Reactions of Acids (AQA GCSE Chemistry)

The half equation for the oxidation of zinc by hydrochloric acid:

Zn → Zn²⁺ + 2e^-

True.

Redox reactions involve the transfer of electrons. Reduction involves gaining electrons, while oxidation involves losing electrons.

Metal-acid reactions are considered redox reactions because both oxidation and reduction occur simultaneously.

Magnesium is oxidised in the following reaction, because it has lost electrons.

Mg + 2H⁺⟶ Mg²⁺ + H₂

Hydrogen ions are reduced in the following reaction, because they gain electrons.

Mg + 2H⁺⟶ Mg²⁺ + H₂

A base is a chemical that neutralises acids.

The products of a neutralisation reaction between an acid and a base are salt and water.

False.

Many bases are insoluble in water.

An alkali is a base that is soluble in water, forming an alkaline solution.

The three products formed when an acid reacts with a metal carbonate are:

• Salt,

• Water,

• Carbon dioxide.

The identity of the salt produced depends on:

• The acid used,

• The positive ions in the base.

False.

Hydrochloric acid produces chloride salts.

Sulfuric acid produces sulfate salts.

Nitric acid produces nitrate salts.

The symbol equation for the reaction of hydrochloric acid and copper(II) oxide is:

2HCl + CuO → CuCl₂ + H₂O

The word equation for nitric acid reacting with magnesium oxide is:

nitric acid + magnesium oxide → magnesium nitrate + water

False.

The balanced symbol equation for the reaction of hydrochloric acid and potassium carbonate is:

2HCl + K₂CO₃ ⟶ 2KCl + H₂O + CO₂

A soluble salt can be made by the reaction of an acid with a solid insoluble substance, such as :

• metal.

• metal oxide.

• metal hydroxide.

• metal carbonate.

The insoluble base is added in excess to ensure that all of the acid has reacted.

The excess insoluble base is removed by filtration.

After removing the excess base, only the salt and water remain in the solution.

The balanced symbol equation is:

CuO (s) + H₂SO₄ (aq) ⟶ CuSO₄ (s) + H₂O (l)

An insoluble base is a base that does not dissolve in water.

The solution is heated gently and then left to cool.

pH measures the concentration of hydrogen ions (H⁺) in a solution.

Acids have pH values below 7.

Alkalis have pH values above 7.

A pH value of 7 is considered neutral.

True.

A lower pH indicates a more acidic solution.

Universal indicator is a wide range indicator made of a mixture of different plant indicators which operate across a broad pH range.

False.

Universal indicator gives only an approximate value for pH.

In solution, an acid ionises, producing protons (H⁺ ions), making the solution acidic.

In solution, a base ionises, producing hydroxide ions (OH^-) which can accept protons, making the solution alkaline.

True.

A pH probe measures pH more accurately than universal indicator.

The ionic equation for neutralisation is:

H⁺  (aq) + OH^– (aq)⟶ H₂O (l)

A strong acid is an acid that completely dissociates (or ionises) in water, producing solutions of a very low pH.

A weak acid is an acid that partially dissociates (or ionises) in water, producing pH values closer to the middle of the pH scale, while still being below 7.

True.

HCl is an example of a strong acid.

The equation for the dissociation of HCl in water is:

HCl (aq) → H⁺ (aq) + Cl^- (aq)

True.

Weak acids form an equilibrium in water between molecules and their ions.

Examples of weak acids are ethanoic, propanoic, citric and carbonic acids.

False.

Strong acids always have a lower pH than weak acids of the same concentration.

The typical pH range for strong acids is 1-3.

The typical pH range for weak acids is 4-6.

False.

"Strong" refers to the degree of dissociation, while "concentrated" refers to the amount of acid present in solution.

A strong acid dissociates completely in water, while a weak acid only partially dissociates.

The ⇌ symbol indicates that the acid is weak, an equilibrium is established and the acid only partially dissociates.

True.

The higher the concentration of hydrogen ions, the lower the pH.

If the pH increases by 1, the concentration of hydrogen ions increases by a factor of 10.

A strong acid will have more hydrogen ions in solution than a weak acid of equal concentration.

The concentration of hydrogen ions in a solution of pH 4 is 100 times (10 x 10) greater than a solution with a pH of 6.

The new pH is 2.

If the hydrogen ion concentration increases by a factor of 10 by the pH decreases by 1.

So, increasing by a factor of 1000 will decrease the pH from 5 to 2.

A titration is a method of analysing the concentration of solutions.

An acid-base titration determines exactly how much alkali is needed to neutralise a quantity of acid, or vice versa.

The endpoint is the point at which the indicator changes color, which means that neutralisation has happened.

False.

Wide range indicators like litmus are not suitable for titrations as they do not give a sharp enough color change at the endpoint.

Phenolphthalein turns pink in an alkaline solution.

Methyl orange turns red in an acidic solution.

False.

Titrations can also be used to prepare salts.

The purpose of using an indicator in a titration is to show the endpoint of the reaction.

Phenolphthalein is colorless in a neutral solution.

Litmus paper turns blue in an alkaline solution.

Concordant results are results that are within 0.1 cm³ of each other.

This piece of apparatus is a pipette.

Repeating a titration and achieving concordant results increases the accuracy of the measurement.

Swirling the flask during a titration ensures thorough mixing of the solutions and helps to identify the endpoint more accurately.

The piece of equipment used to transfer a fixed volume of acid / alkali into a conical flask for a titration is a volumetric pipette.

A white tile placed underneath the conical flask during a titration because it allows the colour change to be observed more easily.

The piece of apparatus that measures a variable volume of acid / alkali during a titration is a burette.

This piece of apparatus is called a burette.

Concentration is the number of moles of solute per unit volume of solution, typically expressed in mol/dm³.

False.

The number of moles of acid and alkali depend on their reaction ratio, which is not always 1:1.

Volume is typically expressed in dm³ (cubic decimeters) in titration calculations.

To convert cm³ to dm³, divide by 1000.

True.

The concentration of a solution can be calculated if you know the number of moles of solute and volume of the solution

The three pieces of information you need to know when calculating the unknown concentration of an acid in an acid-base titration are:

• The volume (average titre) of the acid

• The concentration of the base

• The volume of the base.

It's important to consider the balanced equation to determine the mole ratio between the acid and alkali.

0.25 moles of HCl would be required as this reaction is a 1:1 ratio.

0.20 moles of NaOH would be required as this reaction is a 2:1 ratio.