Acids Flashcards

All 62 cards in this deck

What ion do all acids produce when dissolved in water?

Hydrogen ions, $\mathrm{H^+}$.

What ion do all alkalis produce when dissolved in water?

Hydroxide ions, $\mathrm{OH^-}$.

What colour is litmus in acidic and in alkaline solutions?

Red in acid, blue in alkali (purple in a neutral solution).

What colour is methyl orange in acidic and in alkaline solutions?

Red in acid, yellow in alkali (yellow in a neutral solution).

What colour is phenolphthalein in acidic and in alkaline solutions?

Colourless in acid, pink in alkali (colourless in a neutral solution).

True or false? Phenolphthalein turns pink in acidic solutions.

False. It is colourless in acid and pink in alkali.

What is the pH of a neutral solution, and which pH values are acidic or alkaline?

Neutral is pH 7; acidic solutions have pH below 7, alkaline solutions above 7.

How does the concentration of hydrogen ions in an acidic solution affect its pH?

The higher the hydrogen ion concentration, the lower the pH.

How does the concentration of hydroxide ions in an alkaline solution affect its pH?

The higher the hydroxide ion concentration, the higher the pH.

What happens to the pH when the hydrogen ion concentration of a solution increases by a factor of 10?

The pH decreases by 1.

True or false? A solution of pH 2 has 100 times the hydrogen ion concentration of a solution of pH 4.

True. Each pH unit represents a tenfold change in hydrogen ion concentration.

What are the steps in the core practical investigating the change in pH when a base is added to acid?
e.g. adding powdered calcium hydroxide to dilute hydrochloric acid

1. Measure a fixed volume of acid with a pipette or burette into a beaker and record its pH.
2. Add one weighed spatula of calcium hydroxide and stir.
3. Measure the pH, then repeat the additions until the pH stops changing.

How is the pH found when a drop of the mixture is placed on universal indicator paper?

Observe the colour produced and compare it with the pH chart/scale.

Which piece of equipment measures pH more accurately than universal indicator paper?

A pH meter (pH probe).

Why is the mixture stirred after each addition of the powdered base?

To mix it evenly so the base reacts/dissolves and the pH is uniform.

What happens to the hydrogen ions as calcium hydroxide is added to hydrochloric acid?

They are neutralised by hydroxide ions, so their concentration falls and the pH rises.

What does 'dilute' mean when describing a solution of an acid?

A low concentration of the acid (little solute in a given volume of solution).

What does 'concentrated' mean when describing a solution of an acid?

A high concentration of the acid (a large amount of solute in a given volume of solution).

What is a strong acid?

An acid that is fully dissociated (ionised) in solution, releasing hydrogen ions.

What is a weak acid?

An acid that is only partially dissociated (ionised), so fewer hydrogen ions are released than expected.

True or false? A concentrated acid must be a strong acid.

False. Concentration is how much acid is dissolved; strength is how far it dissociates — a concentrated solution of a weak acid is possible.

Name one example of a strong acid and one example of a weak acid.

Strong: hydrochloric acid (also sulfuric, nitric). Weak: ethanoic acid (also carbonic, citric).

What is a base?

Any substance that reacts with an acid to form a salt and water only.

What is an alkali?

A soluble base — it dissolves in water to give hydroxide ions.

True or false? All bases are alkalis.

False. Only bases that dissolve in water are alkalis, e.g. copper oxide is a base but not an alkali.

What name is given to the reaction between an acid and a base?

Neutralisation.

What is the ionic equation for the neutralisation of an acid by an alkali?

$\mathrm{H^+ + OH^- \rightarrow H_2O}$

Name two types of compound that act as bases.

Metal oxides and metal hydroxides, e.g. copper oxide and sodium hydroxide.

What are the products when a dilute acid reacts with a metal?

A salt and hydrogen.

What are the products when a dilute acid reacts with a metal oxide or a metal hydroxide?

A salt and water only.

What are the products when a dilute acid reacts with a metal carbonate?

A salt, water and carbon dioxide.

Which salts are made by hydrochloric, sulfuric and nitric acid?

Chlorides, sulfates and nitrates, e.g. nitric acid + ammonia gives ammonium nitrate.

What are the steps to write a balanced symbol equation with state symbols for a reaction of an acid?
e.g. magnesium + dilute hydrochloric acid

1. Write the formulae: $\mathrm{Mg + HCl \rightarrow MgCl_2 + H_2}$
2. Balance: $\mathrm{Mg + 2HCl \rightarrow MgCl_2 + H_2}$
3. Add state symbols: $\mathrm{Mg(s) + 2HCl(aq) \rightarrow MgCl_2(aq) + H_2(g)}$

What is the ionic equation for the reaction of magnesium with a dilute acid?

$\mathrm{Mg + 2H^+ \rightarrow Mg^{2+} + H_2}$

What is the chemical test for hydrogen, and the positive result?

Apply a lighted splint; it burns with a squeaky pop.

What is the chemical test for carbon dioxide, and the positive result?

Bubble the gas through limewater; it turns milky (cloudy).

What is limewater?

A solution of calcium hydroxide in water.

True or false? Hydrogen is tested for using a glowing splint that relights.

False. A relighting glowing splint tests for oxygen; hydrogen gives a squeaky pop with a lighted splint.

What are the steps to prepare pure, dry hydrated copper sulfate crystals?
e.g. from copper oxide and dilute sulfuric acid

1. Warm the acid and add copper oxide until it is in excess (no more dissolves).
2. Filter off the excess copper oxide to leave copper sulfate solution.
3. Evaporate some of the water using a water bath, leave to crystallise, then pat the crystals dry.

Why is copper oxide added in excess to the dilute sulfuric acid?

To make sure all the acid reacts, so the solution left is only salt and water.

How is the excess copper oxide removed from the copper sulfate solution?

By filtration — the solid stays on the filter paper.

Why is a water bath used instead of heating the copper sulfate solution strongly with a Bunsen burner?

It heats gently, so the solution does not spit and the crystals are not decomposed (they keep their water of crystallisation).

What colour are hydrated copper sulfate crystals?

Blue.

State one safety precaution needed when preparing copper sulfate from copper oxide and sulfuric acid.

Wear eye protection — the dilute sulfuric acid is an irritant/corrosive (also take care with hot apparatus).

What are the steps to prepare a pure, dry salt from an acid and a soluble base?
e.g. sodium hydroxide solution and dilute hydrochloric acid

1. Pipette the alkali into a flask, add indicator and titrate with acid from a burette; record the volume needed.
2. Repeat with the same volumes but no indicator.
3. Evaporate part of the solution, leave to crystallise, then dry the crystals.

Why must a titration be used when a soluble reactant is used to make a salt?

Excess soluble reactant cannot be filtered off, so the acid and alkali must be mixed in exactly the right proportions.

Which apparatus is used to measure the acid and the alkali in a titration?

A burette for the acid and a (volumetric) pipette for the alkali.

Why is the titration repeated using the same volumes but without indicator?

So the indicator does not contaminate (colour) the salt crystals.

What colour change does phenolphthalein show at the end point when acid is added to an alkali?

From pink/magenta to colourless.

True or false? Universal indicator is the best indicator to use in a titration.

False. It changes through a range of colours, so there is no sharp end point; phenolphthalein or methyl orange is used.

Which salts are always soluble in water?

All common sodium, potassium and ammonium salts, and all nitrates.

What is the solubility rule for common chlorides?

All are soluble except silver chloride and lead chloride.

What is the solubility rule for common sulfates?

All are soluble except lead, barium and calcium sulfate.

What is the solubility rule for common carbonates and hydroxides?

They are insoluble except those of sodium, potassium and ammonium.

True or false? Calcium carbonate is insoluble in water.

True. Carbonates are insoluble except sodium, potassium and ammonium carbonate.

True or false? Silver nitrate is insoluble in water.

False. All nitrates are soluble — it is silver chloride that is insoluble.

What are the steps to predict whether a precipitate forms when two solutions are mixed?
e.g. silver nitrate solution + sodium chloride solution

1. Swap the positive ions to give the two possible products: silver chloride and sodium nitrate.
2. Apply the solubility rules: silver chloride is insoluble, sodium nitrate is soluble.
3. Name the insoluble product as the precipitate: silver chloride.

What are the steps to prepare a pure, dry sample of an insoluble salt?
e.g. barium sulfate from barium chloride and sodium sulfate solutions

1. Mix the two solutions so the precipitate of barium sulfate forms.
2. Filter to collect the precipitate.
3. Wash it with distilled water and leave it to dry (e.g. in a warm oven).

Why is the precipitate washed with distilled water on the filter paper?

To remove the soluble salt solution and other soluble impurities.

True or false? Mixing any two solutions of soluble salts produces a precipitate.

False. A precipitate forms only if one of the possible products is insoluble.

Which two solutions could be mixed to make insoluble lead chloride?

Lead nitrate solution and a soluble chloride such as sodium chloride solution.

True or false? When a dilute acid reacts with a metal carbonate, the only products are a salt and water.

False. Carbon dioxide is also produced: acid + metal carbonate → salt + water + carbon dioxide.