1. Klein Forest High School-Chemistry Jarrod G Collins, M.Ed.
Thermochemistry day 2
Klein Forest High School-Chemistry
Jarrod G Collins, M.Ed.

2. Warm up! Heat flows from warm to cold or cold to warm?
An endothermic process is one where heat flows into or out of a system?
Is ice melting an endothermic or exothermic process?

3. Quantifying Heat Energy
The heat capacity of an object is proportional to the following:
Its mass
The specific heat of the material
So we can calculate the quantity of heat absorbed by an object if we know the mass, the specific heat, and the temperature change of the object.

4. q = m Cp T = m Cp (Tfinal – Tinitial )
Example
How much energy is needed to heat grams of water from 10oC to 55oC at constant pressure?
Specific heat of water (CH20 ) = J/goC
q = m Cp T = m Cp (Tfinal – Tinitial )

5. Example
How much energy is needed to heat 80 grams of water from 10oC to 55oC at constant pressure?
Specific heat of water (CH20 ) = J/goC
q = m Cp T = m Cp (Tfinal – Tinitial )
q = (80grams) ( J/goC) (55oC – 10oC)
q = joules
divide by 1000 to get kilojoules
15062 J 1 kJ = q =15.06 kJ absorbed
1000J

6. Calculating Heat
Practice problem
5 g of copper was heated from 20C to 80C. How much energy was used to heat the Cu?
Specific heat of Cu (CCu ) = 0.38 J/(gC)

7. Calculating Specific Heat Capacity
What is the specific heat of aluminum if the temperature of a 28.4 g sample of aluminum is increased by 8.1 oC when 207 J of heat is added?

8. Heat Capacity and Specific Heat
Specific heat, quantity of heat transferred, mass of substance, and temperature change:
Specific heat =
heat transferred
mass  temperature change
Cs = q
m  T

9. Calculating Specific Heat Capacity
What is the specific heat of aluminum if the temperature of a 28.4 g sample of aluminum is increased by 8.1 oC when 207 J of heat is added? Cs = _____207 J _______ (28.4 g x 8.1 ºC) Cs = J/gºC
Cs = q
m  T

10. Practice on your own!!
DIRECTIONS: Use q = (m)(ΔT)(Cp) to solve the following problems. Show all work and units.
How many joules of heat are needed to raise the temperature of 10.0 g of aluminum from 22°C to 55°C, if the specific heat of aluminum is 0.90 J/g°C?
2. The specific heat of ethanol is 2.46 J/g oC. Find the heat required to raise the temperature of 193 g of ethanol from 19oC to 35oC.

11. Practice on your own!!
3. How much heat (in J) is given out when 85.0 g of lead cools from ºC to 10.0 ºC? (Cp of Pb = J/g ºC)
4. How many joules of heat are needed to change 50.0 g of ice at -15.0ºC to ºC? (Cp of H2O = J/g ºC)
5. A g piece of iron absorbs joules of heat energy, and its temperature changes from 25ºC to 175ºC. Calculate the specific heat capacity of iron.
6. If it takes joules to heat a piece of gold weighing g from 10.0 ºC to 27.0 ºC, what is the specific heat of the gold?
7. A certain mass of water was heated with 41,840 Joules, raising its temperature from 22.0 ºC to 28.5 ºC. Find the mass of the water, in grams.
(Cp of H2O = J/g ºC)
8. Calculate the number of joules given off when 32.0 grams of steam cools from ºC to ice at ºC. (Cp of H2O = J/g ºC)
9. The specific heat of ethanol is 2.46 J/g oC. Find the heat required to raise the temperature of 193 g of ethanol from 19oC to 35oC.
10. When a 120 g sample of aluminum (Al) absorbs 9612 J of energy, its temperature increases from 25oC to 115oC. Find the specific heat of aluminum.