Energy of Reactions

Types of Reactions
Molecular Enthalpy
Why are energy losing reaction favoured?

Do all reactions release heat? Are only reactions that release heat spontaneous? Watch the following video demonstration and answer the questions asked.

click

Demonstration: Reaction types Click on the camera to watch a demonstration related to reactions and energy.

Let's examine energy changes that occur during reactions.

All reactions, whether they are physical, chemical or nuclear undergo some energy change. Reactions can be classified based on whether the change involves an absorption of energy or a release. These reactions are termed:

a) Exothermic: reaction in which energy is released by the system into the surrounding environment.
b) Endothermic: reaction in which energy is absorbed by the system from the surrounding environment

What is meant by the system and the surroundings?

The system refers to the substances undergoing the change, whether the change is physical, chemical or nuclear.
The surrounding refers to the immediate environment that the substances are in. In the demonstration above the surroundings for the Ammonium thiocyanate and barium hydroxide, are the glass beaker, the air, the water below the beaker and the board.

Top of Page

How do molecules in a system absorb or release energy?

The sum of the energies in a molecule is called it's molecular enthalpy. Molecules can store energy in a number of ways or forms. These can be divided into two groups:

Kinetic energies (energies of motion). These energies include:

a) Vibrational energy caused by the movement of atoms back and forth along a bond. Vibrational energy is the only kinetic energy in a solid and it increase with the temperature of the solid.
b) Rotational energy caused by the rotating of atoms about an axis. Molecules gain rotational energy when they enter a liquid state, and this energy increases with temperature of the liquid.
c) Translational energy caused by the molecules moving from one place to another in a straight line. Translational motion first appears in a gaseous state and increases with temperature.

Potential energies (energies of position). These energies include:

a) Potential Energy related to the relative position of the molecules.
These forms of energies are involved when a substance changes state between solid and liquid or liquid and gas or solid and aqueous. These forces of attraction can be divided into the following groups.

i) Van der waals forces (dispersion force) for nonpolar molecules
ii) Dipole- dipole forces in polar molecules and ions including hydrogen bonding.

b) Bond energies: Energy is stored in the formation of bonds or forces of attraction between atoms in the molecule.
These forms of energies are involved when chemical change occurs. Energy is either stored in the form of bonds or released when bonds are broken

When systems absorb energies one of the above energies increases. When systems release energy, the energy comes from one of these forms or types. The laboratory activity below illustrate this idea.

Activity: Molecular Enthalpy click on the test tube to perform the activity

Manufactures use these changes in energy in a useful way to create hot and cold packs that are now used for a wide variety of ailments. Complete the activity below to investigate hot and cold packs.

Hot and Cold Packs Click on the picture and complete the following research.

Top of Page

Why are chemical reactions that lose energy favoured in nature?

In nature changes that maximize the stability of the atoms or ions in a compound are favoured. When a chemical reaction occurs the bonds in the substances involved rearrange themselves. The formation of chemical bonds releases energy, while the breaking of bonds requires energy. If more energy is released by the formation of bonds than absorbed by the breaking of bonds the reaction is exothermic.

Exothermic Reaction	Reactant		Products
	Bonds broken		Bonds formed
	Energy absorbed	<	Energy released

These types of reactions are favoured in nature and provide a drive or reason why reactions occur. (In these systems the total energy of the system decreases as we move from reactant to product.)

Endothermic reactions are the opposite of this type of change. The amount of energy absorbed is greater than the energy released and therefore energy is absorbed overall.

Endothermic Reaction	Reactants		Products
	Bonds broken		Bonds formed
	Energy absorbed	>	Energy released

click

If Exothermic reactions are favoured in nature, why do endothermic reactions occur at all? Click the question mark, to answer this question

Back to Reasons

Top of Page