Looking at chemical reactions

In chemical reactions, atoms form new combinations. Atoms that are on their own may join together with other atoms. Atoms that are bonded with other atoms may separate, forming new combinations with other atoms.

In the reaction between iron and sulfur, the iron and the sulfur atoms that were there at the start of the reaction are still there at the end. They have just rearranged themselves. We can represent this reaction by drawing the atoms, writing a word equation or by writing a symbol equation.

In a chemical reaction, no atoms are lost. No new atoms are produced. The atoms are simply rearranged into new combinations.

When you look at any of the equations for the reactions in this topic you can see that the elements that are present in the reactants are also present in the products.

Here is the equation for the reaction between magnesium and hydrochloric acid.

The magnesium metal in this reaction is a reactant. The magnesium is still present in the products as part of the salt magnesium chloride. The element hydrogen is present in the reactants as part of the compound hydrochloric acid. In the products it is present as hydrogen gas. The element chlorine is present in the reactants as part of the compound hydrochloric acid and in the products it is part of the salt magnesium chloride.

This is an important idea. No element that is present in the reactants disappears from the products. No new element appears in the products.

Look carefully at the symbol equation. Not only does it tell you which elements are present in the reactants and products but how many of each atom is present. In the reactants there is one atom of magnesium and in the production there is one atom of magnesium. In the reactants there are two atoms of hydrogen and in the products there are two atoms of hydrogen. In the reactants there are two atoms of chlorine and in the products there are two atoms of chlorine.

Conservation of mass

Atoms have mass. If no atoms are gained or lost during a chemical reaction, then no mass is gained or lost either.

Zara, Sofia and Arun carry out the reaction between calcium carbonate and hydrochloric acid.

They place some calcium carbonate in a flask, add the hydrochloric acid and place the stopper in the top of the flask. They place the flask on a top pan balance. They each have different ideas about what will happen to the mass in the flask as the reaction takes place.

Sofia: I think the mass will decrease because one of the products is a gas and gases are very light.

Zara: I think the mass will increase because there are two reactants and three products, so there are more products.

Arun: I think the mass will stay the same because there is a stopper in the top and no atoms can enter or leave the flask.

In chemical reactions the elements you begin with are the ones you end the reaction with. Nothing is added or taken away. The mass you begin with is the mass you end with.

This important idea is called the law of conservation of mass.

Not the results you expect?

When you add calcium carbonate to hydrochloric acid, there is a chemical reaction.

Arun places a flask of hydrochloric acid on a top pan balance and carefully adds calcium carbonate. He measures the mass of the flask and the contents at the beginning of the reaction and after 10 minutes. These are his results:

The law of conservation of mass tells you that there must be the same mass at the end of the reaction as at the start. In Arun’s experiment the mass appears to decrease. Why is this?

The word equation for this reaction is:

calcium carbonate + hydrochloric acid → calcium chloride + water + carbon dioxide

The carbon dioxide gas escapes into the air because the flask is open. You then cannot measure its mass. So, it appears as if the mass decreases as the reaction continues. The elements present are all present in the products. Remember, water is made from hydrogen and oxygen atoms.

Another surprising result

Here is another reaction that produces a result that is a surprise to some people.

Some magnesium is placed in a crucible, and the mass of the crucible with the magnesium is recorded. The crucible is heated very carefully. The lid of the crucible is lifted from time to time during the heating to allow the air in. After heating, the mass of the crucible with contents is measured again.

A reaction takes place when the magnesium is heated. After the reaction, the crucible contains white ashes. The mass of the ashes is more than the mass of the magnesium at the start of the reaction.

Some people think that the ashes will be lighter because the ashes look smaller than the magnesium. They see the flames escaping from the crucible, so they think something has been lost.

However, the word equation explains what happens:

magnesium + oxygen → magnesium oxide

There is an increase in mass because oxygen from the air has combined with the magnesium.

A French scientist called Antoine Lavoisier carried out this experiment in 1772. He repeated it many times and found he had an increase in mass every time. He could not explain why this happened. Finally he came up with the idea that when something burns it combines with a gas from the air. He also found that the gas from the air that is involved in burning is involved in respiration as well. He named the gas oxygen.

Energy and chemical reactions

All chemical reactions involve energy.

Energy is used to break bonds in the reactants and energy is released when new bonds are formed in the products. When metals react with water or acids, energy is released, as thermal energy or sometimes as light or sound or kinetic energy. For example, in the reaction between potassium and water the potassium catches fire, gets so hot that it melts, burns with a pinkish purple flame and hisses or explodes. It also moves across the surface of the water.

In this reaction, less energy is needed to break bonds in the reactants than is released when bonds form in the products. This is an exothermic reaction.

In other chemical reactions it takes more energy to break bonds in the reactants than is released when the bonds form in the products. These reactions are called endothermic reactions and they take in energy. For example:

sodium hydrogencarbonate + citric acid → sodium citrate + water + carbon dioxide

This reaction happens when we eat sherbet sweets. They give us a cool refreshing feeling in our mouth when we eat them.

When this reaction takes place, energy is transferred from the environment (your mouth). The energy is transferred to chemical energy stored in the bonds of the products.

Whether a chemical reaction takes in energy or releases energy there is no overall change in the amount of energy during the reaction. This is because energy cannot be created or destroyed. This is the law of conservation of energy.

Energy may be transferred from one form to another but the same amount of energy remains after the reaction as before. This is true of every chemical reaction.