Topic 1 - Matter and Energy - Revsworld

Topic 1 -Matter . and Energy
27. Heat calculations: Solving for heat in heat calculation problems.
Concept Task: Be able to use heat equation to set up and calculate heat absorbed or released by a
substance.
As mentioned earlier, one of the most important steps in solving a heat problem is to choose the correct
heat equation. Once the correct equation is chosen, solving heat problem becomes a matter of plugging
in the right numbers into the equation and calculating your setup. Your set up and calculation must all be
correct to get the right heat value for the problem given.
Which heat equation to choose? Below are example key words and phrases in heat questions that
indicate which of the three heat equations you
Example 14:
should choose. Example heat problem for each
How much heat is released by a 3 gram sample of
equation, and step-by-step instruction of solving
water to change its temperature from 15
are given on the right. Study both sides to learn
o C to 10 o C?
how to solve heat problems.
Step 1. Identify the known and unknown
A. Heat equation for temperature change
Heat = m x C x ∆ T
Choose this equation for heat questions that
have any of the following key words or phrases.
. Changes temperature from…
.Two different temperatures given
. A change in temperature is given
Known:
Unknown
Mass (g) = 3 Heat = ?
∆T ( o C) = 15 – 10 = 5
C (J/g. o C) = 4.18 ( for water – Table B)
Step 2. Choose and write down equation to use
Heat = m x C x ∆T
Step 3. Substitute known into equation & solve
Heat = m x C x ∆T
Heat = 3 x 4.18 x 5 = 62.7 Joules
B. Heat equation for fusion phase change
Heat = m x Hf
Choose this equation for heat questions that
have any of the following key words or phrases.
. To melt or to freeze
. Changes from solid to liquid
. Changes from liquid to solid
. At constant temperature of 0 o C
C. Heat equation for evaporation phase change
Heat = m x Hv
Choose this equation for heat questions that
have any of the following key words or phrases.
. To evaporate or vaporize
. To boil, to condense
. To change from liquid to gas
. To change from gas to liquid
. At constant temperature of 100 o C
Example 15:
What is the number of joules needed to melt a 6 g
sample of ice to water at 0 o C?
Step 1. Identify the known and the unknown.
Known
Unknown
Mass (g) = 6 Heat = ?
Hf (J/g) = 334 (for water – Table B)
Step 2: Choose and write down equation to use:
Heat = m x Hf
Step 3: Substitute known into equation & solve
Heat = m x Hf
Heat = 6 x 334 = 2004 J
Example 16:
Liquid ammonia has a heat of vaporization of
1.35 KJ/g. How many kilojoules of heat are needed
to evaporate a 5 gram sample of ammonia at its
boiling point?
Step 1: Identify the known and the unknown
Known
Unknown
Mass (g) = 5 Heat = ?
Hv (KJ/g) = 1.35 (for ammonia)
Step 2: Choose and write down equation to use
Step 3: Substitute known into equation & solve
Heat = m x Hv
Heat = 5 x 1.35 = 6.75 KJ
Copyright © 2011 E3 Scholastic Publishing. All Rights Reserved. 17