Calculate Energy Released from Chemical Reactions

 To calculate energy (in Joules/Kilojoules) released from chemical reactions, some steps have to be followed.

The general equation to be used is

q = m x Cp x ∆T

Your Cp value refers to specific heat, and is usually given with the problem. The q variable represents the energy lost/gained, which is the value we are solving for. And finally, ∆T (pronounced Delta T) refers to the change in temperature. Usually, sample problems will give a mass, Cp value for various states (liquid, gas, solid), and a temperature. Sometimes, freezing point, melting point, Heat of formation, and chemical equation will be given.


 Let's try a few example problems.

1. How many Joules of heat are needed to melt a 25.00 g ice cube at 0.00 C?

First, set up your equation.

q = m x Cp x ∆T

Since the ice is going through a phase change (from solid to liquid), it is gaining energy, and therefore you need to use a Heat of fusion value (∆Hfus). For liquids going to gas, you will need a heat of vaporization value (∆Hvap). If the substance is losing energy (gas --> liquid), (liquid -->solid), etc. Then the fusion/vaporization values will be negative.

The value for the heat of fusion of water is ∆Hfus = 333.55 J/g

Now, we use the equation to figure out energy from the phase change.

q = (25.00g)(333.55 J/g)

The grams cancel each other out, and your value is 8,338.75 J

Next, we are not finished.We need another equation because there is a temperature change. Usually for reactions or substances that do not go from a phase change, we would just do this step.

q = m x Cp x ∆T

q = (25.00g) x (2.09 J/gC) x (1)

The Cp (specific heat) value for water at a solid is 2.09 J/gC. (the C stands for degrees celsius)

The ∆T value is just 1 because there is no temperature change. It is constant at 0.00 C.

With this, you should get a value of 52.25 J. As you can see, tremendous amounts of energy is generated from phase changing, while not that much is generated through slow, gradual, heating.


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