A chemical reaction involves a chemical change in molecules, atoms, or ions when two or more of these interact. It is important to note that chemical reactions are not the same as state changes. For example, liquid water changing to ice is not a chemical reaction because water and ice have the same chemical properties, just different physical ones. A chemical reaction occurs between two reactants (substances) that form a new substance with different chemical properties than either of the two initial reactants.
Reactants are the substances that are consumed or altered in the chemical reaction, while products are substances formed as a result of the chemical reaction. Equations are usually written with the reactants on the left, the products on the right, and an arrow between them. The state of the chemical compounds are sometimes noted using the labels s (solid), l (liquid), g (gas), or aq (aqueous, meaning a solution).
Chemical reactions generally occur in two directions. A reaction can move “forward” (from reactants to products), and in “reverse” (from products to reactants). Often, the arrow in a chemical equation has a head on each side, signifying that the reaction occurs in both directions.
The equilibrium point of a reaction is defined as the point where both the forward and reverse reactions are occurring at equal rates simultaneously—products are turning into reactants and reactants back into products. This produces a state in which, while the reaction is still taking place, no net change in concentration of reactants or products is occurring.
In the equation below, H2 and O2 are the reactants, while water (H2O) is the product.
2H2 + O2 → 2H2O
In this equation, the number 2 is called a coefficient, and it describes the number of atoms or molecules involved in the reaction. In this reaction, four hydrogen atoms (two molecules of H2) react with two oxygen atoms. Note that the products also contain four hydrogen and two oxygen molecules. When chemical equations are written, they must include the same number of each atom on both the reactant and product side of the arrow. This is an important step because chemical reactions adhere to the law of conservation of matter, which states that matter is neither created nor destroyed in a chemical reaction.
In order to balance the equation above, first examine the initial equation without coefficients, which looks like this:
H2 + O2 → H2O
This equation is unbalanced: there are two H atoms on each side, but the reactant side has two O atoms while the product side only has one. To fix this discrepancy, a coefficient of 2 is added in front of the product, H2O, making the number of O atoms equal on both sides of the equation:
H2 + O2 → 2H2O
Now there are four H atoms on the product side while there are only 2 on the reactant side. This means that in order to finish balancing the equation, a coefficient of 2 must be added in front of H2, so that there are four H atoms on the reactant side as well:
2H2 + O2 → 2H2O
Remember that in a chemical reaction, only the coefficients may be changed in order to balance it; the subscripts must not be changed. This would be like changing the actual chemical in the equation.
There are several common types of chemical reactions, including decomposition, substitution, and combustion reactions.
Decomposition reactions are a common class of reaction, consisting of the separation of a compound into atoms or simpler molecules:
General Reaction: AB → A + B
2H2O2 → 2H2O + O2
Single replacement reactions are those in which a part of one molecule is replaced by another atom or molecule. Reactivity in single substitutions is determined by the activity series: elements on the list will replace any element that is below it on the list.
General Reaction: AB + C → AC + B
CH4 + Cl2 → CH3Cl + HCl
In a double replacement reaction, two parts of two different molecules swap places:
General Reaction: AB + CD → CB + AD
CuCl2 + 2AgNO3 → Cu(NO3)2 + 2AgCl
Combustion or burning reactions are high-temperature reactions in which a great deal of heat is released. In combustion reactions, oxygen is a reactant and carbon dioxide and water are produced. Because of the substantial amount of heat energy produced by combustion reactions, they have been important means of generating energy throughout human history, including combustion of fossil fuels, coal, and oil.
General Reaction: CxHx + O2 → CO2 + H2O
2C8H18 + 25O2 → 16CO2 + 18H2O
Exothermic reactions are defined as those which produce energy, whereas endothermic reactions need energy in order to occur. Regardless of whether energy is absorbed or released overall, every chemical reaction requires a certain amount of energy in order to begin. This amount is referred to as the activation energy.
Collisions of reactant particles supply the activation energy for a reaction. The more particles collide, the more energy will be produced. Thus, the more often particles collide, the more likely a reaction is to occur. However, it is quite possible that though some particles collide, not enough energy is generated for an actual reaction to occur.
Given the variability in activation energies of a reaction, as well as variation in the frequency of reactant particle collisions, not all chemical reactions occur at the same rate. A number of variables affect the rate of reaction, including temperature, pressure, concentration, and surface area. The higher the temperature, pressure and concentration, the more likely particles are to collide and thus the reaction rate will be higher. The same is true of surface area for a reaction between a solid and a liquid in which it is immersed. The larger the surface area, the more solid reactant particles are in contact with liquid particles, and the faster the reaction occurs.