C2.9 Investigate a neutralization reaction | Ontario Grade 11 Chemistry

Ontario Grade 11 Chemistry

Chemical Reactions

From visible indicator changes to a balanced equation and a measured result

A drop of indicator can change colour as an acid is added to a base. That visible change helps identify when the mixture has reached a chosen point in a neutralization investigation. To explain the result, first recall that acids and bases can be represented as substances in water, then connect what is observed to the particles and the balanced reaction. The goal is not to memorize a colour alone. It is to use observations and measurements to investigate how an acid and a base react.

What you will learn

Describe observable evidence that an acid and a base have reacted.
Use a particle model and a balanced equation to represent neutralization.
Outline a safe, fair investigation that measures the volumes needed for neutralization.
Use a balanced equation to interpret a simple neutralization measurement.

1. Review: acids, bases, and indicators

An aqueous solution is a mixture in which a substance is dissolved in water. At this level, an acid is described as a substance that produces hydrogen ions,

ext{H}^+

, in water. A base produces hydroxide ions,

ext{OH}^-

, in water. These ion symbols show electrical charges.

An indicator is a substance that changes colour over a certain range of acidity. It can help show a change during an investigation. For example, phenolphthalein is colourless in an acidic solution and pink in a basic solution. The colour change is evidence to observe; by itself, it does not identify every substance in the mixture.

Neutralization is a reaction between an acid and a base. In the familiar reaction of an acid with a metal hydroxide, the products are water and an ionic compound called a salt. The salt contains the positive ion from the base and the negative ion from the acid. The word “salt” here means an ionic compound, not only table salt.

\mathrm{acid + base \rightarrow salt + water}

Use an indicator consistently and record its colour before and after adding a solution.
In a neutralization reaction, the acid and base react to form water and a salt.

2. What happens in the mixture?

Suppose hydrochloric acid solution is mixed with sodium hydroxide solution. An observable result can be a change in indicator colour as one solution is added to the other. The colour change shows that the mixture has passed through the indicator’s colour-change range. If the base initially makes phenolphthalein pink, adding acid drop by drop eventually makes the pink colour disappear.

At the particle level, the acid supplies hydrogen ions and the base supplies hydroxide ions. These ions combine to form water molecules. The sodium and chloride ions remain in solution as the dissolved salt sodium chloride. This model explains why the acid and base react while the salt remains in the mixture.

The balanced equation shows the substances and their proportions. State symbols identify whether each substance is aqueous, liquid, or another state. In this example, the acid, base, and salt are dissolved in water, while water is liquid. The equation is balanced: there are equal numbers of each type of atom on both sides, and the net charge is the same on both sides.

\mathrm{HCl(aq) + NaOH(aq) \rightarrow NaCl(aq) + H_2O(l)}

The observable colour change is evidence used to identify a chosen endpoint.
Hydrogen ions and hydroxide ions form water; the other ions make up the salt in solution.

3. Plan and carry out an investigation

A neutralization investigation can find the volume of acid needed to react with a measured volume of base. A burette or graduated measuring device can deliver the acid in controlled amounts. A measured volume of base is placed in a flask, and a few drops of a suitable indicator are added. Add the acid while mixing the contents. Near the colour change, add acid one drop at a time. Record the acid volume when the indicator shows the selected endpoint.

An endpoint is the observed colour change used to decide when to stop adding solution. It is a practical signal, not a claim that every particle has disappeared or that the indicator gives an exact measurement without uncertainty. Use the same indicator and the same colour-change rule each time. Record the initial and final burette readings, then find the volume delivered by subtracting the initial reading from the final reading.

For a fair comparison, keep the measured base volume and its concentration the same when repeating trials. Record the acid concentration, base concentration, and volumes with units. Repeat the procedure and compare the results. Do not invent or assume measurements: report only the values actually observed. If results differ, check reading technique, mixing, and whether acid was added too quickly near the endpoint.

Wear splash goggles and follow teacher instructions for handling solutions. Avoid skin and eye contact. If a spill occurs, tell the teacher and follow the classroom cleanup procedure. Rinse equipment as directed. Safe handling and careful recording are part of a reliable investigation.

Identify what you will measure: the acid volume needed for the chosen endpoint.
Control the procedure, record units, and repeat trials rather than treating a proposed result as evidence.

4. Use the balanced equation to interpret a result

The coefficients in a balanced equation give the reacting particle ratio. In the hydrochloric acid and sodium hydroxide example, one unit of acid reacts with one unit of base. In a solution measurement, this means the amount of acid that reacts is equal to the amount of base for this particular reaction. Concentration tells how much solute is present in a volume of solution. When concentration is given in moles per litre and volume in litres, multiplying them gives amount in moles.

Do not assume every acid and base pair has a one-to-one ratio. Read the balanced equation for the specific substances being investigated. A balanced equation is also a check: the atom counts and total charge must match on both sides. For measurements, retain units during calculations and round the final result to match the precision of the supplied data.

n=cV

Use the equation’s coefficients to choose the correct reacting ratio.
Keep volume and concentration units visible until the calculation is complete.

Worked example

Finding the acid volume for a neutralization

A sample contains

20.0\ \mathrm{mL}

of sodium hydroxide solution at

0.100\ \mathrm{mol/L}

. What volume of hydrochloric acid at

0.0800\ \mathrm{mol/L}

is needed to neutralize it? Assume the endpoint corresponds to the balanced reaction shown in this lesson.

Read the reaction ratio
The balanced equation shows that hydrochloric acid and sodium hydroxide react in a one-to-one ratio. First find the amount of sodium hydroxide. Convert its volume to litres so it matches the concentration unit.
$20.0\ \mathrm{mL}=0.0200\ \mathrm{L}$
Find the amount of base
Multiply concentration by volume. The litre units cancel, leaving moles of sodium hydroxide.
$n(\mathrm{NaOH})=(0.100\ \mathrm{mol/L})(0.0200\ \mathrm{L})=0.00200\ \mathrm{mol}$
Use the reaction ratio
Because the ratio is one to one, the acid amount needed is also $0.00200\ \mathrm{mol}$ . Divide this amount by the acid concentration to find its volume.
$V(\mathrm{HCl})=\frac{0.00200\ \mathrm{mol}}{0.0800\ \mathrm{mol/L}}=0.0250\ \mathrm{L}$
Convert and report
Convert litres to millilitres. The given values support three significant figures, so report the volume to three significant figures.
$0.0250\ \mathrm{L}\times\frac{1000\ \mathrm{mL}}{1\ \mathrm{L}}=25.0\ \mathrm{mL}$

Answer: The required hydrochloric acid volume is

25.0\ \mathrm{mL}

.

Check: The acid amount is

0.0800\ \mathrm{mol/L}\times0.0250\ \mathrm{L}=0.00200\ \mathrm{mol}

, equal to the sodium hydroxide amount, as the one-to-one equation requires.

Common mistakes and how to avoid them

Saying the indicator colour change proves that the solution contains only water and salt.

Correction: The indicator signals a chosen endpoint. The mixture also contains water and dissolved ions, and the colour alone does not identify every substance.

Using the same one-to-one ratio for every acid and base reaction.

Correction: Balance the specific reaction and use its coefficients to determine the ratio.

Recording only the final burette reading as the delivered volume.

Correction: Subtract the initial reading from the final reading, and include the volume unit.

Adding a large amount of acid at once near the colour change.

Correction: Add slowly and mix, then use single drops near the endpoint so the chosen colour change is easier to observe.

Lesson summary

Neutralization is a reaction between an acid and a base that forms water and a salt.
Indicator colour change provides observable evidence for a chosen endpoint.
The particle model and balanced equation explain the reaction and its proportions.
A careful investigation measures and records volumes, concentrations, units, and repeated results.

Check your understanding

Question 1

For the reaction of hydrochloric acid with sodium hydroxide, what does the balanced equation show about their reacting ratio?

One acid unit reacts with one base unit.
Two acid units react with one base unit.
One acid unit reacts with two base units.
The equation gives no ratio.

Show answer and explanation

One acid unit reacts with one base unit.

The coefficients are both one, so the reacting ratio is one to one.

Question 2

A learner uses phenolphthalein with a basic solution and adds acid until the pink colour disappears. What is the colour change used for?

It is an endpoint signal for the investigation.
It proves that no ions remain in the solution.
It shows that the acid has become a base.
It identifies the exact salt concentration.

Show answer and explanation

It is an endpoint signal for the investigation.

The indicator’s colour change is used as the chosen endpoint. It does not prove that no ions remain or measure salt concentration.

Key terms

Aqueous solution: A mixture in which a substance is dissolved in water.
Indicator: A substance that changes colour over a range of acidity.
Neutralization: A reaction between an acid and a base that forms water and a salt.
Endpoint: The observed signal used to decide when to stop adding a solution in an investigation.
Concentration: The amount of dissolved substance in a given volume of solution.

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Published by DoAssignment. This AI-assisted lesson follows Ontario Grade 11 Chemistry (SCH3U), expectation C2.9. It is a study resource, not an official curriculum publication.

Before publication, the draft is checked for structure, mathematical or chemical notation, calculations, course boundaries, and readability, and then requires administrator approval. Errors can still occur, so corrections are welcomed.

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C2.9 · Investigate a neutralization reaction