Lesson Explainer: Substitution Reactions Science

In this explainer, we will learn how to describe and explain single and double substitution reactions.

A substitution reaction is a type of chemical reaction that is also known as a displacement reaction. There are two different types of substitution reactions, single substitution and double substitution reactions.

The word substitution means to “swap out,” and this is one way to think about a substitution reaction.

In order to fully understand what is happening in these types of chemical reactions, we first need to look at the series of chemical activity.

The series of chemical activity is shown in the diagram below. The series indicates which elements are more active than others. The elements near the top of the series are described as being more active than the elements further down.

Highly active elements such as sodium and potassium usually have more vigorous reactions than less active elements such as silver and gold.

Example 1: Naming a List of Elements of Different Reactivities

What is the name given to the list of elements shown below when discussing how reactive different elements can be?

  1. The metals of the periodic table
  2. The metal neutralization series
  3. The substitution reaction series
  4. The series of chemical reactions
  5. The series of chemical activity

Answer

If we look at the list, we will see it has more active elements such as potassium and sodium at the top. We also find that it has less active elements such as silver and gold at the bottom.

There is a good chance that this list has something to do with chemical activity.

While substitution reactions depend on the activity of different chemicals, the list does not include examples of neutralization reactions, chemical reactions, or substitutions, and so we can discount answers B, C, and D.

The elements in the list are all metals, but the list does not show all the metals of the periodic table. In addition, the list includes hydrogen, which is a nonmetal element. We can therefore discount answer A.

The statement in answer E refers to the series of chemical activity. Chemical activity relates to the different reactivities of elements. We have already noticed that the list has more reactive elements at the top and less reactive elements at the bottom.

Based on this, we can conclude that answer E, the series of chemical activity, is correct.

In a substitution reaction, a more active metal will replace a less active metal in another compound.

Definition: Substitution Reaction

Substitution reactions happen when an active metal replaces a less active metal in a chemical compound.

We can represent this idea of a substitution reaction using a simple diagram containing cubes.

Imagine that the green cube represents a highly active metal, the blue cube a less reactive metal, and the red cube a negative anion such as chloride. A substitution reaction occurs when the more active metal, the green cube, replaces the less active metal, the blue cube, in a compound.

Example 2: Identifying the Statement That Best Describes a Substitution Reaction

Which of the following statements best describes a substitution reaction?

  1. A more active metal replaces hydrogen in another compound.
  2. A more active metal replaces another less active metal in another compound.
  3. A less active metal forms a compound with another more active metal from another compound.
  4. A less active metal replaces another more active metal in another compound.
  5. A more active metal forms a compound with another less active metal from another compound.

Answer

Substitution reactions are closely linked to the series of chemical activity. Elements at the top of the series are more active than elements at the bottom.

While answer A is correct to an extent, it is not always hydrogen that is replaced, so answer A is incorrect.

Answer B states that a more active metal replaces a less active metal in another compound. This is close to what we consider to be a definition for a substitution reaction, so this may be the correct answer.

Answer C and E describe reactions where a less active metal forms a compound with a more active metal. However, in substitution reactions, it is normally the metals that are swapped, so answers C and E are incorrect.

Answer D is the opposite of answer B. Answer D must be wrong because highly active metals replace less active metals in substitution reactions. We can use this line of reasoning to discount answer D.

Therefore, answer B is the correct answer.

Group 1 metals are considerably more active than hydrogen. A more active metal is able to replace hydrogen in water or acids.

The following reaction shows how group 1 metals can replace the hydrogen in water: groupmetalwatergrouphydroxidehydrogengas1+1+

The next two equations show how one representative group 1 metal (sodium) replaces the hydrogen in water: sodium+watersodiumhydroxide+hydrogen2Na+2HO2NaOH+H22

Example 3: Explaining the Cause of Sodium Metal’s Reaction with Water

When a piece of sodium metal is placed into a large trough of cold water, a chemical reaction takes place. Which of the following statements best explains why this is the case?

  1. Sodium is more active than hydrogen.
  2. Sodium is less active than hydrogen.
  3. Sodium is more active than water.
  4. Sodium is less active than water.

Answer

When a group 1 metal reacts with water, a substitution reaction takes place.

All of the group 1 metals are more reactive than hydrogen.

When a group 1 metal such as sodium reacts with water, the more active sodium takes the place of the less active hydrogen in the water molecule.

The substitution occurs because sodium is more active than hydrogen. Specifically, we mean the hydrogen in the water molecules, not the water molecule itself, as implied by answers C and D.

Therefore, the correct answer to this question is answer A: sodium is more active than hydrogen.

If we look closely at the diagram below, we can see how the atoms have rearranged themselves during this chemical reaction.

The more active sodium atoms have replaced the less active hydrogen atoms in the water molecules:

This reaction is frequently demonstrated in school laboratories by teachers. During the demonstration, only very small pieces of sodium are used. In the photograph below, a larger piece of sodium has reacted violently with water in a large trough, cracking the glass vessel.

This type of single substitution reaction can also be seen with metals when they react with dilute mineral acids.

The chemical equation below shows the reaction between zinc and dilute hydrochloric acid.

During the reaction, the zinc metal replaces the hydrogen atoms from two molecules of hydrochloric acid. The result is the formation of a salt (zinc chloride) and hydrogen gas: metal+acidsalt+hydrogenzinc+hydrochloricacidzincchloride+hydrogenZn+2HClZnCl+Hdilute22

Example 4: Describing the Correct Symbol Equation for the Reaction between Zinc Metal and Hydrochloric Acid

Which of the following equations shows the reaction between zinc metal and hydrochloric acid?

  1. Zn()+2HCl()ZnCl()+H()saqaqg22
  2. Zn()+HCl()ZnH()+Cl()saqaqg
  3. Zn()+HCl()ZnCl()+12H()saqaqg2
  4. Zn()+2HCl()ZnH()+Cl()saqaqg22
  5. Zn()+2HCl()ZnHCl()+HCl()saqaqg

Answer

A substitution reaction happens when a highly active metal is added to some dilute acid. The reaction between the metal and the acid produces a salt and hydrogen gas. The only two answers that have hydrogen gas as a product are answers A and C.

Zinc has two valence electrons and it generally forms 2+ ions. Chlorine has seven valence electrons and it generally forms 1 ions. These two statements can be used to determine that zinc chloride has the chemical formula ZnCl2.

Only answer A includes the chemical formula ZnCl2. This shows that answer A must be the correct answer for this question.

If we consider the series of chemical activity, we can see that elements such as magnesium, zinc, and aluminum are more active than hydrogen. Metals like aluminum and zinc can replace hydrogen atoms in hydrochloric acid.

However, no reaction happens when copper, silver, or gold metals are added to hydrochloric acid because these metals are less active than hydrogen.

The final example of simple substitution reactions involves the substitution of a highly active metal with a salt solution of a less active metal.

Consider the reaction of zinc metal with a copper(II) sulfate solution.

Zinc is more active than copper and so will displace the copper in a copper(II) sulfate solution. In this reaction, the blue copper(II) sulfate solution loses its color as zinc sulfate is produced, and the concentration of copper(II) sulfate decreases.

The zinc metal will gradually be integrated into the solution as it forms zinc sulfate, and brown solid copper metal will collect at the bottom of the beaker.

The chemical equation for this substitution reaction is zinc+copper(II)sulfatezincsulfate+copperZn+CuSOZnSO+Cu44

Other examples of this type of reaction could include magnesium metal replacing lead in lead(II) nitrate solution: magnesium+lead(II)nitratemagnesiumnitrate+leadMg+Pb(NO)Mg(NO)+Pb3232

Magnesium nitrate, like other common solutions of magnesium, such as magnesium sulfate and magnesium chloride, is a colorless solution.

Another form of substitution reaction we are interested in is a double substitution reaction.

A neutralization reaction is a reaction between an acid and alkali that produces water and a salt product: acidalkalisaltwater++

The following equations show an example of a neutralization reaction. The equation shows how hydrochloric acid can be reacted with sodium hydroxide to make water and a sodium chloride salt product: hydrochloricacid+sodiumhydroxidesodiumchloride+waterHCl+NaOHNaCl+HO2

We can also consider this reaction as a substitution. Sodium is more active than hydrogen and can therefore substitute it.

If we look closely at the symbol equation above, we can see that sodium has replaced the hydrogen atoms in the dilute acid, forming the salt sodium chloride. The remaining hydrogen atom combines with the hydroxide to form water.

Double substitution reactions can also occur when certain salt solutions are reacted together.

If two salt solutions are mixed together and all of the possible products are also soluble, then no reaction will take place as all you will have is a mixture of the two solutions.

However, if one combination of the atoms present creates an insoluble salt, then a precipitation reaction occurs, which can be considered a double substitution reaction.

Consider the reaction between solutions of sodium chloride and silver nitrate. The more active sodium replaces the silver in the nitrate solution to form sodium nitrate. The other product formed is a precipitate of silver chloride: sodiumchloride+silvernitratesodiumnitrate+silverchlorideNaCl+AgNONaNO+AgCl33

Overall, the silver and sodium metals have substituted to form two different compounds.

Once again, we can illustrate this idea by using a simple diagram of cubes.

Our final example of a double substitution reaction involves the reaction of an acid with a carbonate: metalcarbonateacidsaltcarbondioxidewater+++

The chemical equation below shows the reaction between sodium carbonate and hydrochloric acid: sodiumcarbonate+hydrochloricacidsodiumchloride+carbondioxide+waterNaCO+2HCl2NaCl+CO+HO2322

In this reaction, sodium replaces the hydrogen in the acid to form sodium chloride. The hydrogen ions that have been replaced end up forming water molecules. Carbon dioxide is made from the carbonate ions in the sodium carbonate.

Let us summarize what we have learned in this explainer.

Key Points

  • Single and double substitution reactions occur when a more active metal replaces a less active metal in a compound.
  • The series of chemical activity can be used to determine which elements are more active than others.
  • More active metals such as sodium will replace less active elements such as hydrogen in water.
  • A single substitution reaction occurs between highly active metals and dilute mineral acids.
  • Single substitution reactions occur between highly active metals and the salts of less active metals.
  • Neutralization–precipitation reactions and the reaction between an acid and a carbonate can be classed as double substitution reactions.

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