White Board Practice
Problems
BC
DE
1.
2.
3.
4.
Increase in PE
Temperature constant
KE is constant
Phase changes occur
S L endothermic
SL exothermic
Q=MHf
Solid
Q=mc T
L G endothermic
LG exothermic
Q=MHv
Liquid
Q=mc T
AB
CD
EF
1.
2.
3.
4.
Increase KE
Endothermic
PE is constant
No Phase changes
Gas
Q=mc T
How much energy
is needed to melt
the ice block?
Heat of Fusion
Q=mHf
Heat of Vaporization
Q=mHv
1. Solid  Liquid (endothermic)
1. Liquid  Gas (endothermic)
2. Solid  Liquid (exothermic)
2. Liquid  gas (exothermic)
temperature
1. The heat quantity for
each step is calculated
separately from the rest.
2. The same formula is
used for each step
regardless if energy is
absorbed or released.
q = mcDt
GAS
q = mHv
LIQUID
q = mHf
q = m cDt
MELT
q = m cDt
SOLID
added energy
VAPORIZE
Problem
1
How much heat is needed to melt 35.0 grams
of water at 0°C?
temperature
Start by planning which formula is
used by the information given in the
problem.
q = mHf
0 °C
0 °C
added energy
Problem
How much heat is need to melt 35.0 grams of
water at 0°C?
temperature
1Substitute and calculate.
Q = mHf
q = grams x Hf
q= (35.0 g)(334.0 J/g)
q melt= 11690 J absorbed
0 °C
0 °C
added energy
Problem
temperature
2
How much heat must be removed to freeze
35 grams of water at its freezing point?
Substitute and calculate.
Q = mHf
q = grams x Hf
q= (35 g)(334 J/g)
q melt= 11690 J released
0 °C
0 °C
added energy
Problem
3
How much heat must be added to
vaporize 200 grams of water at its boiling
point?
temperature
q = mHv
q = grams x Hv
q = (200 g)(2260 J/g)
q = 452,000 J absorbed
added energy
Problem
4
How much heat must be released to
condense 25 grams of water at 373K?
temperature
q = mHv
q = grams x Hv
q = (25 g)(2260 J/g)
q = 56,500 J released
added energy
Problem How much heat is released when 125 g of
temperature
5
water freezes at 0oC?
q = mHf
q = grams x Hf
q = (125 g)(334 J/g)
q = 41750 J released
added energy
Problem
6
How much energy is absorbed when 25 kg
of water vaporize at its Boiling Point?
temperature
q = mHv
q = grams x Hv
q = (25000 g)(2260 J/g)
q = 56,500,000 J released
added energy
Do not worry about your difficulties in
Mathematics.
I can assure you mine are still greater.
Who said this?
Amount of heat needed to increase the temperature of
one gram of a substance by 1 degree Celsius or 1 Kelvin.
2. Because substances have different compositions, each
substance has its own specific heat.
Specific Heat of Water
4.18 J/g 0C or 4.18 J/gK
1.
Q = mcΔT
1) q is the heat gained or lost (J)
2) m is the mass of the heated substance (grams)
3) c is the specific heat capacity (J/g 0C)
4) ΔT (pronounced delta T) is the change in
temperature.
5) ΔT = (Tfinal - Tinitial) in 0C or K
Table T
Unit for q =
2. Unit for mass =
3. Unit for specific heat =
4. Unit for change in temperature =
1.
Water = 4.18 joules /g 0C
Concrete = 0.84 joules /g 0C
How many joules are absorbed by
300.0g of concrete when its temperature
is raised from 20.0 0C to 40.0 0C?
How many joules are absorbed by
300.0g of water when its temperature is
raised from 20.0 0C to 40.0 0C?
How many joules of heat are released by 35 kg of
concrete when its temperature drops from 25 0C to
5 0C?
Problem Calculate the specific heat capacity of
copper given that 204.75 J of energy
raises the temperature of 15g of copper
from 25o to 60o.
temperature
60 °C
q = mcDt
25 °C
added energy
© Mr. D. Scott; CHS
1) The temperature of a sample of water changes
from 10°C to 20°C when the sample absorbs
418 joules of heat. What is the mass of the sample?
(1) 1 g
(3) 100 g
(2) 10 g
(4) 1000 g
2) A 36-gram sample of water has an initial
temperature of 22°C. After the sample absorbs
1200 joules of heat energy, the final temperature
of the sample is
(1) 8.0°C
(3) 30.°C
(2) 14°C
(4) 55°C
Q = mcΔT
Q = 181 J
m=45.0g
c= ___ J/gK
ΔT= 30.0 K
Why is the specific heat value of
water so important in terms of
hurricanes affecting the United
States?
Windows to the Universe
Desecheo Island
Sand = 0.835 J/g°C
Water = 4.18 J/goC
The greater the specific heat capacity of a substance,
the more time and energy it takes to heat it up.