Chem Standard Blogpost :O

ChemStd 7d: Problems about heat flow involving temperature changes can be solved by using known values of specific heat and latent heat of phase change.

Heat is the energy transferred from one object to another because of a difference in temperature.

No Phase Change Problems:

To solve these kinds of problems, use the equation Q= mcΔT.
m= mass of the matter being measured. (g)
c= specific heat, the amount of heat energy required to raise 1g of a substance 1°C. (cal/g°C) or (J/g°C)
ΔT= change in temperature, the increase or decrease in the temperature of the matter (°C), = final temperature - initial temperature

Q= mcΔT must refer to only ONE object. Mixing your references will result in a wrong answer.

The value of Specific Heat is not the same for all types of matter. Heat affects the temperature of those things with a high specific heat much less than those with a low specific heat. Water has the highest specific heat while metals have low specific heats so a small amount of energy raises the temperature of metals a lot.

Here are Specific Heat Capacities of some common substances:



Water (liquid) - 4.184 J/g°C
Water (ice) - 2.03 J/g°C
Water (steam) - 2.0 J/g°C

To convert to J/g°C, use the Calorie-Joule Conversion: 1 cal = 4.184 J.

For example:

A block of lead having a mass of 20.0 grams is heated from a room temperature of 22.4 ^oC to a temperature of 85.4 ^oC. What would be the added heat to the lead?


From:

http://www.neoam.cc.ok.us/~rjones/Pages/online1014/physics/chapter4/pages/heat.html

1. Write the formula
2. Substitute the variables
3. Simplify the equation and cancel the units.
   
4. Calculate the answer
5. Box your answer and don't forget to include the units!
   

You can also find the value of the other variables by rearranging the equation:

m= Q/cΔT

c= Q/mΔT

ΔT= Q/mc


Latent Heat of Phase Change Problems:

Heat of Fusion - ( ΔHfus) - the heat absorbed by a substance that is melting from a solid state to a liquid state at a constant temperature.

Heat of Solidification - (ΔHsolid) - the heat lost when a liquid changes to a solid at a constant temperature.

Heat of vaporization - (ΔHvap) - the heat absorbed by a substance that is vaporizing from a liquid state to a gaseous state at a constant temperature.

Heat of condensation- (ΔHcon) - the heat lost when a gas changes to a liquid at a constant temperature.

Energy is conserved in all physical and chemical changes:
ΔHfus = -ΔHsolid
ΔHvap = -ΔHcon


Changes of State for Water:
Melting: ΔHfus= 80 cal/g
Solidifiction: ΔHsolid = -80 cal/g
Vaporization: ΔHvap = 540 cal/g
Condensation: ΔHcon = -540 cal/g

To solve phase change problems, use the latent heat of fusion or latent heat of vaporization formulas:

For freezing or melting, use:
Latent Heat of Fusion of Water: Q= mΔHfus
ΔHfus(H2O)= 80 cal/g = 334 J/g

For boiling or condensation, use:
Latent Heat of Vaporization: Q= mΔHvap
ΔHvap(H2O) = 540 cal/g = 2260 J/g

For example:
How many calories are needed to melt 225 g of ice at 0°C to liquid at 0°C?

Step 1: Write the formula
Step 2: Substitute the variables and cancel units.
Step 3: Calculate and box the answer. Don't forget to include the units.