Section 3: Energy and Rates of Chemical Reactions

Every reaction involves energy. Chemical reactions involve chemical energy.

Energy is released in an exothermic reaction.

Energy is absorbed in an endothermic reaction.

Exothermic Reaction

If the chemical energy of the reactants is greater than the chemical energy of the products, then the excess energy is released. The word energy is added to the product side of the equation if the reaction is exothermic.

Examples of exothermic reactions are light energy, electrical energy, and thermal energy.

Endothermic Reactions

If the chemical energy of the reactants is less than the energy of the products it is an endothermic reaction.

The word + energy is written on the reactant side of the equation

Energy Is Conserved – It’s A Law!

Mass is never created or destroyed in chemical reactions

This is the law of conservation of energy.

The total amount of energy is the same before and after a reaction.

So how does a reaction get started?

A reaction needs activation energy.

For example, striking a match against a cement sidewalk produces friction between the match and the sidewalk. This friction breaks the bonds of the chemical in the head of a match and the result is fire, and the formation of new bonds.

Factors affecting the rate of reaction

The rate of reaction is a measure of how rapidly the reaction takes place.

Four factors can affect this rate:TemperatureConcentrationSurface areaPresence of a catalyst or inhibitor

Factors affecting the rate of reaction

Temperature:

An increase in temperature increases the rate of reaction, particles more quickly, thus breaking bonds to form new substances.

Factors affecting the rate of reaction

Concentration:

Generally, increasing the concentration or reactants (left side) increases the rate of reaction because there are more reactant particles to collide so more particles react per second.

Concentration is the measure of the amount of one substance dissolved in another.

Factors affecting the rate of reaction

Surface Area:

Increasing the surface area (the amount of exposed surface) of solid reactants increases the rate of reaction.

Let’s look at this QuickLab.

Factors affecting the rate of reaction

Catalysts and Inhibitors:

A catalyst is a substance that speeds up the reaction without being permanently changed by lowering the activation energy needed to begin the reaction which allows the reaction to happen more rapidly.

Enzymes are a good example of a catalyst.

Factors affecting the rate of reaction

Catalysts and Inhibitors:

An inhibitor is a substance that slows down or stops a chemical reaction.

Preservatives in food are a good example of inhibitors.