How to Calculate the Amount of Heat Released

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Some chemical reactions release energy by heat. In other words, they transfer heat to their surroundings. These are known as exothermic reactions: "Exo" relates to external, or outside, and "thermic" means heat.

Some examples of exothermic reactions include combustion (burning), oxidation reactions (rusting) and neutralization reactions between acids and alkalis. Many everyday items like hand warmers and self-heating cans for coffee and other hot beverages undergo exothermic reactions.

TL;DR (Too Long; Didn't Read)

To calculate the amount of heat released in a chemical reaction, use the equation Q = mc ΔT, where Q is the heat energy transferred (in joules), m is the mass of the liquid being heated (in kilograms), c is the specific heat capacity of the liquid (joule per kilogram degrees Celsius), and ΔT is the change in temperature of the liquid (degrees Celsius).

Difference Between Heat and Temperature

It's important to remember that temperature and heat are not the same thing. Temperature is a measure of how hot something is, measured in degrees Celsius or degrees Fahrenheit, while heat is a measure of the thermal energy contained in an object measured in joules.

When heat energy transfers to an object, its temperature increase depends on:

  • the mass of the object
  • the substance the object is made from 
  • the amount of energy applied to the object

The more heat energy transferred to an object, the greater its temperature increase.

Specific Heat Capacity

The specific heat capacity (c) of a substance is the amount of energy needed to change the temperature of 1 kg of the substance by 1 unit of temperature. Different substances have different specific heat capacities, for example, water has a specific heat capacity of 4,181 joules/kg degrees C, oxygen has a specific heat capacity of 918 joules/kg degrees C, and lead has a specific heat capacity of 128 joules/kg degrees C.

Heat Energy Calculator

To calculate the energy required to raise the temperature of a known mass of a substance, you use the specific heat formula:

Q = m × c × ΔT

Q is the energy transferred in joules, m is the mass of the substances in kg, c is the specific heat capacity in J/kg degrees C, and ΔT is the temperature change in degrees C in the specific heat formula.

Heat Released Calculator

Imagine 100 g of an acid was mixed with 100 g of an alkali, which resulted in the temperature increase from 24 degrees C to 32 degrees C.

The equation for a neutralization reaction between an acid and an alkali can be reduced to:

H+ + OH- --> H2O

The formula to use: Q = mc ∆T

Mass = m = 100 g + 100 g / 1000 g per kg = 0.2 g (one significant figure)

Specific heat capacity of water = c = 4,186 J/kg degrees C
Change in temperature = ΔT = 24 degrees C - 32 degrees C = -8 degrees C

Q = (0.2 kg) (4,186 J/kg degrees C) (-8 degrees C )
Q = -6,688 J, which means 6,688 joules of heat is released.

References

About the Author

Claire is a writer and editor with 18 years' experience. She writes about science and health for a range of digital publications, including Reader's Digest, HealthCentral, Vice and Zocdoc.