Acids Flashcards

All 48 cards in this deck

What ions do acids produce in solution?

Hydrogen ions, H⁺.

What ions do alkalis produce in solution?

Hydroxide ions, OH⁻.
e.g. sodium hydroxide is a source of OH⁻ ions.

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

Neutralisation (it is exothermic).

What is the ionic equation for acid–alkali neutralisation?

$H^+ + OH^- \rightarrow H_2O$

What pH value does a neutral solution have, and what pH ranges are acidic and alkaline?

Neutral = pH 7; acidic = below 7; alkaline = above 7.

How does pH change as the concentration of hydrogen ions in a solution increases?

The pH decreases (gets lower).

If the hydrogen ion concentration decreases by a factor of 10, how does pH change?

The pH increases by 1.

What colour is litmus in acid and in alkali?

Red in acid, blue in alkali.

What colour is phenolphthalein in acidic and in alkaline solution?

Colourless in acid, pink in alkali.

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

Red in acid, yellow in alkali.

Why is litmus unsuitable for measuring the pH of a solution?

It only shows whether a solution is acidic or alkaline — it does not give a pH value or show how acidic/alkaline it is.

What are the steps to find the pH of a mixture using universal indicator paper?
e.g. acid after adding calcium hydroxide

1. Place a drop of the mixture on the universal indicator paper.
2. Observe the colour produced on the paper.
3. Compare the colour to a pH chart/scale to read the pH.

In the core practical adding powdered calcium hydroxide to dilute hydrochloric acid, how should the acid and the base each be measured accurately?

Acid: with a burette, pipette or measuring cylinder. Base: weighed on a balance.

What does 'dilute' mean when describing a solution?

It has a low concentration of solute (e.g. of acid / hydrogen ions) in the solution.

What is a strong acid?

An acid that fully dissociates (ionises) in solution to form H⁺ ions.

What is a weak acid?

An acid that only partially dissociates (ionises), so the concentration of H⁺ ions is lower than in a strong acid of the same concentration.

True or false? A dilute acid must be a weak acid.

False. Dilute refers to concentration; weak refers to the degree of dissociation — an acid can be dilute and strong.

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 OH⁻ ions).

True or false? All bases are alkalis.

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

What is the general equation for acid + metal?

acid + metal → salt + hydrogen
e.g. magnesium + sulfuric acid → magnesium sulfate + hydrogen

What is the general equation for acid + metal oxide?

acid + metal oxide → salt + water
e.g. copper oxide + sulfuric acid → copper sulfate + water

What is the general equation for acid + metal carbonate?

acid + metal carbonate → salt + water + carbon dioxide
e.g. $MgCO_3 + 2HCl \rightarrow MgCl_2 + H_2O + CO_2$

What are the steps to name the salt made when an acid reacts with a base?
e.g. calcium hydroxide + hydrochloric acid

1. First part = metal in the base: calcium.
2. Second part from the acid (HCl → chloride, H₂SO₄ → sulfate, HNO₃ → nitrate): chloride.
3. Salt = calcium chloride (plus water).

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

Hold a lighted splint at the mouth of the tube — a squeaky pop is heard.

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

Bubble the gas through limewater — the limewater turns cloudy (milky).

True or false? A gas that turns damp blue litmus paper red has been shown to be carbon dioxide.

False. That test only shows the gas is acidic; carbon dioxide must be tested with limewater.

When making copper sulfate from copper oxide and dilute sulfuric acid, how do you know to stop adding copper oxide?

When some powder remains in the beaker after stirring — the copper oxide is in excess, so all the acid has reacted.

Why is an excess of the insoluble base added when preparing a soluble salt from an acid?

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

How is the excess insoluble base removed after reacting it with an acid?

By filtration — the unreacted solid stays in the filter paper and the salt solution passes through.

In the copper sulfate core practical, how are dry crystals obtained from the filtrate?

Heat the filtrate in an evaporating basin over a water bath to evaporate some of the water, leave it to cool so crystals form, then pat the crystals dry with filter paper.

Why must a titration be used to prepare a salt from an acid and a soluble reactant?

The excess soluble reactant cannot be filtered off, so the exact volumes must be found to mix them in the correct proportions.

In a titration, which apparatus measures the alkali and which measures the acid added to it?

A pipette measures a fixed volume of alkali into the flask; a burette adds the acid.

What are the steps to prepare a pure, dry salt by titration?
e.g. sodium chloride from NaOH and HCl

1. Titrate the alkali with the acid using an indicator to find the exact volume needed.
2. Repeat with the same volumes but no indicator, so only salt and water are left.
3. Evaporate/crystallise the solution and dry the crystals.

Which common salts of sodium, potassium and ammonium are soluble in water?

All of them.

State the solubility rule for nitrates and for common chlorides.

All nitrates are soluble; common chlorides are soluble except silver chloride and lead chloride.

State the solubility rule for common sulfates.

Soluble except lead, barium and calcium sulfate.

State the solubility rule for common carbonates and hydroxides.

Insoluble, except those of sodium, potassium and ammonium.

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

1. Swap the partners to get the two possible products: silver chloride and sodium nitrate.
2. Check each against the solubility rules: silver chloride insoluble, sodium nitrate soluble.
3. The insoluble one is the precipitate: silver chloride.

Why is the precipitate washed with distilled water when preparing an insoluble salt?

To remove soluble impurities (the leftover solution) from the precipitate.

True or false? Both solutions mixed to make an insoluble salt by precipitation must be soluble in water.

True. The ions must be in solution to meet and form the precipitate.

True or false? When a metal carbonate reacts with a dilute acid, the gas given off is hydrogen.

False. The gas is carbon dioxide; only acid + metal reactions give off hydrogen.

In an alkaline solution, how does the pH change as the concentration of hydroxide ions (OH⁻) increases?

The pH increases (the solution becomes more alkaline).

Which two products form when an acid neutralises a metal hydroxide?

A salt and water.
e.g. nitric acid + calcium hydroxide → calcium nitrate + water

When making copper sulfate from copper oxide, why is the dilute sulfuric acid warmed in a water bath first?

Warming the acid makes it react faster with the insoluble copper oxide.

Why is universal indicator not suitable for a titration?

It passes through many colours as the pH changes, so you cannot see exactly when the acid has been neutralised. Methyl orange or phenolphthalein is used, because each changes colour suddenly at the end point.

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

Strong: hydrochloric acid (nitric and sulfuric acid are also strong).
Weak: ethanoic acid, the acid in vinegar (citric and carbonic acid are also weak).

How do the concentrations of H⁺ and OH⁻ ions compare in a neutral solution (pH 7)?

They are equal.