Chemical Reactions

What Happens During a Chemical Reaction?

Recall that during a physical change, such as melting or freezing of a substance, the physical properties of the substance change, but its chemical properties do not. In contrast, a chemical change occurs when the chemical properties of matter change as new substances form. Rusting is an example of a chemical change. When iron rusts, metallic iron reacts with oxygen and a substance made up of iron and oxygen atoms forms. That substance is rust. The chemical properties of rust are different from those of the pure iron. The physical properties of rust, such as its color and hardness, are also different from those of iron. When matter changes chemically, both its physical properties and its chemical properties change.

Changes in Matter

How do chemical changes occur? The process by which one or more substances change into new substances is called a chemical reaction. In a chemical reaction, the atoms of the reactants-the substance or substances that existed before the reaction-are rearranged and form products-the substance or substances that exist after the reaction is completed. A forest fire is an example of chemical reactions on a very large scale. Molecules that make up trees and other organic matter in the forest combine with oxygen in the atmosphere and produce carbon dioxide, water vapor, and other compounds. At the same time, the chemical reaction releases a lot of energy as heat.

When a chemical reaction takes place, atoms rearrange as the chemical bonds between some atoms are broken and new chemical bonds form between other atoms. This rearrangement of atoms causes new substances to form.

The rearrangement of the atoms of hydrogen molecules and oxygen molecules is illustrated here. The models to the left of the arrow show the bonds that exist between hydrogen atoms and between oxygen atoms. To the right of the arrow, each new bond connects two hydrogen atoms to an oxygen atom. The chemical and physical properties of water are very different from those of hydrogen or oxygen.


Law of Conservation of Mass

Atoms do not appear or disappear during a chemical reaction. All the atoms in the reactants will be in the products in exactly the same numbers as before the reaction, but they will be in a different arrangement. This idea is expressed in one of the most important laws of chemistry, the law of conservation of mass, which states that in a chemical reaction, the combined mass of all the reactants is equal to the combined mass of all the products. Matter is conserved; it is neither created nor destroyed.

The illustration of the formation of water shows that two molecules of hydrogen react with each molecule of oxygen and form two molecules of water. You can see that the reactants must react in these proportions to follow the law of conservation of mass. What would happen if the ratio of hydrogen and oxygen molecules in the reaction mixture were equal, for example, 50 molecules of hydrogen and 50 molecules of oxygen? In this case some of the molecules cannot react. After 25 molecules of oxygen have ‘ reacted, forming 50 molecules of water, no hydrogen molecules remain.

In this situation, hydrogen is called the limiting reactant, which is the reactant that is completely consumed when the reaction goes to completion and limits the amount of product formed. The final mixture will contain 50 molecules of water and 25 molecules of unreacted oxygen. The actual numbers of molecules involved in a reaction would be many orders of magnitude higher. These numbers are used only for demonstration.

Think about Science

Directions: Answer the following questions.

  1. What change always indicates that a chemical reaction has occurred?
  2. What is the limiting reactant when 2,000 molecules of hydrogen and 750 molecules of oxygen are available for the formation of water?

How Are Chemical Reactions Represented?

Hydrogen and oxygen molecules react to form water. That is one way to describe the chemical reaction that occurs between hydrogen and oxygen. Chemists do not explain chemical processes in sentences. They use a form of notation that represents the reactants and the products and shows the reaction process, similar to the way that a mathematical equation represents an operation in mathematics. This notation uses specific symbols that can be recognized by all chemists.

Symbol Meaning
produce or forms
aqueous, a substance is dissolved in water
the reactants are heated
the reactants are exposed to light
an electric current is passed through the reactants
the reaction is carried out at 0° C

Chemical Equations

A chemical equation is a form of notation that shows the reactants and products involved in a chemical reaction. The reactants are shown on the left, and the products are shown on the right, with an arrow between to indicate the direction in which the reaction proceeds. The arrow indicates that something is produced or formed. Words above the arrow can describe the conditions under which the reaction takes place. They can show, for example, if the reaction requires heat to occur. Each reactant is separated in the equation by a plus sign ( + ), as shown in the equation below. If there are two or more products, each product is also separated by a plus sign. The equation for the formation of water is written as follows:

Notice that in this equation, a number appears in front of some of the chemical formulas. These numbers, called coefficients, indicate how many molecules of each substance are involved in the reaction. If there is no coefficient in front of a formula, the coefficient is assumed to be 1. The coefficient shows the ratio of molecules in a reaction. In this reaction, the ratio is two hydrogen atoms to one oxygen atom, so two hydrogen molecules will react with one oxygen molecule and produce two water molecules.

Balancing Chemical Equations

Look at the following chemical equation written without coefficients:

Count the number of atoms on each side of the equation Recall that the subscripts indicate the number of atoms in one molecule of a compound. There are two hydrogen atoms and two oxygen atoms on the reactant side of the equation There are two hydrogen atoms but only one oxygen atom on the product side of the equation The number of atoms on each side of the equation is not the same. This means that the reaction as written cannot take place, because it violates the law of conservation of mass.

Coefficients are used to write a balanced chemical equation which is an equation with the same number of atoms on both sides of the arrow. Adding coefficients to a chemical equation does not change the reactants or products. It simply shows the ratios of substances involved in the reaction. Balancing a chemical equation is often a trial-and-error process.

To balance the equation place the coefficient 2 in front of the formula for water. This signifies that two molecules of water are produced for every one molecule of oxygen that reacts.

Notice that oxygen is now balanced in the equation There are two oxygen atoms on the left side of the equation and two on the right. Hydrogen, however, is now unbalanced. To balance hydrogen, place the coefficient 2 in front of hydrogen on the reactant side of the equation.

The number of atoms of each element on the left is now equal to the number of atoms of that element on the right. The equation is balanced.

  How To Balance a Chemical Equation
Step 1 Identify the reactants and products and write their chemical formulas on the appropriate sides of the equation.
Step 2 Count the number of atoms of each element on each side of the equation. If you need to, use a table to keep track of the numbers.
Step 3 Try coefficients that will balance the equation. Start with elements that appear in only one molecule on each side of the equation, if possible. Never change the subscripts in a chemical formula to balance an equation. That changes the identity of the substances.
Step 4 Check to be sure that you have the same number of atoms of each element on both sides of the equation.
Think about Science

Directions: Write a balanced equation for the chemical reactions.

Energy Changes in Chemical Reactions

What causes the bonds between hydrogen atoms in a hydrogen molecule and between the oxygen atoms in an oxygen molecule to break at the start of the chemical reaction that forms water? It takes energy to break the chemical bonds between atoms or ions in a compound. This means that all chemical reactions require some energy to get started. The minimum amount of energy required for a chemical reaction to occur is called activation energy. Different reactions required different amounts of this energy to get started. Light, heat, and electricity are some common sources of activation energy.

Exothermic Reactions

While energy can be released in the form of light, motion, or electricity, in most chemical reactions, energy is released in the form of heat. Reactions that release heat are called exothermic reactions. The energy given off in a chemical reaction comes from the original bonds that are broken (the reactant bonds) and the new bonds that are formed (the product bonds) during the reaction. Although all reactions require activation energy, some reactions give off energy once they are started. If the products of a reaction are more stable-that is, their bonds require less energy- than the reactants energy will be released as the products are formed. Think of logs burning in a campfire or fireplace. Logs do not start burning spontaneously. It takes energy to get them to start burning. Once burning, however, they give off a great deal of energy in the form of light and heat. All combustion reactions are exothermic and involve a release of energy as heat.

Endothermic Reactions

Some chemical reactions require not only activation energy but also a continuous supply of energy in order for the reaction to continue. Such reactions stop when that energy source is removed. The reactants no longer react, and the products no longer form. Overall these reactions absorb more energy than they give off. Reactions that absorb heat are described as endothermic reactions. Chemical reactions involved in baking cookies, for example, are endothermic reactions driven by thermal energy. In this case, the heat is from the oven. Reactions absorb energy when the products formed are less stable-their bonds require more energy- than the reactants.

Representing Energy Changes

The energy changes that occur during a chemical reaction can be modeled on a graph plotting the energy stored in chemical bonds against the progress of the reaction. These diagrams show that the products of an endothermic reaction have more chemical energy than the reactants, while the products of an exothermic reaction have less energy than the reactants.

Endothermic Reaction

Exothermic Reaction

Think about Science

Directions: Fill in the blank.

  1. When vinegar and a solution of baking soda are mixed, the final solution is colder than either of the original solutions. This observation indicates that the reaction between vinegar and baking soda is an [ blank ] reaction.
  2. The thermal energy released by a forest fire indicates that combustion is an [ blank ] reaction.

Chemical-Reaction Rate

Some reactions happen very quickly in just fractions of a second. Others take hours or even years to complete. A reaction’s rate depends on the nature of the substances involved. It also depends on how often the particles of those substances collide with one another. The more the particles collide, the more likely the substances are to react, and the faster the reaction will be.

Effect of Temperature on Reaction Rate

Heating a substance increases the speed at which the particles move. The faster the particles move, the more frequently they will collide with the particles of other substances. Adding heat to most reactions will cause the reactions to occur more quickly. Cooling reactions tends to cause them to occur more slowly.

Effect of Concentration on Reaction Rate

Concentration describes how much of a substance is in a given unit of volume. For example, if 20 g of sodium chloride is dissolved in 11 of water, the solution contains 20 g of salt per liter of solution. If an additional 20 g of salt is added to the solution, the change in volume is too small to notice. The concentration of the solution is 40 g of salt per liter of solution.

A high concentration of reactants means there are a great many particles per unit volume. There are more reactant particles available for collisions. More collisions occur, and more product is formed in a certain amount of A low concentration of reactants means there are fewer particles per unit volume. With fewer reactant particles available, there will be fewer collisions. Less product will form in a certain amount of time.