Energy Relations in Chemistry: Thermochemistry

The Nature of Energy

Sugar you eat is "combusted" by your body to produce CO2 and H2O. During this process energy is also released.

This energy is used (among other things) to:

Chemical reactions involve changes in energy:

Our society as an "organism" requires energy: 90% of our energy comes from chemical reactions involving the combustion of petroleum products.

The study of energy and its transformations is known as thermodynamics

This area of study began when steam engines were developed during the industrial revolution and the relationships between heat, work and energy for different fuels was being studied.

The relationship between chemical reactions and energy changes is known as thermochemistry

The Nature of Energy

A Force is any kind of push or pull exerted on an object.

If you move an object against some force, work is being done.

The amount of work (w) being done is relative to the distance (d) the object is moved and the strength of the force (F) against the object:

w = F * d

Energy, in the form of work, must be used to move an object against a force.

When we do work, our body temperature increases (and we sweat to cool us down). Our bodies are generating Heat energy.

Heat is an energy which is transferred from one object to another depending on the relative temperature:

Energy is the capacity to do work or to transfer heat

Objects can possess energy due to their motions and positions, as kinetic energy and potential energy.

Kinetic and Potential Energy

Kinetic energy is the energy of motion. The magnitude of the kinetic energy (Ek) of an object depends upon its mass (m) and velocity (v):

In other words, both the mass and the speed of an object determines how much energy it has, and thus, how much work it can accomplish.

An object can also possess energy based upon its position relative to other objects - a type of stored up energy, or "potential energy"

Potential energy is the result of the attractions and repulsion between objects. An electron has potential energy when located near a proton due to the attractive electrostatic force between them.

Chemical and thermal energy are terms which relate to potential and kinetic energy at the atomic level

Energy Units

The SI unit for energy is the joule ("J"). In honor of James Prescot Joule (1818-1889) a British Scientist who investigated work and heat. (Note: SI is short for the French term Systeme International d'Unites. Which defines metric standards).

Kinetic energy for example is defined as:

Thus, the joule must have units of:


and, in fact, 1 joule is defined as:

Traditionally, energy changes accompanying chemical reactions have been expressed in calories, which is a non-SI unit (though still widely used).

1 calorie = 4.184 J

Systems and surrounding

When we focus on a study of energy changes we look at a small, well defined and isolated part of the universe - the flask or container the reactants are in. This is called the system.

Everything else is called the surroundings.

Usually the system is isolated from its surroundings such that there will be an exchange of energy between system and surroundings, but not matter.

Thus, the system will contain the same mass after an experiment, but the system can lose or gain energy (in the form of heat, work, or both).

Lowering the energy of the system

Systems tend to attain as low an energy as possible

Systems with a high potential energy are less stable and more likely to undergo change than systems with a low potential energy.

Like a shopping cart at the top of a hill, chemical reactants move spontaneously toward a lower potential energy when possible.

1996 Michael Blaber