Chem Ch. 11 - Acids and Bases

What is an acid?

A substance that produces H⁺ ions in aqueous solution.

What is a base?

An oxide or hydroxide of a metal that can neutralise an acid.

What is an alkali?

A soluble base; alkalis are soluble metal hydroxides.

What ions are responsible for acidity?

H⁺ ions.

What ions are responsible for alkalinity?

OH⁻ ions.

What is an indicator?

A substance that changes colour depending on whether a solution is acidic or alkaline.

What happens to blue litmus in an acid?

It turns red.

What happens to red litmus in an alkali?

It turns blue.

What colour is litmus in an acid?

Red.

What colour is litmus in an alkali?

Blue.

What colour is methyl orange in an acid?

Red.

What colour is methyl orange in an alkali?

Yellow.

What colour is thymolphthalein in an acid?

Colourless.

What colour is thymolphthalein in an alkali?

Blue.

What are the four common laboratory acids and their formulae?

Hydrochloric acid HCl; sulfuric acid H₂SO₄; nitric acid HNO₃; ethanoic acid CH₃COOH.

Which acid is found in vinegar?

Ethanoic acid.

Which acid is found in lemons?

Citric acid.

Which acid is found in yoghurt?

Lactic acid.

Which acid is present in fizzy drinks?

Carbonic acid.

Give three common alkalis and their formulae.

Sodium hydroxide NaOH; potassium hydroxide KOH; calcium hydroxide Ca(OH)₂.

What is the pH of a neutral solution?

pH 7.

What pH values represent acidic solutions?

pH below 7.

What pH values represent alkaline solutions?

pH above 7.

What happens to H⁺ concentration as pH decreases?

H⁺ concentration increases.

What happens to OH⁻ concentration as pH increases?

OH⁻ concentration increases.

What does universal indicator show?

The approximate pH of a solution from its colour.

What is the best way to measure pH precisely?

Use a pH meter.

What is a strong acid?

An acid that is completely dissociated into ions in aqueous solution.

What is a weak acid?

An acid that is partially dissociated into ions in aqueous solution.

Is hydrochloric acid strong or weak?

Strong.

Is ethanoic acid strong or weak?

Weak.

Write the ionisation equation for hydrochloric acid.

HCl(aq) → H⁺(aq) + Cl⁻(aq).

Write the ionisation equation for ethanoic acid.

CH₃COOH(aq) ⇌ H⁺(aq) + CH₃COO⁻(aq).

Does strong mean concentrated?

No. Strong refers to the degree of ionisation; concentrated refers to the amount of solute per volume.

Two acids have the same concentration. Which has the lower pH: the one with more H⁺ ions or fewer H⁺ ions?

The one with more H⁺ ions.

What is the general reaction between an acid and a metal?

Acid + metal → salt + hydrogen.

What is the general reaction between an acid and a base?

Acid + base → salt + water.

What is the general reaction between an acid and a carbonate?

Acid + carbonate → salt + water + carbon dioxide.

What gas is produced when an acid reacts with a metal?

Hydrogen.

What gas is produced when an acid reacts with a carbonate?

Carbon dioxide.

How do you test for hydrogen?

Use a lighted splint; hydrogen produces a squeaky pop.

How do you test for carbon dioxide?

Bubble the gas through limewater; it turns milky/cloudy.

Write the reaction between magnesium and sulfuric acid.

Mg + H₂SO₄ → MgSO₄ + H₂.

Write the reaction between hydrochloric acid and sodium hydroxide.

HCl + NaOH → NaCl + H₂O.

Write the reaction between calcium carbonate and hydrochloric acid.

CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂.

Write the reaction between sulfuric acid and copper(II) oxide.

H₂SO₄ + CuO → CuSO₄ + H₂O.

What is neutralisation?

A reaction involving an acid that produces a salt and water.

Which reactions are neutralisations?

Reactions of acids with bases, alkalis and carbonates.

Why is acid + metal not called neutralisation?

It produces hydrogen rather than water.

What is the ionic equation for neutralisation between an acid and an alkali?

H⁺(aq) + OH⁻(aq) → H₂O(l).

What happens to H⁺ ions during neutralisation?

They combine with OH⁻ ions to form water.

What are salts?

Ionic compounds formed when the H⁺ ions of an acid are replaced by positive ions such as metal ions or ammonium ions.

What type of salts are formed from hydrochloric acid?

Chlorides.

What type of salts are formed from sulfuric acid?

Sulfates.

What type of salts are formed from nitric acid?

Nitrates.

What are bases generally?

Oxides and hydroxides of metals.

Give examples of bases.

Calcium oxide CaO; magnesium oxide MgO; sodium hydroxide NaOH; magnesium hydroxide Mg(OH)₂.

Are most bases soluble in water?

No. Most bases are insoluble in water.

What are alkalis?

Soluble bases, usually soluble metal hydroxides.

What happens when sodium hydroxide dissolves in water?

It forms an alkaline solution containing Na⁺ and OH⁻ ions.

How do bases react with acids?

Base + acid → salt + water.

What happens when a metal hydroxide reacts with an ammonium salt?

Salt + water + ammonia are produced.

Write the reaction between calcium hydroxide and ammonium chloride.

Ca(OH)₂ + 2NH₄Cl → CaCl₂ + 2H₂O + 2NH₃.

What is an oxide?

A compound containing oxygen and one other element.

What do metals generally form when they react with oxygen?

Basic oxides.

What do non-metals generally form when they react with oxygen?

Acidic oxides.

Give examples of basic oxides.

CaO, CuO and MgO.

Give examples of acidic oxides.

CO₂, SO₂ and phosphorus(V) oxide.

What is a basic oxide?

An oxide that can neutralise an acid to form a salt and water.

What is an acidic oxide?

An oxide that shows acidic behaviour, often forming an acidic solution with water.

What is an amphoteric oxide?

An oxide that reacts with both acids and alkalis.

Give two examples of amphoteric oxides.

Aluminium oxide Al₂O₃ and zinc oxide ZnO.

What is a neutral oxide?

An oxide that is neither acidic nor basic.

Give two examples of neutral oxides.

Carbon monoxide CO and dinitrogen oxide N₂O.

What happens when carbon dioxide dissolves in water?

It forms weak carbonic acid: CO₂ + H₂O ⇌ H₂CO₃.

What happens when sulfur dioxide dissolves in water?

It forms an acidic solution.

How can you make a soluble salt from a reactive metal such as zinc?

Add excess metal to dilute acid, filter off excess metal, evaporate some water and cool to crystallise the salt.

Why is excess metal added when making a salt?

To ensure all the acid is used up.

Why is excess metal filtered off?

The unreacted metal is insoluble and can be removed by filtration.

Why can't sodium, potassium and calcium generally be used in this method?

They react too violently with acids.

How can copper sulfate be made from copper(II) oxide and sulfuric acid?

Add excess CuO to warm dilute H₂SO₄ until no more dissolves, filter off excess CuO, evaporate some water and cool to crystallise CuSO₄.

Why is excess copper(II) oxide added?

To ensure all the sulfuric acid is neutralised.

Why is excess copper(II) oxide removed by filtration?

CuO is insoluble.

What is the equation for making copper sulfate from copper(II) oxide?

CuO + H₂SO₄ → CuSO₄ + H₂O.

Why is titration needed when making a salt from two soluble reactants?

Any excess soluble reactant cannot be removed by filtration.

What is a titration?

A method where one reactant is slowly added to another until the reaction is complete, using an indicator to identify the end point.

Why is an indicator used in titration?

It shows when the reaction is complete by changing colour.

Why is the titration repeated without indicator when making a pure salt?

The indicator would contaminate the final salt.

What apparatus is used to add acid accurately during titration?

A burette.

What apparatus accurately measures a fixed volume such as 25 cm³?

A volumetric pipette.

What is a precipitate?

An insoluble solid formed when two solutions react.

What is precipitation?

The formation of an insoluble solid when two solutions are mixed.

How do you make an insoluble salt by precipitation?

Mix two solutions containing the positive and negative ions needed to form the insoluble salt.

What ions must be present to form an insoluble salt by precipitation?

The required positive ion and negative ion.

How can barium sulfate be prepared by precipitation?

Mix a solution containing Ba²⁺ ions with a solution containing SO₄²⁻ ions.

Give one pair of solutions that can make barium sulfate.

Barium chloride solution and magnesium sulfate solution.

Write the equation for barium sulfate precipitation using barium chloride and magnesium sulfate.

BaCl₂(aq) + MgSO₄(aq) → BaSO₄(s) + MgCl₂(aq).

What is the ionic equation for barium sulfate precipitation?

Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s).

What are spectator ions?

Ions that are present in solution but do not take part in the reaction.

What are the spectator ions in BaCl₂ + MgSO₄ → BaSO₄ + MgCl₂?

Mg²⁺ and Cl⁻ ions.