2.2 Acids, Bases & Salts (WJEC GCSE Chemistry): Flashcards

Acids contain hydrogen ions (H⁺), while alkalis contain hydroxide ions (OH^-). The presence of H⁺ ions is what makes a solution acidic.

True.

Alkalis are water-soluble bases that contain hydroxide ions (OH^-). It is the presence of OH^- ions that makes an aqueous solution alkaline.

HCl (aq) produces H⁺ (aq) + Cl^- (aq). The HCl molecule dissociates fully, releasing hydrogen ions that make the solution acidic.

Hydroxide ions are OH^- ions released when an alkali dissolves in water. Their presence is what makes an aqueous solution alkaline.

True.

A base is any substance that neutralises an acid. An alkali is specifically a base that is soluble in water. So all alkalis are bases, but insoluble bases (e.g. copper oxide) are not alkalis.

Sodium hydroxide releases Na⁺ (aq) and OH^- (aq) ions when it dissolves. The hydroxide ions make the solution alkaline.

The products are a salt and hydrogen gas. The general equation is: metal + acid → salt + hydrogen.

False.

Only metals above hydrogen in the reactivity series react with dilute acids. Copper is below hydrogen in the reactivity series, so it does not react with dilute acids.

The reactivity series is a list ranking metals in order of reactivity. Only metals placed above hydrogen in the series will react with dilute acids to produce a salt and hydrogen gas.

When magnesium reacts with hydrochloric acid, the products are magnesium chloride and hydrogen gas. The more reactive the metal, the more vigorous the reaction.

Hydrogen gas is tested using a burning splint held near the mouth of the tube. A squeaky pop confirms the presence of hydrogen.

Zn (s) + H₂SO₄ (aq) → ZnSO₄ (aq) + H₂ (g). The products are zinc sulfate solution and hydrogen gas.

True.

Metals high in the reactivity series such as potassium and sodium react explosively with dilute acids and are therefore too dangerous to use in standard laboratory reactions.

Excess zinc is used to ensure all the acid is used up. This means no acid remains in the final solution, giving a pure salt product.

A neutralisation reaction is a reaction between an acid and a base that produces a salt and water. The reaction is exothermic.

The products are a salt and water. The general equation is: acid + base → salt + water.

True.

All acid-base neutralisation reactions release energy to the surroundings. This means the temperature of the reaction mixture rises during the reaction.

Hydrochloric acid produces chloride salts, sulfuric acid produces sulfate salts, and nitric acid produces nitrate salts.

The ionic equation is H⁺ (aq) + OH^- (aq) → H₂O (l). The H⁺ ions from the acid react with the OH^- ions from the base to form water.

2HCl (aq) + CuO (s) → CuCl₂ (aq) + H₂O (l). The products are copper chloride solution and water.

False.

Neutralisation requires both a salt and water to be produced. When a metal reacts with an acid, hydrogen gas is produced rather than water, so it is not a neutralisation reaction.

Neutralisation is used to raise the pH of acidic soils. Bases such as limestone or quicklime are added so that crops can grow in conditions closer to neutral pH.

Carbon dioxide (CO₂) is produced. This causes fizzing (effervescence) during the reaction.

The carbonate ion is CO₃^2-, a negatively charged ion found in all carbonate compounds. It reacts with acids to produce carbon dioxide gas, water, and a salt.

Carbon dioxide is tested by bubbling the gas through limewater. If CO₂ is present, the limewater turns milky or cloudy due to the formation of a white precipitate of calcium carbonate.

True.

Fizzing (effervescence) is the correct observation. Stating that carbon dioxide is produced is an explanation, not an observation. The fizzing is caused by CO₂ gas being released.

When carbon dioxide is bubbled through limewater, a white precipitate of calcium carbonate forms, causing the limewater to turn milky.

CO₃^2- (aq) + 2H⁺ (aq) → CO₂ (g) + H₂O (l). The carbonate ion reacts with hydrogen ions to form carbon dioxide gas and water.

True.

Fizzing (effervescence) is the correct observation. Stating that carbon dioxide is produced is an explanation, not an observation. The fizzing is caused by carbon dioxide gas being released.

A soluble salt is made by reacting an acid with an excess of an insoluble base. The excess base is then removed by filtration, and the solution is heated until crystals form.

Adding a reactant in excess means adding more than is needed to react with all of the other reactant. This ensures the other reactant is fully used up.

The base is added in excess to ensure all the acid has reacted. Any unreacted acid would become dangerously concentrated during the evaporation step.

True.

All the acid has reacted with the excess base, and the unreacted solid base has been removed by filtration. The remaining solution therefore contains only salt and water.

To make copper(II) sulfate, copper(II) oxide is added in excess to sulfuric acid. The excess solid is removed by filtration, and the solution is heated until crystals begin to appear.

Large crystals are obtained by allowing the filtered salt solution to evaporate slowly over several days. Rapid heating produces smaller crystals.

False.

The excess base has not all reacted. Filtration removes the unreacted excess base. It is added in excess deliberately so that none of the acid remains, and then the surplus solid is filtered off.

CuO (s) + H₂SO₄ (aq) → CuSO₄ (aq) + H₂O (l). The product copper sulfate can then be crystallised from the solution.

Heating the acid speeds up the rate of reaction between the acid and the insoluble base, making the preparation more efficient.

True.

The base is added a little at a time and stirred until it stops dissolving and a suspension forms. This confirms the base is now in excess, ensuring all the acid has reacted.

A salt is a compound formed when the hydrogen atom in an acid is replaced by a metal ion. For example, replacing H in HCl with potassium gives KCl.

After filtering out the excess base, the solution is heated to evaporate half of the water. It is then left in a warm place to crystallise.

Dip a cold glass rod into the solution. If crystals form on the rod, the solution is saturated and ready to be left to crystallise.

A glass rod is touched onto indicator paper after adding the base. A neutral colour change confirms that the acid has been fully neutralised by the excess base.

False.

Heating strongly produces smaller crystals. Allowing the solution to evaporate slowly over several days produces larger, better-formed crystals.

CuCO₃ (s) + H₂SO₄ (aq) → CuSO₄ (aq) + H₂O (l) + CO₂ (g). Carbon dioxide gas is also produced because a carbonate was used.

The first part comes from the metal, metal oxide, or metal carbonate used. The second part comes from the acid used in the reaction.

Hydrochloric acid produces chloride salts, sulfuric acid produces sulfate salts, and nitric acid produces nitrate salts.

Magnesium chloride (MgCl₂) is formed. The metal comes from magnesium oxide and the acid is hydrochloric acid, which gives chloride salts.

True.

Sulfuric acid always produces sulfate salts. The sulfate ion SO₄^2- forms the second part of the salt name. For example, sulfuric acid reacting with calcium carbonate gives calcium sulfate.

The nitrate ion is NO₃^-, the negatively charged ion found in all nitrate salts. Nitrate salts are formed whenever nitric acid reacts with a base, metal, or carbonate.

Potassium nitrate (KNO₃) is formed. Potassium comes from the base and nitric acid produces nitrate salts.

True.

The metal part of the salt (calcium) comes from the calcium carbonate base, and the acid part (sulfate) comes from the sulfuric acid. The other products are water and carbon dioxide.

2HNO₃ (aq) + Ca(OH)₂ (aq) → Ca(NO₃)₂ (aq) + 2H₂O (l). The salt formed is calcium nitrate.

The sample is first acidified with dilute hydrochloric acid, then aqueous barium chloride is added. A white precipitate of barium sulfate confirms sulfate ions are present.

The sulfate ion test gives a positive result when a white precipitate of barium sulfate (BaSO₄) forms after adding acidified barium chloride solution to the sample.

True.

The acid is added first to remove any carbonate ions that may be present. Carbonates would also form a precipitate with barium ions, giving a false positive result.

Ba²⁺ (aq) + SO₄^2- (aq) → BaSO₄ (s). The white precipitate of barium sulfate confirms sulfate ions are present.

When testing for sulfate ions, the sample is acidified with dilute hydrochloric acid, then barium chloride solution is added. A white precipitate confirms a positive result.

True.

Barium nitrate solution also provides barium ions (Ba²⁺), which will react with sulfate ions to form the same white precipitate of barium sulfate.

Carbonate ions also form a white precipitate with barium ions. If carbonates were present, a false positive result would be obtained, so the acid removes them first.

A titration is used to find the exact volume of acid needed to neutralise a given volume of alkali. This ensures the final solution contains only salt and water, with no excess acid or alkali.

True.

The indicator would contaminate the salt product. The exact volume of acid found in the first titration is added to a fresh portion of alkali, with no indicator, to produce a pure salt solution.

In a titration, the acid is added from a burette to the alkali in a conical flask. A few drops of indicator are used to show when neutralisation is complete.

The alkali is more concentrated. Only half the volume of alkali is needed to neutralise the acid, so the alkali must be twice as concentrated as the acid.

A titration is a technique used to find the exact volume of one solution needed to completely react with a measured volume of another solution, often used to determine concentration.

The concentration of HNO₃ is 0.59 mol dm^-3 (to 2 d.p.).

Moles Ba(OH)₂ = 0.15 × 0.025 = 3.75 × 10^-3 mol

Moles HNO₃ = 3.75 × 10^-3 × 2 = 7.5 × 10^-3 mol

Concentration = 7.5 × 10^-3 ÷ 0.0128 = 0.59 mol dm^-3

True.

A burette is a calibrated glass tube that allows accurate measurement of the volume of liquid delivered. It is used in titrations to add solution dropwise and record the exact volume used.

The volume of HNO₃ needed is 40 cm³.

Moles KOH = 0.80 × 0.025 = 0.02 mol

Moles HNO₃ = 0.02 mol (1:1 ratio)

Volume = 0.02 ÷ 0.50 = 0.040 dm³ = 40 cm³

Concordant results are two titre values that are within 0.1 cm³ of each other. Only concordant results are used to calculate the mean titre.

A rough titration is used to estimate the approximate end-point volume. This allows the student to add the solution more carefully (drop by drop) when approaching the end-point in accurate repeat titrations.

True.

Burettes are calibrated in 0.1 cm³ divisions, allowing readings to be estimated to 0.05 cm³. Recording to this precision is required for accurate titre values.

In an acid-base titration, the alkali is placed in the conical flask using a pipette. The acid is added from a burette.

The mean titre is 14.95 cm³.

Mean = (14.90 + 15.00) ÷ 2 = 14.95 cm³. These two results are concordant as they are within 0.1 cm³ of each other.

The funnel can drip liquid into the burette, making the initial volume reading inaccurate. It should only be used to fill the burette and then removed before readings are taken.

False.

The rough titre is only used to estimate the end-point. Only concordant accurate titres (within 0.1 cm³ of each other) are used to calculate the mean titre.

Phenolphthalein is commonly used. It changes from pink to colourless when acid is added to alkali, indicating that the end-point has been reached.

Concentration is the amount of solute dissolved in a given volume of solution. It can be expressed in g dm^-3 or mol dm^-3.

Concentration (mol dm^-3) = moles of solute ÷ volume of solution (dm³). Rearranging: moles = concentration × volume.

To convert cm³ to dm³, divide by 1000. 1 dm³ is equal to 1000 cm³, which is the same as 1 litre.

The concentration is 5 g dm^-3.

Concentration = mass ÷ volume = 10 ÷ 2 = 5 g dm^-3

True.

Concentration measures how much solute is present per unit volume. Adding more solute to the same volume increases the concentration.

The number of moles is 0.5 mol.

Moles = concentration × volume = 0.2 × 2.5 = 0.5 mol

True.

Concentration can be given as mass per unit volume (g dm^-3) or as moles per unit volume (mol dm^-3). Chemists often prefer mol dm^-3 as it relates directly to the amount of substance.

The volume is 200 cm³.

Volume = moles ÷ concentration = 0.1 ÷ 0.5 = 0.2 dm³. Multiplying by 1000 gives 200 cm³.

The three variables are moles, concentration, and volume of the acid or alkali. These are linked by: concentration (mol dm^-3) = moles ÷ volume (dm³).

The concentration of HCl is 0.0484 mol dm^-3.

Moles NaOH = 0.0121 × 0.100 = 1.21 × 10^-3 mol

Moles HCl = 1.21 × 10^-3 mol (1:1 ratio)

Concentration = 1.21 × 10^-3 ÷ 0.025 = 0.0484 mol dm^-3

True.

At the end-point of a 1:1 neutralisation reaction, the moles of acid and alkali are equal. This is the basis for calculating unknown concentrations from titration data.

To calculate moles from concentration and volume: moles = concentration × volume (in dm³). To convert cm³ to dm³, divide by 1000.

A neutralisation calculation uses titration data to find the moles, concentration, or volume of an acid or alkali, based on the balanced equation for the neutralisation reaction.

18.50 cm³ is equal to 0.01850 dm³. To convert from cm³ to dm³, divide by 1000.

True.

The moles of each reactant must be adjusted using the stoichiometric ratio from the balanced equation. For example, in a 1:2 ratio, you multiply or divide moles accordingly.

0.00400 mol of NaOH are neutralised.

Moles HCl = 0.200 × 0.020 = 0.00400 mol. Since the ratio is 1:1, moles NaOH = 0.00400 mol.

A dilute acid is an acid solution that contains a large amount of water relative to the number of acid molecules. The acid molecules are spread out in a high volume of water.

A concentrated acid is an acid solution containing little or no water. The acid molecules are present in a high proportion relative to the solvent.

True.

A dilute acid has a large proportion of water, while a concentrated acid has little or no water. These terms describe the proportion of solute to solvent, not the strength of the acid.

Yes. Hydrochloric acid is a strong acid that can be either dilute or concentrated. Strong and weak refer to how fully the acid ionises, while dilute and concentrated refer to the amount of acid dissolved in water.

A dilute acid has acid molecules mixed with a large amount of water. A concentrated acid has little or no water mixed with the acid molecules.

False.

Weak and strong refer to the degree of ionisation. A weak acid such as ethanoic acid can be either dilute or concentrated. These are separate, independent descriptions.

The key difference is the amount of water present. A dilute acid has a large amount of water relative to acid molecules, while a concentrated acid has very little or no water.

A strong acid is an acid that completely dissociates in water, producing a high concentration of H⁺ ions and a very low pH. Examples include HCl and H₂SO₄.

A weak acid is an acid that only partially dissociates in water. An equilibrium is established between the acid molecules and their ions, resulting in a pH closer to 7 (but still below 7).

True.

Strong acids such as hydrochloric acid and sulfuric acid fully ionise in water to produce a high concentration of H⁺ ions. This gives them a very low pH.

For a weak acid, the equilibrium lies to the left. This means there is a lower concentration of H⁺ ions compared to a strong acid of the same concentration.

A strong acid has a greater concentration of H⁺ ions because it fully dissociates. More reacting particles means a faster, more vigorous reaction, producing gas and heat more rapidly.

A strong acid produces gas more rapidly (e.g. hydrogen or carbon dioxide) and shows a greater temperature increase during the reaction compared to a weak acid.

False.

Strong and weak refer to degree of ionisation, while concentrated and dilute refer to amount of solute. A dilute strong acid can have a lower pH than a concentrated weak acid, depending on the specific concentrations.

CH₃CH₂COOH ⇌ H⁺ + CH₃CH₂COO^-. The reversible arrow shows that equilibrium is established, with the equilibrium lying to the left.

An insoluble salt is prepared by mixing two soluble salt solutions that react to form an insoluble precipitate. The precipitate is then filtered, washed, and dried.

A precipitation reaction is a reaction between two solutions that produces an insoluble solid called a precipitate. It is used to prepare insoluble salts.

To prepare an insoluble salt, two soluble salts are dissolved in water and mixed. The precipitate is filtered off, washed with distilled water, and dried in an oven.

True.

Both reactants must be soluble so they can be dissolved and mixed in solution. The insoluble salt then forms as a precipitate when the two solutions are combined.

Pb(NO₃)₂ (aq) + K₂SO₄ (aq) → PbSO₄ (s) + 2KNO₃ (aq). Lead(II) sulfate forms as an insoluble precipitate.

Washing with distilled water removes any traces of the other soluble salts (e.g. potassium nitrate) from the surface of the precipitate, giving a purer product.

True.

The insoluble precipitate is collected by filtration. The soluble salts in the mixture pass through the filter paper, while the insoluble salt is retained as the residue.

A soluble salt is made by reacting an acid with an excess insoluble base, then filtering and crystallising. An insoluble salt is made by mixing two soluble solutions to form a precipitate, then filtering and drying.