What is the relationship between the atoms of the reactants and the atoms of the products after a chemical reaction?

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Chris
Chemical reactions: They’re fundamental to chemistry; they make new things by rearranging other things. They can blow stuff up ... or freeze things quickly. In short, they are awesome.

Brittny
But why do some chemical reactions release massive amounts of energy, while others absorb energy? In a chemical reaction, the main change that occurs relates to the way atoms are connected (or bonded) to each other. In order to change those connections, bonds must be broken and new bonds must be formed. Let’s break down how energy is transferred in these reactions.

Chris
To understand the energy implications of chemical reactions, it’s important to keep in mind two key ideas:

It takes energy to break bonds.
Energy is released when bonds are formed.

To understand this, consider the chemical reaction between vinegar and baking soda. That’s right —the classic baking soda volcano experiment. The chemical reaction behind this science fair favorite involves baking soda—also known as sodium bicarbonate to chemists—and vinegar, otherwise known as acetic acid. These compounds react to form the molecules sodium acetate, water, and carbon dioxide. The baking soda and vinegar are called the reactants. The sodium acetate, water, and carbon dioxide that are formed are called the products. Before the atoms in acetic acid and sodium bicarbonate can be rearranged to form the products, some of the bonds between the atoms in those molecules must be broken, and because the atoms are attracted to one another, it takes energy to pull them apart. Then, when the products are formed (sodium acetate, water, and carbon dioxide) energy is released because atoms that have an attraction for one another are brought back together.

By comparing the energy absorbed when bonds in the reactants are broken with the energy released when bonds in the products are formed, you can determine whether a chemical reaction releases energy or absorbs energy overall.

Brittny Chemical reactions that release energy are called exothermic. In exothermic reactions, more energy is released when the bonds are formed in the products than is used to break the bonds in the reactants. Exothermic reactions are accompanied by an increase in temperature of the reaction mixture.

Chemical reactions that absorb (or use) energy overall are called endothermic. In endothermic reactions, more energy is absorbed when the bonds in the reactants are broken than is released when new bonds are formed in the products. Endothermic reactions are accompanied by a decrease in temperature of the reaction mixture.

Chris You can use energy level diagrams to visualize the energy change during a chemical reaction. To understand these diagrams, compare the energy level of the reactants on one side with that of the products on the other side.

Consider, for example, a diagram that charts the energy change when a candle burns. Wax (C34H70) combusts in the presence of oxygen (O2) to yield carbon dioxide (CO2) and water (H2O). Because more energy is released when the products are formed than is used to break up the reactants, this reaction is exothermic.

Brittny All of this stuff relates to thermodynamics—the study of heat and its relationship to energy and work. Using thermodynamics, you’ll learn how to calculate the precise amount of energy used or released by chemical reactions. Classifying a chemical reaction as exothermic or endothermic is simple. It comes down to weighing the energy needed to break bonds in the reactants with the energy released when the products are formed. It’s a simple idea, but one with a lot of power.

In a chemical reaction the total mass of all the substances taking part in the reaction remains the same. Also, the number of atoms in a reaction remains the same. Mass cannot be created or destroyed in a chemical reaction.

Law of conservation of mass

The law of conservation of mass states that the total mass of substances taking part in a chemical reaction is conserved during the reaction.

Table 13.1 illustrates this law for the decomposition of hydrogen peroxide.

We will use the reaction of hydrogen and oxygen to form water in this activity.

Coloured modelling clay rolled into balls or marbles and prestik to represent atoms. Each colour will represent a different element.

Build your reactants. Use marbles and prestik or modelling clay to represent the reactants and put these on one side of your table. Make at least ten ($\text{H}_{2}$) units and at least five ($\text{O}_{2}$) units.
Place the $\text{H}_{2}$ and $\text{O}_{2}$ units on a table. The table represents the “test tube” where the reaction is going to take place.
Now count the number of atoms ($\text{H}$ and $\text{O}$) you have in your “test tube”. Fill in the reactants column in the table below. Refer to Table 13.1 to help you fill in the mass row.
Let the reaction take place. Each person can now take the $\text{H}$ and $\text{O}$ unit and use them to make water units. Break the $\text{H}$ and $\text{O}$ units apart and build $\text{H}_{2}\text{O}$ units with the parts. These are the products. Place the products on the table.
When the “reaction” has finished (i.e. when all the $\text{H}$ and $\text{O}$ units have been used) count the number of atoms ($\text{H}$ and $\text{O}$) and complete the table.
What do you notice about the number of atoms for the reactants, compared to the products?
Write a balanced equation for this reaction and use your models to build this equation.

	Reactants	Products

Number of molecules
Mass
Number of atoms

You should have noticed that the number of atoms in the reactants is the same as the number of atoms in the product. The number of atoms is conserved during the reaction. However, you will also see that the number of molecules in the reactants and products are not the same. The number of molecules is not conserved during the reaction.

To prove the law of conservation of matter experimentally.

Reaction 1:

3 beakers; silver nitrate; sodium iodide; mass meter

Reaction 2:

hydrochloric acid; bromothymol blue; sodium hydroxide solution; mass meter

Reaction 3:

any effervescent tablet (e.g. Cal-C-Vita tablet), balloon; rubber band; mass meter; test tube; beaker

Always be careful when handling chemicals (particularly strong acids like hydrochloric acid) as you can burn yourself badly.

Reaction 1

Solution 1: In one of the beakers dissolve $\text{5}$ $\text{g}$ of silver nitrate in $\text{100}$ $\text{mL}$ of water.
Solution 2: In a second beaker, dissolve $\text{4,5}$ $\text{g}$ of sodium iodide in $\text{100}$ $\text{mL}$ of water.
Determine the mass of each of the reactants.
Add solution 1 to solution 2. What do you observe? Has a chemical reaction taken place?
Determine the mass of the products.
What do you notice about the masses?
Write a balanced equation for this reaction.

Reaction 2:

Solution 1: Dissolve $\text{0,4}$ $\text{g}$ of sodium hydroxide in $\text{100}$ $\text{mL}$ of water. Add a few drops of bromothymol blue indicator to the solution.
Solution 2: Measure $\text{100}$ $\text{mL}$ of $\text{0,1}$ $\text{mol·dm$^{-3}$}$ hydrochloric acid solution into a second beaker.
Determine the mass of the reactants.
Add small quantities of solution 2 to solution 1 (you can use a plastic pipette for this) until a colour change has taken place. Has a chemical reaction taken place?
Determine the mass of hydrochloric acid added. (You do this by weighing the remaining solution and subtracting this from the starting mass)
Compare the mass before the reaction to the total mass after the reaction. What do you notice?
Write a balanced equation for this reaction.

Reaction 3

Half fill a large test tube with water.
Determine the mass of the test tube and water.
Break an effervescent tablet in two or three pieces and place them in a balloon.
Determine the mass of the balloon and tablet.
Fit the balloon tightly to the test tube, being careful to not drop the contents into the water. You can stand the test tube in a beaker to help you do this.
Determine the total mass of the test tube and balloon.
Lift the balloon so that the tablet goes into the water. What do you observe? Has a chemical reaction taken place?
Determine the mass of the test tube balloon combination.
What do you observe about the masses before and after the reaction?

Fill in the following table for the total mass of reactants (starting materials) and products (ending materials).

Reaction 1

Reaction 2

Reaction 3

Reactants

Products

Add the masses for the reactants for each reaction. Do the same for the products. For each reaction compare the mass of the reactants to the mass of the products. What do you notice? Is the mass conserved?

In the experiment above you should have found that the total mass at the start of the reaction is the same as the mass at the end of the reaction. Mass does not appear or disappear in chemical reactions. Mass is conserved, in other words, the total mass you start with is the total mass you will end with.

Textbook Exercise 13.2

Complete the following chemical reactions to show that atoms and mass are conserved. For each reaction give the total molecular mass of the reactants and the products.

Hydrogen gas combines with nitrogen gas to form ammonia.

Solution not yet available

Hydrogen peroxide decomposes (breaks down) to form hydrogen and oxygen.

Solution not yet available

Calcium and oxygen gas react to form calcium oxide.

Solution not yet available