Acids & Bases (SQA National 5 Chemistry): Flashcards

True.

Adding water to an acid decreases the concentration of H⁺ ions, making the solution less acidic and moving the pH upwards towards 7.

Diluting an alkali with water decreases the concentration of hydroxide ions (OH^-). With fewer OH^- ions present, the solution becomes less alkaline, so the pH falls towards 7.

Soluble non-metal oxides dissolve in water to form acidic solutions because they produce H⁺ ions.

Sodium oxide reacts with water to form sodium hydroxide, which releases OH^- ions into solution. This produces an alkaline solution with a pH above 7.

False.

Diluting an alkali decreases the concentration of OH^- ions, making the solution less alkaline. The pH decreases towards 7, not away from it.

An alkali is a base that is soluble in water. Alkalis dissolve to produce hydroxide ions (OH^-) in solution, giving a pH above 7.

Pure water produces equal numbers of H⁺ and OH^- ions, so it is neither acidic nor alkaline — it has a pH of 7.

Water produces equal concentrations of H⁺ and OH^- ions. Because neither type of ion is in excess, water is neutral with a pH of 7.

When an acid reacts with a base, a salt and water are produced. The general equation is: acid + base → salt + water.

Neutralisation is the reaction between an acid and a base that produces a salt and water. These reactions are exothermic.

True.

The reaction of an acid with a metal carbonate produces a salt, water and carbon dioxide gas. The CO₂ causes effervescence (fizzing).

Hydrochloric acid always produces salts ending in chloride, while nitric acid always produces salts ending in nitrate.

Effervescence occurs because carbon dioxide gas is produced during the reaction. The CO₂ forms bubbles, causing the mixture to fizz.

False.

For neutralisation to occur, water must be formed. Acid-metal reactions do not produce water, so they are not neutralisation reactions.

The name of a salt has two parts. The first part comes from the metal, metal oxide or metal carbonate used. The second part comes from the acid: hydrochloric acid gives chloride, sulfuric acid gives sulfate and nitric acid gives nitrate.

A salt is an ionic compound formed when an acid reacts with a base. Its name is determined by the acid used and the metal or base it reacted with.

Sulfuric acid always produces salts that end in sulfate and contain the sulfate ion, SO₄^2-.

When an acid reacts with a metal oxide, a salt and water are produced. For example, hydrochloric acid reacting with copper oxide produces copper chloride and water.

A spectator ion is an ion that does not change its state or charge during a chemical reaction. It is present in the solution but takes no part in forming the product.

1. Write a full balanced equation with state symbols. 2. Split all aqueous (aq) ionic compounds into separate ions. 3. Identify ions that appear unchanged on both sides — these are the spectator ions. 4. Write the equation without spectator ions — this is the net ionic equation.

False.

Only aqueous (aq) compounds are split into separate ions. Solids (s), liquids (l) and gases (g) are left in their formula form and are not separated.

The ionic equation for an acid reacting with a metal hydroxide is: H⁺ (aq) + OH^- (aq) → H₂O (l).

The net ionic equation shows the actual chemical change that occurs — only the ions that react to form products are shown. Spectator ions that do not participate in the reaction are removed.

False.

Na⁺ (aq) and Cl^- (aq) appear unchanged on both sides of the equation, so they are spectator ions. The net ionic equation is H⁺ (aq) + OH^- (aq) → H₂O (l).

The ionic equation is: 2H⁺ (aq) + O^2- (s) → H₂O (l). The oxide ion has state symbol (s) because insoluble metal oxides are not split into aqueous ions.

For an acid reacting with a soluble carbonate, the carbonate ion has state symbol (aq), but for an insoluble carbonate the state symbol is (s).

Spectator ions are removed because they do not participate in the chemical reaction. Excluding them makes the net ionic equation show only the species that actually change — giving a clearer picture of the real chemical change.

A titration is an analytical method used to determine the exact concentration of a solution by reacting it with a solution of known concentration.

The purpose of the indicator is to show when the reaction is complete. The end-point is the moment the indicator changes colour, showing that the acid and base have completely reacted with each other.

True.

A burette allows a precise but adjustable volume of solution to be added, making it ideal for titrations where the exact volume needed is unknown at the start.

1. Calculate the moles of the known solution using n = c × v.

2. Deduce the moles of the unknown solution using the mole ratio from the balanced equation.

3. Calculate the unknown concentration (c = n ÷ v) or volume (v = n ÷ c).

When using any titration calculation method, all volumes must be converted from cm³ to litres by dividing by 1000.

The end-point is the point in a titration at which the indicator changes colour, signalling that the acid and base have completely reacted with each other.

In the SQA formula, c is the concentration in mol l^-1, v is the volume in litres and n is the number of moles from the balanced equation (the balancing number). The subscripts 1 and 2 refer to the two different solutions.

False.

All volumes must be converted to litres before substituting into the formula. Divide any volume given in cm³ by 1000 first.

Step 1 — moles of NaOH:

n = 0.0121 l × 0.100 mol l^-1 = 1.21 × 10^-3 mol

Step 2 — moles of HCl (1:1 ratio):

n = 1.21 × 10^-3 mol

Step 3 — concentration of HCl:

c = (1.21 × 10^-3) ÷ 0.025 = 0.0484 mol l^-1

The SQA formula for titration calculations is: c₁v₁ ÷ n₁ = c₂v₂ ÷ n₂.

A salt is a compound formed during a neutralisation reaction, in which the hydrogen atom in an acid is replaced by a metal ion.

1. Measure a fixed volume of alkali into a conical flask and add a few drops of indicator.

2. Fill the burette with acid and record the starting volume.

3. Add acid slowly until the indicator changes colour.

4. Record the final volume and calculate the volume of acid added.

5. Repeat with the exact same volumes but without indicator to produce a pure salt solution.

False.

The first titration only determines the exact volume of acid needed. The pure salt is made in a second run using the same volumes but without indicator, so no dye contaminates the product.

Repeating without indicator is necessary because indicator is a coloured dye. If it remained in the solution and the water was evaporated, the final salt crystals would be coloured and impure.

To check that a salt solution is saturated, dip a cold glass rod into the solution and see if crystals form on the end.

The pure salt solution is gently heated to evaporate water until it becomes saturated, then left in a warm dry place to allow the remaining water to evaporate slowly, forming pure crystals.

True.

At the end-point of a neutralisation titration, the acid and alkali have completely reacted, leaving only the salt (sodium chloride) and water in solution.

A volumetric pipette is used to measure the fixed, precise volume of alkali into the conical flask.

During crystallisation, the saturated salt solution is left in a warm, dry place so that the water evaporates slowly and pure crystals form.

A titration cannot be used because the insoluble solid base would block the burette, making it impossible to add the base as a solution.

True.

Adding excess base guarantees that the acid is the limiting reactant and is completely used up. Any leftover solid base is then easily removed by filtration.

1. Warm dilute acid and slowly add excess insoluble base while stirring until no more reacts.

2. Filter the mixture to remove excess solid base.

3. Gently heat the filtrate in an evaporating basin until saturated.

4. Leave in a warm place to crystallise, then decant and dry the crystals.

The filtrate is the clear liquid that passes through the filter paper after filtration, leaving the insoluble solid behind.

In the excess method, you know the base is in excess when unreacted solid remains visible and the reaction (e.g. fizzing) has stopped.

Filtration removes the unreacted solid so that only the salt solution passes through, preventing the excess base from contaminating the final salt crystals.

False.

Chloride salts are made using hydrochloric acid. Sulfuric acid produces sulfate salts, and nitric acid produces nitrate salts.

A suitable acid is sulfuric acid (H₂SO₄) because sulfate salts come from sulfuric acid. A suitable insoluble base is zinc oxide or zinc carbonate, as both contain the zinc needed for the salt.

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

You check by dipping a cold glass rod into the hot solution. If crystals form on the end of the rod, the solution is saturated and ready to be left to crystallise.