Solving Heat Problems
COPY DOWN THIS PROBLEM:
The specific heat of ethanol is
2.46 J/g oC. Find the heat energy
required to raise the temperature of
193 g of ethanol from 19oC to 35oC.
Step 1
• What type of problem is it?
Heat
• What is the question?
Find the heat energy
• What equation to use?
Q = mCpDT
• Does the equation need to be
manipulated?
No manipulation needed
Step 2
•
•
•
•
Plug in the numbers
m = 193 g
Cp = 2.46 J/g-oC
DT = 35 – 19 = 16 oC
• Soooo, Q = 193g x 2.46 J/g-oC x 16 oC
Step 3
• Calculate
Answer is 7596.48
• Cancel Units (if you don’t end up with the right
units, you haven’t used appropriate equation!)
All cancel except J, which is the unit for energy
• Determine Significant Figures
There are 2 significant figures in the temp., so two
should be in the answer.
• Write your final answer.
7600 J or 7.6 x 103 J
Problem 2
• When a 120 g sample of aluminum
absorbs 9612 J of heat energy, its
temperature increases from 25oC to
115oC. Find the specific heat of
aluminum. Include the correct unit.
• Type of Problem?
• Question?
• Equation? Does it Need to Be
Manipulated?
Equation Manipulation
•
Q = mCpDT, but you need Cp, so
divide both sides by m and DT.
So the final equation is
Cp =
Q
m DT
You may notice the equation mirrors
the units for Cp of J/g-oC.
Plug in the Numbers
Cp = Q/mDT so, Cp = 9612 J/(120g x 90 oC)
The mathematical answer is 0.89, but there is
only one sig. fig. in 90, so the answer is
0.9 J/g-oC
Check for Understanding
• Complete conversion problems #1-3 to
see if you are able to convert from one
heat unit to another.
• Then, complete heat problems #1 and #4
on the worksheet to check your
understanding of working that type of
problem. Work more of these for practice
at home (answers will be on the blog for
you to self check). Quiz Monday!
A Challenge Problem
• The specific heat of lead is 0.129 J/goC.
Find the amount of heat released when
2.4 mol of lead are cooled from 37.2oC to
22.5oC. WATCH THE UNITS!!