Learning Objectives

Chemical Reactions

Chemical changes take place when molecules or elements interact with other elements or molecules to form new chemical compounds. In order for a reaction to take place between molecules and or atoms, these molecules must come into contact with each other.

An example of a chemical reaction can be shown by the reaction of ammonia with hydrogen chloride to form ammonium chloride.  This reaction is usually shown by a shorthand method called a chemical equation. The chemical equation for this reaction is...

NH3   +   HCl      NH4Cl

This equation does not clearly show what has happened. In order for these two molecules to react, the pair of electrons on nitrogen must collide with the hydrogen atom of the hydrogen chloride on the side exactly opposite of the chlorine atom. 

This collision must not only be precise as to the angle of the collision, it must have enough energy to break the bond between the hydrogen atom and the chlorine atom and form a new bond between the hydrogen atom and the nitrogen atom. Energy is released when a bond is formed. If all of these requirements are met, a reaction occurs forming a new compound.

The rate of a chemical reaction depends on all of the above factors.  The reaction rate is measured by the change in concentration of one of the reactants or products over a measured period of time.

If some reaction condition is changed, the reaction rate will be changed.

Reaction coordinate diagrams are used to visualize the energy changes in chemical reactions. Some initial energy must be applied to any reaction in order to get the reaction started. This energy is called the energy of activation Ea.

If a reaction releases more energy than it takes to keep it going, it is called an exothermic reaction.

If a reaction requires a constant application of energy to keep it going, it is called an endothermic reaction.

A catalyst is something that, when added to a chemical reaction, will increase the reaction rate without undergoing a permanent change.  Although it appears that only Ea is lowered for a catalyzed reaction, the actual reaction pathway must change due to the involvement of the catalyst with the reactants. The energy released for the reaction remains the same. Catalysts are used extensively in biochemical reactions in order to decrease the energy demands for the animal or plant.

Matter can neither be gained nor lost in a chemical reaction. The number and type of atoms in the reactants must exactly equal the number and types of atoms in the products. The arrangement of the atoms will be different because new compounds are formed. Therefore, we must balance chemical equations with respect to the numbers of all of the atoms that are involved in the reaction.

Balancing Chemical Equations

Chemical equations must be balanced with respect to all atoms and the atoms must exist in real compounds.

Balance equations by using the following rules...

1.Select the compound with the largest number of atoms of an element not including H or O or groups.

2.Balance this number of atoms with the corresponding atom on the other side of the equation with a coefficient (temporarily a fraction if necessary)...never change the compound.

3.Next balance any groups if they are present.

4.Now balance any H and O if they are present.

Hydrogen reacts with oxygen to form water. Let's try to balance this reaction.

Antacids contain compounds such as CaCO3, which will react with excess stomach acid, HCl. The unbalanced reaction equation is...

CaCO3   +   HCl    CaCl2   +   H2O   +   CO2

Let's balance this reaction.

Another example is the burning of methane gas. Methane reacts with oxygen to form carbon dioxide and water.  The unbalanced equation can be written as...

CH4   +   O2      CO2   +   H2O

Try balancing this reaction.

Equilibrium Reactions

Some reactions will yield essentially 100% products. These reactions either have very reactive reactants or something happens in the reaction to drive it to completion. An example would be the formation of a gaseous product or a solid that precipitates out of the reaction mixture. These reactions are designated with a single arrow that points to the products as in the following equation.

reactants     products

Most reactions are called equilibrium reactions because the reactants form products and then the products reverse the reaction and go back to the original reactants. In the following example all the compounds represented by the letters A, B, C and D will exist at any given time.

The above reaction equation does not mean that there are equal concentrations of the reactants and the products.  The ratio of the concentrations of the products versus the reactants is given by the equilibrium constant for the reaction.  The Equilibrium Constant is a number representing the ratio of products to reactants in a reversible reaction and must be experimentally determined for each reaction.

The equilibrium expression for the above reaction can be written in the following manner.

K = the equilibrium constant and the expressions in the square brackets represent the concentrations of the reactants and products.

If a coefficient greater than one is used in the balanced reaction equation, this coefficient will appear in the equilibrium equation as an exponent as shown in the following example.

In an equilibrium reaction, if the concentration of one of the products is increased by adding some of that compound to the reaction mixture, the concentration of the reactants must increase because the constant k does not change. What this means is that less of the reactants will react to form product.

Categories of Chemical Reactions

There are many categories of chemical reactions. The broadest category consists of acid-base reactions and the second broadest category consists of oxidation-reduction reactions.

Chemists also use the category terms; combination reactions, decomposition reactions and single and double replacement reactions.  Almost all of the reactions in these categories are really either acid-base or oxidation-reduction reactions.

Acid-base reactions could be defined in the broadest sense to include all ionic reactions, but usually are considered reactions between what chemists traditionally define as acids and bases. A neutralization reaction is defined as a reaction in which a proton (hydrogen cation) is transferred between two reactants. The reaction between HF and NH3 is a neutralization reaction where a hydrogen atom from hydrogen fluoride is transferred over to the ammonia.

An oxidation-reduction reaction is a reaction where an electron or electrons are transferred between two reactants. An example of this was the reaction between chlorine and sodium to form sodium chloride.

A combination reaction is a reaction where two or more reactants (elements or compounds) react to form only a single product.

Some examples of this type of reaction include the following reactions, both of which are oxidation-reduction reactions.

metal   +   oxygen      metal oxide

4 Na   +   O2      2 Na2O

nonmetal   +   oxygen     nonmetal oxide

C   +   O2      CO2

A decomposition reaction is a reaction where a single compound undergoes a reaction to form two or more products.

Some examples of this type of reaction include the following. The first two are also oxidation-reduction reactions.

2 H2O   +   direct electric current      2 H2   +   O2

C12H22O11 sugar   +   heat      12 C   +   11 H2O

Caramelized sugar is brown due to the carbon that is present.

2 NaHCO3   +   heat      Na2CO3   +   CO2   +   H2O

Sodium bicarbonate can be used to extinguish fires because the heat from the fire liberates CO2 which then smothers the flames. This is not an oxidation-reduction reaction.

A single replacement reaction is a reaction where one element reacts with a compound and replaces another element in that compound. An example of this type of reaction is...

Fe   +   CuSO4       FeSO4   +   Cu

If you immerse an iron nail in a copper sulfate solution, it will come out coated with copper metal.

A double replacement reaction is a reaction where two positive ions exchange partners. These reactions usually involve the formation of a precipitate (the most common examples) or a gas as one of the products. The formation of a precipitate or a gas will drive the reaction to completion. Some examples of this type of reaction include the following...

AgNO3   +   NaCl     AgCl    +   NaNO3

CaCO3   +   2 HBr     CaBr2   +   CO2   +   H2O

The arrow pointing down means that a precipitate is formed. The arrow pointing up means that a gas is formed.

Problems

1. The reaction rate will _?_ for every ten degree increase in temperature.

2. The reaction rate will _?_ if the concentration of one of the reactions is tripled.

3. The following reaction coordinate diagram illustrates a _?_ reaction.

4. Try to balance the following equations.

Mg   +   O2      MgO answer

Na  +   H2O      NaOH   +   H2 answer

S   +   O2      SO3 answer

Al   +   O2      Al2O3 answer

C2H6   +   O2      CO2   +   H2O answer

6. Write an equilibrium equation for the following reaction.

7. If sodium phosphate is added to the reaction mixture of the previous reaction, what will happen to the overall reaction?

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Copyright January 2001 by Richard C. Banks...all rights reserved.