Acids Flashcards

All 49 cards in this deck

What do acids produce in solution?

Hydrogen ions, $H^+$.

What do alkalis produce in solution?

Hydroxide ions, $OH^-$.

How is hydrogen chloride gas converted into hydrochloric acid?

Dissolve it in water (add water).

True or false? When sulfur dioxide gas dissolves in water, the solution turns blue litmus paper red.

True. The sulfur dioxide solution is an acid.

What is the pH of a neutral solution?

What range of pH values do acidic solutions have?

Lower than 7 (pH < 7).

What range of pH values do alkaline solutions have?

Higher than 7 (pH > 7).

True or false? Adding an alkali to dilute hydrochloric acid could change the pH from 1 to 12.

True. The mixture starts acidic and ends alkaline, passing through neutral.

What should be used to measure the pH of a mixture most accurately in the core practical?

A pH meter (pH probe).

In the pH core practical, what are the steps for each addition of powdered calcium hydroxide to the acid?

e.g. one spatula added to 50 cm³ of dilute hydrochloric acid

1. Add a measured mass of the solid to the fixed volume of acid.
2. Stir the mixture so it is evenly spread and reacts fully.
3. Measure the pH with a pH meter and repeat the additions.

What improvement to "add half a spatula of solid" gives more accurate results in the pH core practical?

Measure the mass of solid added / use a specified mass each time.

In the pH core practical, why does the pH level off once excess calcium hydroxide has been added?

All the acid has been neutralised and no more calcium hydroxide can dissolve, so the extra solid stays at the bottom and the pH stays constant.

What is a base?

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

What is an alkali?

A base that is soluble in water.

What type of reaction occurs when an acid reacts with a base or alkali?

Neutralisation (it is exothermic).

In an acid–alkali neutralisation, which ions react and what do they form?

$H^+$ from the acid reacts with $OH^-$ from the alkali to form water.

What is the general word equation for an acid reacting with a metal?

metal + acid → salt + hydrogen

What is the general word equation for an acid reacting with a metal carbonate?

metal carbonate + acid → metal salt + carbon dioxide + water

What is the general word equation for an acid reacting with a metal oxide or a metal hydroxide?

metal oxide (or hydroxide) + acid → metal salt + water

Which acid is used to make chloride salts, and which acids make sulfates and nitrates?

Hydrochloric acid makes chlorides; sulfuric acid makes sulfates; nitric acid makes nitrates.

True or false? Fizzing is seen when a metal oxide reacts with dilute acid.

False. No gas is produced; the solid just disappears/dissolves. Fizzing happens with metals or carbonates.

Describe the chemical test for hydrogen and its positive result.

Apply a lighted splint to the gas; a squeaky pop is heard.

Describe the chemical test for carbon dioxide and its positive result.

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

True or false? A glowing splint that relights shows a gas is hydrogen.

False. That is the test for oxygen; hydrogen gives a squeaky pop with a lighted splint.

A gas from a reaction does not turn limewater cloudy. What does this tell you?

The gas is not carbon dioxide.

Why must excess insoluble base be added to the acid when preparing a soluble salt?

To make sure all of the acid reacts, so only salt and water remain in solution.

How can you tell the insoluble base added to an acid is in excess?

Solid remains at the bottom and no more of it dissolves (no more fizzing if a carbonate).

What are the steps of the core practical to make pure, dry hydrated copper sulfate crystals?

e.g. starting from copper oxide and dilute sulfuric acid

1. Add excess copper oxide to warm dilute sulfuric acid and stir until no more dissolves.
2. Filter off the unreacted copper oxide.
3. Evaporate the filtrate in an evaporating basin over a water bath, then leave to crystallise and pat dry.

Why is a water bath used instead of direct heating when evaporating copper sulfate solution?

It heats gently and slowly, so the crystals form properly and the hydrated crystals are not decomposed.

Why must a titration be used to make a soluble salt from an acid and a soluble reactant (alkali)?

Excess alkali cannot be filtered off, so the exact volumes needed must be found to mix them in the correct proportions.

What are the steps of an acid–alkali titration to find the volumes needed for a salt?

e.g. hydrochloric acid and sodium hydroxide solution

1. Use a pipette to measure a fixed volume of the alkali into a conical flask and add a few drops of indicator.
2. Add acid from a burette, swirling, until the indicator just changes colour.
3. Record the volume of acid used (the titre).

After a titration, how is a pure, dry sample of the salt obtained?

Repeat using the same volumes without indicator, then evaporate/crystallise the solution.

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

The indicator would contaminate the salt crystals.

True or false? After a correctly done acid–alkali titration, the solution left in the flask contains only salt and water.

True. The acid and alkali have been mixed in exactly the right proportions, so neither is in excess.

Which common salts are always soluble, whatever the other ion?

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

Which common chlorides are insoluble?

Silver chloride and lead chloride (all other common chlorides are soluble).

Which common sulfates are insoluble?

Lead, barium and calcium sulfate (all other common sulfates are soluble).

What is the solubility rule for carbonates and hydroxides?

Common carbonates and hydroxides are insoluble, except those of sodium, potassium and ammonium.

What are the steps to predict whether a precipitate forms when two solutions are mixed?

e.g. barium chloride solution + dilute sulfuric acid

1. Swap the partners of the two compounds: barium sulfate and hydrochloric acid.
2. Use the solubility rules on each new compound: barium sulfate is insoluble.
3. Name any insoluble product as the precipitate: barium sulfate (white).

What is seen when sodium hydroxide solution is added to copper sulfate solution?

A blue solid (precipitate) of copper hydroxide forms, so the mixture goes cloudy.

Why is the precipitate washed with distilled water before drying?

To remove the soluble salt solution left on it, so the sample is pure.

In a balanced equation, what state symbols are used for powdered calcium hydroxide and for dilute hydrochloric acid?

Calcium hydroxide: $(s)$; hydrochloric acid: $(aq)$.

True or false? In a balanced symbol equation, hydrogen gas can be written as $2H$.

False. Hydrogen gas is made of molecules, so it is written $H_2$; $2H$ means two separate hydrogen atoms.

What colour is litmus in an acid, in a neutral solution and in an alkali?

Red in an acid, purple when neutral, blue in an alkali.

What colour is methyl orange in an acid, and in an alkali?

Red in an acid; yellow in an alkali (it is also yellow in a neutral solution).

What colour is phenolphthalein in an acid, and in an alkali?

Colourless in an acid (and in a neutral solution); pink in an alkali.

Why is universal indicator not used in a titration?

Its colour changes gradually over the pH range, so the end point is not clear. An indicator with one sharp colour change, such as phenolphthalein or methyl orange, is used instead.

In the pH core practical, describe the shape of the graph of pH against the amount of base added to the acid.

The pH rises slowly at first, then rises steeply around neutral (pH 7), then levels off at a high pH.

How is an insoluble salt made?

e.g. silver chloride

By precipitation: mix solutions of two soluble salts, one containing the wanted positive ion and one the wanted negative ion — e.g. silver nitrate and potassium chloride solutions.