Endothermic Versus Exothermic Reactions

Energy can either be absorbed or released during a chemical reaction.



B10.3 Flame
If energy is released during a chemical reaction, it is identified as an exothermic change and an energy term can be written on the product side of the chemical reaction equation. For example, the combustion of natural gas (methane) in figure B10.3 has the reaction equation as follows:

CH4(g) + 2O2(g) → CO2(g) + 2H2O(g) + energy

If energy is absorbed during a chemical reaction, it is identified as an endothermic change and an energy term can be written on the reactant side of the chemical reaction equation. For example, the process of photosynthesis is an endothermic reaction and requires the input of energy (sunlight); its reaction equation is as follows:

6CO2(g) + 6H2O(l) + energy → C6H12O6(g) + 6O2(g)

Heat is one form of energy; energy can also take the form of light. Some reactions will produce or absorb both light and heat.
B10.4 Leaf absorbing sunlight

  Read This

Please read page 81 in your Science 10 textbook. Make sure you take notes on your readings to study from later. You should focus on identifying endothermic and exothermic reactions. Remember, if you have any questions or you do not understand something, ask your teacher!

  Practice Questions

Complete the following practice question to check your understanding of the concept you just learned. Make sure you write a complete answer to the practice question in your notes. After you have checked your answer, make corrections to your response (where necessary) to study from.

  1. Identify the following reactions as endothermic or exothermic.

    1. Farai is holding onto an Erlenmeyer flask containing a solution. When a second solution is poured in, the Erlenmeyer flask starts to feel warmer. Would this reaction be endothermic or exothermic?

      Exothermic; energy must be being released or given off if it feels warmer.
    2. S8(s) + 8O2(g) → 8SO2(g) + heat

      Exothermic; the energy term is on the product side.
    3. Water can be decomposed into hydrogen gas and oxygen gas when an electric current is applied to it.

      Endothermic; energy, in the form of an electric current, is added.
    4. N2(g) + O2(g) + heat → 2NO(g)

      Endothermic; the energy term is a reactant.