Some Uses of Fire

How do we measure the energy released by burning things? The amount of energy is generally reported in calories (you've probably heard of calories related to the foods that you eat) or Joules. A calorie is defined as the amount of heat it takes to raise the temperature of 1 gram of water by 1 degree Celsius. If all the heat released by our reaction was absorbed by a known number of grams of water the amount of heat that the reaction released would be calculated by:

amount of heat = (1 calorie/gram of water) x (number of grams of water) x (number of degrees that the temperature of the water increased)

So, how many or calories have been released by a reaction that raises the temperature of 225 grams of water by 20.0 degrees? You can see the answer here, but try it before you look!

What about those Joules? The Joule is just a different unit for measuring the amount of heat - just like yards and meters are different units for measuring distance. It turns out that 1 calorie is the same as 4.18 Joules, so if we want to measure energy in Joules, we can multiply the calories by 4.18 to get Joules. How many Joules does the reaction release in our example of heating 225 grams of water by 20.0 degrees? Just multiple the previous answer by 4.18. It's that easy!!