Objectives for this post are:
- State the definition of the calorie; state the definition of the joule (1 kg∙m2/s2).
- Solve calorimetry problems including use of the formulas Q = mc∆T, Q = mHf, Q = mHv. (formulas listed on NYS Reference Table, Page 12).
The calorie is a unit of energy based on heat -- it is the amount of heat required to raise the temperature of 1 gram of water 1 degree Celsius. BTW, food calories are kilocalories -- you'll notice that food labels report Calories, the capital C being used in place of "kilo" .
A joule is an energy unit based on motion and is defined as 1 kilogram x ∙meter2/second2
This is the unit of energy in the SI system (System International) and is based on force and motion.
FYI, 1 calorie = 4.18 joules
Many worksheets have both calorie and joule problems.
Here's a calorimetry worksheet:
Here is a similar calorimetry worksheet with answers -- I can't find it w/o the answers:
Here is a brief video solving some calorimetry problems:
In the second problem, again the measurements all 3 significant figures -- following the rule for multiplying & dividing, the answer also is reported with 3 significant figures.
Here is lab measuring the specific heat of a metal sample:
1) Using Q = mc∆T, you can calculate the amount of heat released by the hot cadmium object to increase the temperature of the water. Use the mass of the water and the temperature change of the water.
Q = (100.0 g) (1 cal / g C) (3.0 C) --> the water changed in temperature by 25.0 C - 22.0 C = 3.0 C
The grams cancel, and the C cancel, and the answer if 300 calories.
In joules it would be:
Q =(100.0 g) (4.18 J / g C) (3 C) = 1254 J
2) Rearrange Q = mc∆T to calculate the specific heat (c) for the cadmium.
The Law of Conservation of Energy is the basis of calorimetry. The heat gained by the water to increase its temperature is the same amount of heat lost by the hot cadmium object.
We calculated the heat gained by the water as 300 calories. It is now the heat lost by the hot cadmium and is used along with the mass of the cadmium and its temperature change.
c = Q / (m ∆T)
c = 300 calories / (58.953 g x 75 C) --> the temp change of the metal is 100 C - 25C = 75 C
c = 0.67 cal / g C
This lab handout is a little long, but the illustrations are a useful reference:
This lab handout is shorter:
Be careful transferring the hot object from the boiling water! The video I posted above shows the object attached to a string for easier transfer from the boiling water to the cooler sample.
This reminds me of my first lab accident in the chemistry lab. A student accidentally knocked over his boiling water bath. I rushed over to make sure he was OK or needed medical assistance -- he said, "No, I'm ok. My lab apron protected me."
It is very important for your students to wear proper protective gear-- laboratory safety goggles and rubberized laboratory aprons.
Eye protection is very important for labs involving chemicals. They're called splash-proof goggles for a reason!
If your department doesn't have lab aprons, start finding ways the department can purchase them! Talk to the head of your department! Could it be a donation to the school? It's very important -- don't stop until your department has the proper safety equipment!
*I'd love to hear from you -- I know that some people are reading this blog regularly, but I don't know who! I'm writing for the chemistry teacher so I hope this information is useful. Your feedback would really help me to focus on your needs, so please write to me! Simply click on the "Contact" tab.
For other lab ideas, check out my lab book "Chemistry on a Budget" at amazon.com:
Each lab is presented with two possible report formats -- both with the same procedure -- one with 10 questions to be answered as a conclusion, the other with a full laboratory report required. This was to give the teacher the option of what type of report is desired!
Have a good rest of the week!