Acids, bases & salt preparations (Edexcel IGCSE Chemistry (Modular): Unit 1): Flashcards

Hydrochloric acid acts as a proton donor by ionising in solution: HCl (aq) → H⁺ (aq) + Cl^- (aq). The released H⁺ ions make the solution acidic.

A proton acceptor is a substance that accepts protons (H⁺ ions) in solution. Bases (alkalis) act as proton acceptors.

Acids are proton donors. They ionise in solution to produce H⁺ ions. Bases are proton acceptors.

Sodium hydroxide acts as a proton acceptor by ionising in solution: NaOH (s) → Na⁺ (aq) + OH^- (aq). The OH^- ions can accept protons, making the solution alkaline.

When a metal reacts with a dilute acid, a salt and hydrogen gas are produced. The general equation is: metal + acid → salt + hydrogen.

Neutralisation is the reaction between an acid and a base that produces a salt and water. The general equation is: acid + base → salt + water.

Complete the table showing products when acids react with metal oxides or hydroxides.

False.

Only metals above hydrogen in the reactivity series react with dilute acids. Metals below hydrogen, such as copper and silver, do not react.

Metals high in the reactivity series, such as potassium and sodium, react explosively with acids because they are extremely reactive. The reaction releases hydrogen gas very rapidly.

Effervescence is the rapid release of gas bubbles during a chemical reaction. In acid-carbonate reactions, the gas produced is carbon dioxide.

True.

When an acid reacts with a metal carbonate, the products are a salt, carbon dioxide gas and water. The carbon dioxide causes effervescence.

Hydrochloric acid produces chloride salts. Sulfuric acid produces sulfate salts. Nitric acid produces nitrate salts.

A metal carbonate is a compound containing a metal ion and the carbonate ion (CO₃^2-). When reacted with an acid, it produces a salt, carbon dioxide gas and water.

Nitric acid always produces a nitrate salt when it reacts with a metal, base or carbonate. For example, reacting with potassium hydroxide gives potassium nitrate.

A base is a substance that can neutralise an acid to form a salt and water. Bases are usually oxides, hydroxides or carbonates of metals.

False.

Not all bases are alkalis. An alkali is a base that is soluble in water. All alkalis are bases, but bases that do not dissolve in water are not alkalis.

Alkaline solutions have a pH above 7 because they contain hydroxide ions (OH^-). These ions are produced when a base dissolves in water, making the solution alkaline.

Complete the table of common alkalis and their ions in solution.

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

True.

Ammonia solution is unusual because most bases are metal oxides, hydroxides or carbonates. When ammonia dissolves in water it produces hydroxide ions (OH^-), making it alkaline.

When ammonia (NH₃) dissolves in water, it reacts to form ammonium ions (NH₄⁺) and hydroxide ions (OH^-). These hydroxide ions make the solution alkaline.

In basic (alkaline) conditions, red litmus paper turns blue.

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

Filtration is the process used to remove the excess insoluble base from the salt solution after the acid has fully reacted. Only the salt solution and water pass through the filter.

False.

The excess solid base is removed by filtration, not evaporation. Evaporation is used afterwards to concentrate the salt solution and produce crystals.

To prepare a soluble salt, an insoluble base is added in excess to the acid. The excess solid is then removed by filtration.

After filtration, the salt solution is heated until small crystals begin to appear. For larger crystals, the solution is left to evaporate slowly over several days.

True.

A metal can be used to prepare a soluble salt if it is above hydrogen in the reactivity series and not too reactive. The reaction produces a salt and hydrogen gas.

Complete the table showing stages in soluble salt preparation.

If a metal carbonate is used as the base, any carbon dioxide (CO₂) produced is released into the atmosphere as a gas. The reaction still produces a salt and water.

The titration step is repeated without indicator so that the indicator does not contaminate the final salt. The exact volume of acid needed is already known from the first titration. A clean repeat gives a pure product.

A titration is a technique used to find the exact volume of one solution needed to react completely with a measured volume of another solution. In salt preparation, acid is added to alkali until the indicator shows the reaction is complete.

False.

The indicator is removed by repeating the titration without any indicator added. This keeps the final salt free from contamination.

A pipette is used to measure out the alkali into a conical flask, and a few drops of indicator are added before acid is run in from the burette.

The solution is only partially evaporated so that some water remains, allowing water of crystallisation to form in the salt. Fully drying on a hotplate could also cause the salt to decompose.

True.

Both phenolphthalein and methyl orange are suitable indicators for detecting the endpoint of the titration. Both change colour clearly when the acid has exactly neutralised the alkali.

The second salt solution is added in slight excess to ensure that the maximum amount of precipitate is obtained. This ensures all of the first solution has reacted, leaving no unreacted starting material in the product.

A precipitation reaction is a reaction in which two soluble solutions are mixed and produce an insoluble solid, called the precipitate, which separates from the solution.

To prepare a salt by precipitation, soluble salt 1 is mixed with soluble salt 2, forming an insoluble salt that falls out of solution.

False.

All nitrates are soluble in water. This makes them useful starting materials when preparing insoluble salts of silver or lead(II).

The precipitate is washed with distilled water to remove any traces of the reactant solutions that may be contaminating the solid. This ensures the final product is pure.

Match each step to its purpose when preparing an insoluble salt.

A double decomposition reaction is another name for a precipitation reaction, in which two soluble salts swap their ions to form an insoluble salt and a new soluble salt.

True.

The precipitate is the insoluble solid that forms when two soluble solutions are mixed. It is recovered by filtration.

Copper(II) oxide is added in excess to ensure that all of the acid is used up. If acid remained, it would become dangerously concentrated during the evaporation stage.

Excess describes a reactant that is present in greater quantity than needed to completely react with the other reactant. The excess reactant is not fully used up by the reaction.

False.

Hydrated copper(II) sulfate crystals are bright blue and regularly shaped.

The filtrate is gently heated to evaporate the water until the solution is saturated, then left in a warm place to crystallise.

The solution is tested by dipping a cold glass rod into it. If crystals form on the end of the rod, the solution is saturated and ready to crystallise.

True.

The excess copper(II) oxide is removed by filtration before the solution is evaporated. This prevents solid base from contaminating the final crystals.

Match each step in the copper(II) sulfate preparation to its purpose.

A precipitation reaction is used because lead(II) sulfate is insoluble in water, so it forms as a solid (precipitate) when two soluble solutions are mixed. The starting materials, lead(II) nitrate and potassium sulfate, are both soluble.

False.

Lead(II) sulfate is insoluble in water. This is why it forms as a precipitate when lead(II) nitrate and potassium sulfate solutions are mixed.

Lead(II) nitrate solution is mixed with potassium sulfate solution to form lead(II) sulfate as an insoluble precipitate.

The precipitate is washed with distilled water to remove any traces of the soluble reactants that contaminate the solid. This ensures the final product is pure.

A precipitate is an insoluble solid that forms when two solutions react together. It can be separated from the solution by filtration.

True.

Lead salts are toxic, so care must be taken when handling them in practical work.