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Lesson Video: Neutralization Reactions Chemistry

In this video, we will learn how to define and identify a neutralization reaction and how to write and interpret neutralization reaction equations.

18:06

Video Transcript

Neutralization Reactions

Hydrochloric acid is a pungent solution that can erode living tissue. Lye is a white powder that in solution is extremely corrosive and is used for industrial cleaning purposes. What would happen if we mixed these two hazardous chemicals together? Well, we’d end up with salt water. Please do not try this at home. This reaction is an example of a neutralization reaction, where we mix an acid and a base to produce water and a salt.

In this lesson, we will learn how to define and identify a neutralization reaction and write an interpret neutralization reaction equations. In general terms, a neutralization reaction is what happens when we mix an acid and a base to produce water and a salt. For the specific case of hydrochloric acid and lye, also known as sodium hydroxide, the chemical equation looks like this. HCl plus NaOH produces H2O plus NaCl. In this case, the salt produced is sodium chloride, which we know in the kitchen as common table salt. But in chemistry, a salt simply refers to any ionic compound made from a positive ion and a negative ion.

At this point, we might wonder, how do we know what is an acid and what is a base? Well, we can help answer that question by seeing what happens when they dissolve. Acids are any substance that dissolves to produce hydrogen ions. We sometimes call hydrogen ions protons, as there are no electrons or neutrons present in this ion. In addition to the hydrogen ion, acids will also produce a negatively charged anion, in the case of hydrochloric acid, a chloride ion. Bases dissolve to produce hydroxide ions, or OH− ions.

It’s worth noting that there are several definitions of acids and bases that focus on different characteristics. The definition that focuses on the production of hydrogen ions by acids and hydroxide ions by bases is called the Arrhenius definition. Alongside the hydroxide ion, they also produce a positive cation when dissolved. In the case of sodium hydroxide, that positive cation is a sodium ion.

When the two solutions are mixed together, the hydrogen ion from the acid and the hydroxide ion from the base combine to form H2O, water. In our initial equation, we said that an acid plus a base produces a water and a salt. We can see here with the breakdown of ions that the end form of the salt is that it is dissolved in the water as a solution. Because the sodium ions and the chloride ions are dissolved ions at the beginning of the reaction and at the end of the reaction, we leave them out of the net ionic equation.

Net ionic equations include only the ions that participate in the reaction. In this case, the hydrogen ion from the acid and the hydroxide ion from the base combine to form water. So, overall, in a neutralization reaction, we will have an acid, supplying the hydrogen ion, that combines with the base, supplying a hydroxide ion, to form water and a salt.

The pH scale is a measure of acidity or basicity, and it runs from zero to 14. A strong acid, like 0.1-molar hydrochloric acid, will have a very low pH value of about one. On the other side of the scale, a strong base, like 0.1-molar sodium hydroxide, will have a pH value of about 13. Substances lower on the pH scale are more acidic and have higher concentrations of hydrogen ions, while substances on the higher end of the pH scale are more basic with higher hydroxide ion concentrations. When we mix a strong acid and a strong base, what will be the pH of the resulting neutralized solution?

The answer is that the resulting solution’s pH will be right around seven. If we have the same amount of the two acids, the hydroxide ions and the hydrogen ions will combine to form water until there are no more hydroxide or hydrogen atoms left. However, not all acids and bases are made equal. Some acids and bases are considered weak acids or weak bases. This means that not all of the available hydrogen or hydroxide ions will dissolve into solution.

This means that if we combine, say, a strong base and a weak acid, more of the hydroxide ions from the strong base will dissolve into solution than the corresponding hydrogen ions from the weak acid. The extra hydroxide ions will raise the pH of the resulting solution.

It is also important to recognize that one mole of hydrogen ions will neutralize one mole of hydroxide ions. This means that, in general, one mole of acid neutralizes one mole of base. There are a couple exceptions. For example, there are some acids and bases that produce more than one mole of ions per mole of the acid or base. And as we mentioned before, depending on the strength of the acid or base, the solutions will neutralize but maybe not to a perfectly neutral pH of seven.

Since the ions pair up particle by particle to form water molecules, it is important to get the same number of moles of acid and base and not, say, the same volume or mass. If we attempted to mix a basic solution and an acidic solution of unequal amounts, the resulting solution would have a pH further toward one end or the other of the pH scale. To end up with a neutral solution, the amount in moles of acid needs to equal the amount in moles of base.

We can use this relationship to our advantage in titration experiments. If we have an acid or base, in this case let’s say a base of unknown concentration, we can use a burette to slowly add in a solution of the opposite type with a known concentration. We will keep adding the second solution until the solution in the beaker is just neutralized.

By taking the volume of the acid added, we can calculate the amount in moles of the acid. This will be the same as the amount in moles of the base in the beaker, which will let us calculate the concentration of the starting basic solution. But how do we know when the solution is neutralized?

At the beginning, when we were first adding acid to the base, the solution will gradually change in pH up to a certain point. Then the pH will rapidly change when the solution has been neutralized, before leveling off once again. We call the point where the pH changes rapidly the equivalence point. This is because at this point the amount in moles of acid and the amount in moles of base in the solution in the beaker are equal. Once there are no more hydroxide ions in the solution, the addition of hydrogen ions from the acid changes the pH rapidly.

If we include the right indicator in the beaker at the beginning of the experiment, the solution will change color when the equivalence point has been reached. A popular choice is phenolphthalein, which will be pink when the solution is more basic and clear when the solution is more acidic, changing color somewhere between 8.2 and 10 on the pH scale. Overall, this neutralization reaction that occurs during the process we call titration allows us to determine the unknown concentration of an acid or base in solution.

One more thing to recognize about a neutralization reaction is that the temperature will increase. The formation of bonds between the hydroxide ions and the hydrogen ions to make water will release energy into the surroundings that we can observe as heat. We call reactions that release heat into the surroundings exothermic reactions.

Now that we’ve gone over the basics of neutralization reactions, let’s go over one exceptional case to push our understanding a little farther. The combination of magnesium oxide and hydrochloric acid to form water and magnesium chloride is a neutralization reaction. This might seem a little odd considering that magnesium oxide is a base, but it does not have a hydroxide ion in its chemical formula, nor is it soluble in water.

To explain the first of these interesting characteristics, we can say that our previous definition of a base was a simplification. While most bases have an oxygen and a hydrogen in their chemical formula, as long as they accept protons, to make water, they are considered bases. Defining an acid as something that can donate protons and a base as something that can accept protons is called the Brønsted–Lowry definition of acids and bases.

The other interesting characteristic of this base, the fact that it’s insoluble in water, is explained by the fact that this reaction occurs on the surface of magnesium oxide, as opposed to occurring between the ions in the solution. This exceptional neutralization reaction proves that not all bases have oxygen and hydrogen in their chemical makeup and not all bases are soluble in water.

Now that we’ve learned about neutralization reactions, let’s do some practice problems to review.

A neutralization reaction is shown below. Aqueous HNO3 plus aqueous KOH turns into aqueous KNO3 plus liquid H2O. What is the acid in this equation? What is the base in this equation? What is the salt in this equation?

The general form of a neutralization reaction is that an acid and a base added together produce water and a salt. This multipart question is asking us to look at the four compounds listed in the chemical equation and identify which one is the acid, which one is the base, and which one is the salt.

If we look again at the general equation for a neutralization reaction, we can see that the acid and base are the reactants of the reaction on the left side of the equation, while the salt is one of the products on the right side of the equation. Note that we cannot simply say that the acid comes first, the base comes second, the water comes third, and the salt comes fourth. The reactants on the left-hand side of the equation could be listed in any order, and the products on the right-hand side of the equation could be listed in any order.

Let’s start by identifying the salt. In chemistry, a salt is an ionic compound consisting of a positive ion and a negative ion. It’s a product of a neutralization reaction. So it will appear on the right-hand side of the equation. Thankfully, it is not too difficult to identify which of the products is which. Liquid H2O is also known as water, leaving aqueous KNO3, the ionic compound made up of the positive potassium ion and the negative nitrate ion, to be the salt.

Next, we need to identify which is the acid and which is the base. Based on the general formula, we know that the acid and the base are the reactants of the neutralization reaction. So of the two reactants on the left-hand side of the equation, HNO3 and KOH, one will be the acid and one will be the base. How do we determine which one is an acid and which one is a base?

Well, acids produce hydrogen ions when dissolved, while bases produce hydroxide or OH− ions when dissolved. If we break down these compounds into their constituent ions, we can see that HNO3 produces hydrogen ions, while KOH produces hydroxide ions. This means that HNO3 is our acid and KOH is our base.

Note that there are some bases that don’t produce hydroxide ions. However, the acid will still produce hydrogen ions. So you can identify the acid and base in this case by first identifying the acid. Neutralization reactions will always follow the general formula of acid plus base produces water and a salt. If we can identify the reactant that produces hydrogen ions, the reactant that produces hydroxide ions, and the product that is an ionic compound, we can identify the acid, base, and salt, respectively.

In the neutralization reaction shown below, the acid is HNO3, the base is KOH, and the salt is KNO3.

What product or products are formed during the following neutralization reaction?

This question is asking about a neutralization reaction. The general form of neutralization reactions is that a base and an acid combine to produce water and a salt. This question is specifically asking about the products formed during the reaction. Or if we combine these two substances on the left-side of the equation, what chemicals appear on the right-hand side of the equation?

If we look at our general formula, we can more specifically ask, water and what salt will be produced by this reaction? To answer this question, we need to know what ions are present in the reactants and how will they combine to form the products. Barium hydroxide breaks down into a barium ion and two hydroxide ions. H2CO3, also known as carbonic acid, breaks down into two hydrogen ions and one carbonate ion.

Listing the ions gives us a good breakdown of what will happen when the products are formed. The hydroxide ion supplied by the base and the hydrogen ion supplied by the acid will combine to form water, H2O. Meanwhile, the positive cation supplied by the base and the negative anion supplied by the acid will combine to form the salt. Written out, the products of the reaction look like this: 2H2O plus BaCO3.

Since the base and the acid each supply two ions, the reaction as a whole will produce two molecules of water, indicated by the two before H2O. It’s also worth noting that our salt, barium carbonate, is formed with one barium ion and one carbonate ion, because the two plus charge from barium cancels out the two minus charge from the carbonate ion. If the two ions that make up our salt had different charges, we would need to use different numbers of each of them to balance out the charges.

In a neutralization reaction, the acid and the base each supply one ion to help make water. And they each supply one ion to help make the salt. In the neutralization reaction given here, the products that are formed are two molecules of water and barium carbonate salt.

Now that we’ve done some practice problems, let’s review the key points of the video. The general form of a neutralization reaction is that an acid and a base combine to form water and a salt. The acid supplies the hydrogen ion, also known as a proton, and the base typically supplies an OH− ion, or a hydroxide ion. The net ionic equation gives the ions that take part in the reaction. For a neutralization reaction, the net ionic equation is H+ plus OH− produces H2O.

For the net ionic equation, the ions that produce the salt are left out, as they remain aqueous ions in solution at the beginning and end of the reaction. In some exceptional cases, we may see a base that does not dissolve in water. We may also see a base that does not supply a hydroxide ion. According to the Brønsted–Lowry definition of a base, since it can still accept a proton to create water, it is still considered a base.

Since the particles match up ion by ion to produce water molecules, one mole of acid neutralizes one mole of base. Lastly, neutralization reactions are used in titration experiments to determine the unknown concentration of an acid or base.

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