If it lost energy, where did that energy go? ____________________________________________________________
3. What will happen to the temperature of the water? _________________________________________
4. Will the water gain or lose energy? If it gained energy, where did that energy come from? If it lost energy, where did that energy go? _____________________________________________________________
C: An ice cube is placed on the counter top and sits for a couple of hours.
1. What happens to the ice cube? _________________________________________________________
2. What happens to the temperature of the water? (remember ice is solid water) ______________________
3. Will the water gain or lose energy over time? If it gained energy, where did that energy come from? If it lost energy, where did that energy go? ________________________________________________________
4. If you could measure it, what do you expect would happen to the temperature of the air right around the ice cube (water) as the change happened? _____________________________________________________
