Exercises: Neutralisation reactions

Complete the general scheme of a neutralisation reaction with the missing words: "An $\dots$ reacts with a $\dots$ to form a $\dots$ and $\dots$". Then give an example using the reaction between hydrochloric acid and sodium hydroxide.

In the laboratory, hydrochloric acid ($HCl$) is mixed with sodium hydroxide ($NaOH$) to form sodium chloride ($NaCl$), common salt, and water. Write the reaction equation and check by counting atoms that it is balanced.

Classify each of these substances as an acid, a base or a salt:

1. a) $HNO_3$ (nitric acid)
2. b) $KOH$ (potassium hydroxide)
3. c) $NaCl$ (sodium chloride)
4. d) $H_2SO_4$ (sulfuric acid)
5. e) $Ca(OH)_2$ (calcium hydroxide)

pH measures how acidic or basic a solution is. These are the positions of the four solutions on the pH scale:

State whether each of these solutions is acidic, neutral or basic:

1. a) lemon juice, $pH = 2,5$
2. b) pure water, $pH = 7$
3. c) hand soap, $pH = 10$
4. d) crushed tomato, $pH = 4,5$

We have a beaker with dilute vinegar at $pH = 3$ and we gradually add a solution of sodium bicarbonate, which is a base.

1. a) Does the pH in the beaker rise or fall when adding the bicarbonate?
2. b) If we add just the right amount to neutralise all the acid, around what value will the pH be?

Give reasons for your answers.

Gastric juice in the stomach contains hydrochloric acid. In cases of heartburn, antacids from the pharmacy, which contain bases such as magnesium hydroxide, relieve it. Explain, using the concept of neutralisation, how an antacid works in the stomach.

Litmus paper turns red in acidic conditions and blue in basic conditions.

1. a) What colour will it turn in a solution of $HCl$?
2. b) And in a solution of $NaOH$?
3. c) We mix the two solutions in exact amounts for complete neutralisation: explain why the litmus paper no longer indicates acid or base in the resulting mixture.

Write the formula and name of the salt formed in each neutralization reaction:

1. a) Hydrochloric acid ($HCl$) with potassium hydroxide ($KOH$).
2. b) Nitric acid ($HNO_3$) with sodium hydroxide ($NaOH$).

Sulfuric acid ($H_2SO_4$) reacts with calcium hydroxide ($Ca(OH)_2$) to form calcium sulfate ($CaSO_4$) and water. Write the equation and balance it; pay close attention to how many water molecules are formed for each calcium sulfate.

Write and balance the equation for the neutralisation between hydrochloric acid ($HCl$) and calcium hydroxide ($Ca(OH)_2$), forming calcium chloride ($CaCl_2$) and water.

Write and balance the equation for the neutralisation of sulfuric acid ($H_2SO_4$) with sodium hydroxide ($NaOH$), forming sodium sulfate ($Na_2SO_4$) and water.

Milk of magnesia is a suspension of magnesium hydroxide ($Mg(OH)_2$) taken for heartburn. Its reaction with stomach acid is $Mg(OH)_2 + 2\ HCl \rightarrow MgCl_2 + 2\ H_2O$.

1. a) How many moles of $HCl$ does $1\ \text{mol}$ of $Mg(OH)_2$ neutralise?
2. b) If the stomach has an excess of $0,1\ \text{mol}$ of $HCl$, how many moles of $Mg(OH)_2$ are needed to neutralise it?

An effervescent antacid contains sodium bicarbonate ($NaHCO_3$), which reacts with hydrochloric acid as follows: $NaHCO_3 + HCl \rightarrow NaCl + H_2O + CO_2$.

1. a) Check by counting atoms that the equation is balanced.
2. b) Why do bubbles form when the tablet is dropped into a glass with acid?
3. c) How does this reaction differ from a "classic" neutralisation such as that of $NaOH$ with $HCl$?

The core of any neutralisation is the combination of the $H^+$ ion from the acid with the $OH^-$ ion from the base to form water: $H^+ + OH^- \rightarrow H_2O$, as shown in the diagram.

1. a) Check that this ionic equation is balanced in terms of atoms and electrical charge.
2. b) Using this idea, explain what these two reactions have in common: $HCl + NaOH \rightarrow NaCl + H_2O$ and $HNO_3 + KOH \rightarrow KNO_3 + H_2O$.

The soil in a vegetable garden has $pH = 5$, and tomatoes grow best with a pH between $6$ and $7$.

1. a) Is it an acidic or basic soil?
2. b) Farmers add "lime" (calcium hydroxide) to acidic soils. Assuming the acidity of the soil were due to hydrochloric acid, write the balanced equation for what the lime does in the soil.
3. c) What would happen to the pH of the soil if too much lime were added? Would it be good for tomatoes?

Three beakers contain dissolved salts left behind after three complete neutralisations: potassium chloride ($KCl$), sodium nitrate ($NaNO_3$) and calcium sulphate ($CaSO_4$). Deduce which acid and which base have neutralised each other in each beaker and write the balanced equation for each reaction.

Which antacid makes the best use of every gram? These are the reactions of three antacids with stomach acid:

1. a) magnesium hydroxide ($58\ \text{g/mol}$): $Mg(OH)_2 + 2\ HCl \rightarrow MgCl_2 + 2\ H_2O$
2. b) calcium carbonate ($100\ \text{g/mol}$): $CaCO_3 + 2\ HCl \rightarrow CaCl_2 + H_2O + CO_2$
3. c) sodium hydrogen carbonate ($84\ \text{g/mol}$): $NaHCO_3 + HCl \rightarrow NaCl + H_2O + CO_2$

Calculate the moles of $HCl$ neutralised by $1\ \text{g}$ of each antacid (you can give your answer as a fraction or rounded to three decimal places), order them from highest to lowest capacity per gram and explain why calcium carbonate neutralises less acid per gram than magnesium hydroxide, even though both neutralise the same amount of acid per mole.

In a laboratory flask, there are $0,2\ \text{mol}$ of sulphuric acid ($H_2SO_4$).

1. a) Write the balanced equation for its neutralisation with sodium hydroxide ($NaOH$), which produces sodium sulphate ($Na_2SO_4$) and water.
2. b) Calculate how many moles of $NaOH$ are needed to neutralise all the acid.
3. c) If we were to add only $0,2\ \text{mol}$ of $NaOH$, would the solution be acidic, neutral or basic? Justify your answer using stoichiometry.

Marble facades and statues (calcium carbonate, $CaCO_3$) deteriorate with acid rain, which contains sulphuric acid ($H_2SO_4$).

1. a) Write and balance the equation for the reaction between sulphuric acid and calcium carbonate, in which calcium sulphate ($CaSO_4$), water and carbon dioxide are formed.
2. b) Explain why this reaction gradually "eats away at" statues.
3. c) Granite does not contain carbonates: reason why granite buildings withstand acid rain better than marble ones.

In the laboratory, a solution of $NaOH$ is added slowly to $25\ \text{mL}$ of a solution of $HCl$ while measuring the pH, as shown in the diagram.

This is what is observed: at first, the pH is $1$ and hardly increases during the first $20\ \text{mL}$ added; between $24$ and $26\ \text{mL}$, the pH jumps from $3$ to $11$; from then on, it rises slowly, approaching $13$.

1. a) Draw in your notebook the graph of pH against the volume of $NaOH$ added, marking those three sections.
2. b) For what added volume is the neutralisation exactly complete? What is the pH of the solution at that point?
3. c) Explain, using the reaction $H^+ + OH^- \rightarrow H_2O$, why the pH changes so rapidly around that exact point.

You can do it on paper and send a photo with your answer.

We mix $0,1\ \text{mol}$ of sulphuric acid ($H_2SO_4$) with $0,1\ \text{mol}$ of sodium hydroxide ($NaOH$).

1. a) Write the balanced equation for the neutralisation, which produces sodium sulphate ($Na_2SO_4$) and water.
2. b) Reason which of the two reactants is used up first and how many moles of the other remain unreacted.
3. c) Will the final pH be acidic, basic or neutral? What if we mix $0,1\ \text{mol}$ of $H_2SO_4$ with $0,2\ \text{mol}$ of $NaOH$? Justify both answers.

Ammonia ($NH_3$) in cleaning products is not a hydroxide, but it reacts with hydrochloric acid as follows: $NH_3 + HCl \rightarrow NH_4Cl$.

1. a) Check that the equation is balanced.
2. b) Why does water not appear among the products, unlike what happens in neutralisations with hydroxides?
3. c) The modern definition states that a base is any substance capable of accepting $H^+$ ions. Using this definition, explain why ammonia is indeed a base and what has happened to its nitrogen atom upon forming $NH_4Cl$.

A bee sting injects an acid (methanoic acid) and a wasp sting, a basic substance. In the first-aid kit, there is vinegar (dilute acetic acid) and a sodium hydrogen carbonate paste.

1. a) Which of the two remedies would you apply to each sting? Justify your answer using the concept of neutralisation.
2. b) A friend claims that "vinegar works for all stings". Explain to them why they are wrong and what would happen if vinegar were applied to a bee sting.

Phosphoric acid ($H_3PO_4$), present in some cola soft drinks, reacts with calcium hydroxide to form calcium phosphate ($Ca_3(PO_4)_2$) and water.

1. a) Write and balance the equation (you will need the coefficients $2$, $3$ and $6$).
2. b) How many moles of water are formed per mole of phosphoric acid?
3. c) Explain why, when balancing this equation, it is not enough to look at a single element, and why it is advisable to start by balancing calcium and phosphorus before hydrogen and oxygen.