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Endothermic and Exothermic Reactions

Written by tutor Oleg Y.

All processes can be classified into one of two categories: exothermic and endothermic. In an exothermic process, energy is released, while in an endothermic process, energy is stored. This section will specifically cover exothermic and endothermic chemical reactions, but almost any process can be described as releasing or storing energy.

The concept of giving off or storing energy can sometimes be a bit confusing, so let's go over some of the basic types of energy that you'll encounter in your chemistry class, and what it means to give off and store each type of energy.

Heat: Heat energy is the energy that accompanies temperature changes. If heat energy is being released then the reaction from which it is released will become hotter. If heat energy is being stored, then reaction will become colder.

Light: If light energy is being given off, then the reaction will glow. If it's being stored, then the reaction will seemingly proceed on its own without any catalyst present without any heat being evolved or absorbed.

Mechanical energy: If mechanical energy is being stored, then the volume and/or pressure of the reaction will get smaller. If mechanical energy is being given off, then the opposite will be true.

The most common change in energy that you'll witness in your chemistry class will be changes in heat energy. It can be measured with a bomb calorimeter. Energy released or stored in a reaction will often be expressed written as ΔH, or a change in enthalpy. A positive ΔH means that energy is stored and the reaction is endothermic. A negative ΔH means that energy is released and the reaction is exothermic. It is usually expressed in kilojoules (kJ) or joules (J).

Why Exothermic Or Endothermic?

If you understand the above section, then you can now identify whether a reaction is exothermic or endothermic. If it gives off one of the above three types of energy then it's exothermic, if it absorbs it, then it's endothermic. The question that still hasn't been answered, though is why? Why are some reactions exothermic while others are endothermic, and why does energy have to be absorbed or released at all?

The answer lies in chemical bonds. Chemical bonds have bond energies associated with them. This bond energy is the amount energy that it takes to break the bonds, and also the amount of the energy that is released when the bonds are formed. Consequently, if the bonds in your reactants have a higher total bond energy than your products, the reaction will be endothermic. If they have a lower total bond energy, it will be exothermic.

The reason for this is the law of conservation of energy, which states that energy cannot be created or destroyed; it can only change forms. In this case, it would mean that whatever energy was used to break the bond will be released if the bond is reformed. For example:

Suppose we have a C-H bond somewhere, and we wanted to break that bond apart into a C and an H. We'd have to put in some amount of energy. Let's call this amount 'x'. Once we put in x energy, by say, adding heat, the C-H bond will break apart. What happened to 'x' though? The conservation of energy law says that 'x' didn't just disappear; it just took on another form, in this case exciting the electrons in C and H. Some of the energy went to the C atom and some went to the H atom. If the C-H bond reformed, then 'x' would be released again. If the C went off and recombined with a different molecule (let's say a Cl), and so did the H (with an F, for instance). Then the energy released from the new pairings would be 'x' plus whatever energy the Cl and F had stored.

Bond energy is usually expressed in kJ/mol, where the 'mol', is a mole, or 6.02*1023, of the bonds in question, (e.g. a mole of C-H bonds). Several things can affect bond energies:
1. Type of atoms
2. Whether it's a single, double, or triple bond
3. Other atoms in the molecule (an O-H bond will have a different bond energy in H2O than in NaOH)

Endothermic and Exothermic Reactions Practice Quiz

Reactions are always either Endothermic or Exothermic (even if only slightly):

A. True
B. False
The correct answer here would be A.

Bond energy is the amount of energy that must be added to make a bond:

A. True
B. False
The correct answer here would be B.
When bonds are formed, energy is released. Bond energy is either the amount of energy that is released when a bond is made, or added when a bond is broken.

Positive ΔH means energy is being released, while negative ΔH means energy is being stored.

A. True
B. False
The correct answer here would be B.
It's the exact opposite of that. Positive ΔH means energy is being stored, while negative ΔH means energy is being released.

If a reaction is absorbing heat, it will feel colder.

A. True
B. False
The correct answer here would be A.
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