Specific Heat and Calculating Heat Absorbed

Mr. ChapmanChemistry 30

Recall the definition of a calorie:

The amount of energy required to raise the temperature of 1 gram of pure water by 1 degree Celsius.

Also recall that the calorie is equal to 4.184 J.

That quantity, 4.184 J/g°C, is known as the specific heat of water. The symbol for specific heat is (c).

From the Last Class...

The specific heat of any substance is the amount of heat required to raise one gram of the substance by one degree Celsius.

To raise the temperature of water by 1 degree Celsius requires 4.184 J of energy.

You will find that other substances require much less energy to raise their temperatures by 1 degree Celsius.

Specific Heat

It turns out that water has a much higher specific heat capacity than concrete does.

The specific heat of concrete is 0.84 J/g°C, whereas the specific heat of water 4.184 J/g°C.

If you have 1 kg of each substance at 0°C, which of them will take more energy to raise to a temperature of 50°C?

Water vs. Concrete

Water vs. Concrete

It takes more energy to heat the water to 50 degrees Celsius than it takes to heat the

concrete block to 50 degrees Celsius!

It seems like if we know the specific heats of many substances, we should be able to tell how much heat is required to raise the substances to a certain temperature.

And guess what? We can! Aren’t you happy?!

It turns out that three important things matter when calculating the heat lost or gained by a substance:

Heat Calculations

MassThere is a direct relationship betweenmass and the amount of heat required toraise a substance to a specifictemperature. The symbol for mass is m.

In other words, the relationship between mass and heat required is linear.

3 Important Factors

Temperature ChangeThe temperature that you want to raise asubstance to is equally important as themass of the substance that you have.

Temperature change is often given the symbol Δt, and is equal to final temperature – initial temperature.

3 Important Factors Continued..

Specific HeatThe specific heat of the substance is

important because it tells you exactly how much heat is required to raise the temperature of the substance.

The symbol for specific heat is (c).

3 Important Factors Continued...

These 3 important factors lead us to the heat equation:

q = mc Δt

Where:q = heat (energy in J)m = mass (in grams)

c = specific heat (in J/g°C)Δt = temperature change (in °C)

The Heat Equation

1. 12 g of ethanol were heated from an initial temperature 6.2 °C to a final temperature of 19.8 °C. How much heat was absorbed by the ethanol?

2. The temperature of a sample of iron with a mass of 10.0 g changed from 50.4 °C to 25.0 °C with the release of 114 J. What is the specific heat of iron?

Examples