Scientific Tutor
Chem – Heat of Fusion and Heat of Vaporization
Author: Andrew
What is the heat of fusion and heat of vaporization?
In the previous section of specific heat capacity we only discussed and did calculations for how energy affects substances within the same state (gas, liquid, solid) to change their temperature. However, in this section we will discuss how to use energy to cause chemical substances to go from one state to another (example: solid to liquid). This is called the heat of fusion or heat of vaporization.
Fusion = going between solid and liquid
Vaporization = going between liquid and gas
When you are either evaporating or condensing. The formula for energy and how it is involved with the heat of fusion or vaporization is below:
Heat Energy =
|
Mass * Heat of Fusion
|
Heat Energy =
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Mass * Heat of Vaporization
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| Q = | m * Δ H |
The letter Q represents heat energy (with units of J or cal), the letter m represents mass (with units of g), the symbol Δ H represents specific heat capacity (with units of J/g C or cal/g C). NOTICE that whether you are using heat of fusion or heat of vaporization the equation is the same. The only thing that changes is what column of the table you look at to obtain the number for heat of fusion or heat of vaporization. Those heat of fusion or heat of vaporization reference tables can be found here. This table also has melting and boiling points that will not be used in this section but will be used in a later section.
VIDEO Heat of Fusion or Vaporization Example 1: If 123g of H2O is boiled (from liquid to gas), how much heat energy is required? (Use this table for reference)
What information does the problem give you?
Answer:
Q = ?
m = 123g
Δ Hvap = 2260 J/g (vaporization because it says boiled)
What formula do we use to connect this information?
| Q = | m * Δ Hvap |
How do we fill in the formula with the information?
| Q = | 123g * 2260 J/g |
Calculate —-> (123 * 2260) = 277980
| Q = | 277980 J |
What is the complete answer?
COMPLETE ANSWER: 277980 J …..or….. 2.78 * 105 J
VIDEO Heat of Fusion or Vaporization Example 2: If 4kJ is released when freezing (from liquid to solid) CO2, what is the amount of mass of CO2 that is frozen? (Use this table for reference)
What information does the problem give you?
Answer:
Q = 4kJ —-> 4000 J
m = ?
Δ HFus = 2260 J/g (fusion because it says freezing)
What formula do we use to connect this information?
| Q = | m * Δ HFus |
How do we fill in the formula with the information?
| 4000 = | m * 184 |
How do we solve for m?
Answer: divide both sides by 184.
| 4000 = | m * 184 |
| 184 | 184 |
Cross out 184 on the right side
| 4000 = | m * 184 |
| 184 | 184 |
Simplify
| 4000 = | m |
| 184 |
Calculate —-> (4000 / 184) = 21.7
| 4000 = | 21.7 |
| 184 |
What is the complete answer?
COMPLETE ANSWER: 21.7 g
PRACTICE PROBLEMS: Calculate the missing information in the heat of fusion or vaporization equation. (Use this table for reference)
If 50g of H2O is frozen (from liquid to solid), how much heat energy is required?
Answer: 16700 J
If 42g of CO2 is evaporated (from liquid to gas), how much heat energy is required?
Answer: 24108 J
3200J is applied to melt H2O. What mass of H2O is melted?
Answer: 9.58g
If it takes 8600J to condense 700g of an unknown substance. What is the substance’s heat of vaporization?
Answer: 12.29 J/g
Posted in Chemistry
