White Board Practice Problems

1
Heating Cooling Curves

• White Board Practice Problems

2
Review
q m x Cgas x Dt
The heat quantity for each step is calculated
separately from the rest.
GAS
q DHvap x grams
VAPORIZE
LIQUID
q DHfus x grams
q m x Cliquid x Dt
temperature
MELT
The total energy amount is found by adding the
steps together.
q m x Csolid x Dt
SOLID
added energy
3
Problem 1
How much heat is need to change 22.0 grams of
water from -14.0 C to 77.0 C?
Start by planning how many steps are needed.
77.0 C
q melt ?
q liquid ?
temperature
0 C
0 C
q solid ?
-14.0C
added energy
4
Problem 1
How much heat is need to change 22.0 grams of
water from -14.0 C to 77.0 C?
Next, calculate each step.
q DHfus x grams q (333 J/g)(22.0 g) q melt
7326 J
77.0 C
q m x Csolid x Dt q (22.0 g)(4.184
J/gC)(77.0 C) q liquid 7088 J
temperature
0 C
0 C
q m x Csolid x Dt q (22.0 g)(2.05
J/gC)(14.0 C) q ice 631 J
-14.0C
added energy
5
Problem 1
How much heat is need to change 22.0 grams of
water from -14.0 C to 77.0 C?
Finally, add the steps together.
q total q ice q melt q liquid q total 631
J 7326 J 7088 J q total 15045 J 15.0 kJ
77.0 C
q melt 7326 J
q liquid 7088 J
temperature
0 C
0 C
q ice 631 J
-14.0C
added energy
6
Problem 2
How much heat must be removed from 9.00 grams of
34.0 C water to freeze it?
Start by planning how many steps are needed.
34.0 C
q freeze ?
temperature
q liquid ?
0 C
0 C
added energy
7
Problem 2
How much heat must be removed from 9.00 grams of
34.0 C water to freeze it?
Next, calculate each step.
q m x Cliquid x Dt q (9.00 g)(4.184
J/gC)(34.0 C) q liquid -1280 J
34.0 C
temperature
0 C
0 C
q DHfus x grams q (333 J/g)(9.00 g) q melt
-2997 J
added energy
8
Problem 2
How much heat must be removed from 9.00 grams of
34.0 C water to freeze it?
Finally, add the steps together.
q total q liquid q freeze q total -1280 J
-2997 J q total -4277 J - 4.28 kJ
q liquid -1280 J
temperature
0 C
0 C
q freeze -2997 J
added energy
9
Problem 3
Starting with a 13.0 g ice cube at 0.00 C, how
much heat is needed to completely boil it?
Start by planning how many steps are needed.
100. C
100. C
q liquid ?
q boil ?
temperature
0 C
0 C
q melt ?
added energy
10
Problem 3
Starting with a 13.0 g ice cube at 0.00 C, how
much heat is needed to completely boil it?
Next, calculate each step.
100. C
q DHfus x grams q (333 J/g)(13.0 g) q melt
4329 J
100. C
q DHvap x grams q (2260 J/g)(13.0 g) q
liquid 29,380 J
temperature
0 C
0 C
q m x Cliquid x Dt q (13.0 g)(4.184
J/gC)(100.0 C) q ice 5439 J
added energy
11
Problem 3
Starting with a 13.0 g ice cube at 0.00 C, how
much heat is needed to completely boil it?
Finally, add the steps together.
q boil 29,380 J
100. C
q liquid 54349 J
100.C
q melt 4329 J
temperature
q total q melt q liquid q boil q total
4329 J 5439 J 29,380 J q total 39148 J
39.1 kJ
0 C
0 C
added energy
12
Problem 4
Starting with a 11.5 g liquid water at 0.00
C, how much heat is needed to raise its
temperature to 145C?
145 C
Start by planning how many steps are needed.
q steam ?
100. C
100. C
q liquid ?
q boil ?
temperature
0 C
added energy
13
Problem 4
Starting with a 11.5 g liquid water at 0.00
C, how much heat is needed to raise its
temperature to 145C?
145 C
Next, calculate each step.
100. C
q m x Cliquid x Dt q (11.5 g)(4.184
J/gC)(100.0 C) q liq 4812 J
100. C
q m x Cgas x Dt q (11.5 g)(2.02 J/gC)(45.0
C) q gas 1045 J
temperature
0 C
q DHvap x grams q (2260 J/g)(11.5 g) q boil
25,990 J
added energy
14
Problem 4
Starting with a 11.5 g liquid water at 0.00
C, how much heat is needed to raise its
temperature to 145C?
145 C
q gas 1045 J
Finally, add the steps together.
q boil 25,990 J
100. C
q liquid 4812 J
100.C
temperature
q total q liquid q boil q gas q total 4812
J 25,990 J 1045 J q total 31847 J 31.8 kJ
0 C
added energy