Exercises: Neutralisation reactions

Complete the general scheme of a neutralization 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 lab, 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 neutralize 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 drugstore, which contain bases such as magnesium hydroxide, relieve it. Explain, using the concept of neutralization, how an antacid works in the stomach.

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

1. a) What color 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 neutralization: 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 neutralization between hydrochloric acid ($HCl$) and calcium hydroxide ($Ca(OH)_2$), forming calcium chloride ($CaCl_2$) and water.

Write and balance the equation for the neutralization 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 $Mg(OH)_2$ neutralize?
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 neutralize 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" neutralization such as that of $NaOH$ with $HCl$?

The core of any neutralization 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 neutralizations: potassium chloride ($KCl$), sodium nitrate ($NaNO_3$) and calcium sulfate ($CaSO_4$). Deduce which acid and which base have neutralized 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 $HCl$ neutralized 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 neutralizes less acid per gram than magnesium hydroxide, even though both neutralize the same amount of acid per mole.

In a laboratory flask, there are $0.2\ \text{mol}$ of sulfuric acid ($H_2SO_4$).

1. a) Write the balanced equation for its neutralization with sodium hydroxide ($NaOH$), which produces sodium sulfate ($Na_2SO_4$) and water.
2. b) Calculate how many moles of $NaOH$ are needed to neutralize 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 sulfuric acid ($H_2SO_4$).

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

We mix $0.1\ \text{mol}$ of sulfuric acid ($H_2SO_4$) with $0.1\ \text{mol}$ of sodium hydroxide ($NaOH$).

1. a) Write the balanced equation for the neutralization, which produces sodium sulfate ($Na_2SO_4$) 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 neutralizations 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 bicarbonate paste.

1. a) Which of the two remedies would you apply to each sting? Justify your answer using the concept of neutralization.
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.