Reactions of Acids Flashcards

All 59 cards in this deck

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

metal + acid → salt + hydrogen

What is the balanced symbol equation, with state symbols, for magnesium reacting with dilute hydrochloric acid?

$\mathrm{Mg(s) + 2HCl(aq) \rightarrow MgCl_2(aq) + H_2(g)}$

What is the balanced symbol equation, with state symbols, for zinc reacting with dilute sulfuric acid?

$\mathrm{Zn(s) + H_2SO_4(aq) \rightarrow ZnSO_4(aq) + H_2(g)}$

What is the balanced symbol equation, with state symbols, for iron reacting with dilute hydrochloric acid?

$\mathrm{Fe(s) + 2HCl(aq) \rightarrow FeCl_2(aq) + H_2(g)}$

Give two observations that can be made when a reactive metal is added to dilute acid.

Effervescence (fizzing/bubbles) and the metal disappears (gets smaller); the temperature also increases.

True or false? When zinc reacts with dilute sulfuric acid the salt formed is zinc sulfide.

False. Sulfuric acid produces sulfates, so zinc sulfate ($\mathrm{ZnSO_4}$) is formed.

What do oxidation and reduction mean in terms of electrons?

Oxidation is loss of electrons; reduction is gain of electrons (OIL RIG).

In $\mathrm{Mg + 2HCl \rightarrow MgCl_2 + H_2}$, which species is oxidised?

Magnesium — it loses 2 electrons to form $\mathrm{Mg^{2+}}$.

In $\mathrm{Mg + 2HCl \rightarrow MgCl_2 + H_2}$, which species is reduced?

The hydrogen ions ($\mathrm{H^+}$) — each gains an electron to form $\mathrm{H_2}$.

Write the half equation for the oxidation of magnesium when it reacts with an acid.

$\mathrm{Mg \rightarrow Mg^{2+} + 2e^-}$

Write the half equation for the reduction of hydrogen ions in a metal–acid reaction.

$\mathrm{2H^+ + 2e^- \rightarrow H_2}$

True or false? When iron reacts with hydrochloric acid, the iron is reduced.

False. Iron is oxidised (it loses electrons); the hydrogen ions are reduced.

What are the products when an acid is neutralised by an alkali or a metal oxide?

A salt and water.

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

A salt, water and carbon dioxide.

Which type of salt is produced by hydrochloric acid, by nitric acid and by sulfuric acid?

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

What is the difference between a base and an alkali?

A base is any substance that neutralises an acid (e.g. insoluble metal oxides and hydroxides); an alkali is a base that is soluble in water.

What are the steps to deduce the formula of a salt from the formulae of its ions?

e.g. magnesium chloride, from $\mathrm{Mg^{2+}}$ and $\mathrm{Cl^-}$

1. Write the two ion formulae with charges: $\mathrm{Mg^{2+}}$, $\mathrm{Cl^-}$
2. Balance the charges with whole numbers: one $\mathrm{Mg^{2+}}$ needs two $\mathrm{Cl^-}$
3. Write the formula without charges: $\mathrm{MgCl_2}$

True or false? Neutralising nitric acid with potassium hydroxide produces potassium nitrate and water.

True. Acid + alkali → salt + water, and nitric acid produces nitrates.

Which types of insoluble solid can be added to a dilute acid to make a soluble salt?

Metals, metal oxides, metal hydroxides and metal carbonates.

Why is the insoluble solid added to the acid in excess when preparing a soluble salt?

So that all the acid reacts, and the excess solid can then be filtered off.

Give one observation showing that the insoluble solid added to an acid is in excess.

Solid remains in the mixture (it no longer disappears), or effervescence stops with a carbonate.

Why is the reaction mixture filtered when preparing a soluble salt?

To remove the excess insoluble solid, leaving a solution of the salt.

What are the steps to obtain solid crystals from a salt solution?

e.g. from zinc chloride solution

1. Heat the solution gently to evaporate some water, until crystallisation point (crystals start to form).
2. Leave the solution to cool and crystallise.
3. Pat the zinc chloride crystals dry with filter paper.

True or false? Any metal can be added to a dilute acid to make a soluble salt.

False. Unreactive metals such as copper do not react with dilute acids, so a copper oxide or copper carbonate must be used instead.

What are the steps of the required practical to prepare a pure, dry sample of a soluble salt?

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

1. Warm the dilute sulfuric acid with a Bunsen burner and add copper oxide until it is in excess.
2. Filter off the excess copper oxide.
3. Evaporate the filtrate with a water bath/electric heater, then leave it to crystallise and dry the crystals.

In the salt preparation required practical, what is used to heat the dilute acid before the solid is added?

A Bunsen burner — warming the acid makes the solid react faster.

In the salt preparation required practical, what equipment is used to evaporate the filtrate gently?

A water bath or an electric heater.

How are the salt crystals dried at the end of the salt preparation practical?

Pat them dry between sheets of filter paper, or leave them in a warm place/warm oven.

True or false? In the salt preparation practical, excess acid is used rather than excess solid.

False. Excess solid is used because it can be filtered off, leaving a pure salt solution.

Which ion do acids produce in aqueous solution?

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

Which ion is present in aqueous solutions of alkalis?

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

Write the ionic equation for the neutralisation of an acid by an alkali.

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

What does the pH scale measure, and what do pH 7, below 7 and above 7 mean?

Acidity or alkalinity, from 0 to 14: pH 7 is neutral, below 7 is acidic, above 7 is alkaline.

What two methods can be used to measure the pH of a solution?

Universal indicator (or a wide range indicator) for an approximate pH, or a pH probe/meter for an accurate value.

What colour is universal indicator in a strongly acidic, a neutral and an alkaline solution?

Red in strongly acidic, green in neutral, blue/purple in alkaline solution.

What is a strong acid, and name three examples?

An acid that is completely ionised in aqueous solution — hydrochloric, nitric and sulfuric acids.

What is a weak acid, and name three examples?

An acid that is only partially ionised in aqueous solution — ethanoic, citric and carbonic acids.

What do 'dilute' and 'concentrated' mean when describing an acid?

They describe the amount of acid dissolved in a given volume of solution: concentrated has a large amount per unit volume, dilute a small amount.

True or false? A weak acid and a dilute acid mean the same thing.

False. Weak refers to partial ionisation; dilute refers to a small amount of acid per unit volume.

For solutions of the same concentration, how does the pH of a strong acid compare with that of a weak acid?

The strong acid has the lower pH, because it is fully ionised so has a higher $\mathrm{H^+}$ concentration.

How does the hydrogen ion concentration change as the pH decreases by one unit?

It increases by a factor of 10.

What is a titration used to find?

The volumes of acid and alkali solutions that react exactly with each other.

Name an indicator used in an acid–alkali titration and give its colour change as acid is added to alkali.

Phenolphthalein: pink → colourless (or methyl orange: yellow → red).

Why is universal indicator not used in a titration?

It changes through a range of colours, so it does not give a sharp end point.

Which pieces of apparatus are used to measure the alkali and the acid in a titration?

A (volumetric) pipette measures a fixed volume of alkali into the conical flask; a burette holds and measures the acid added.

Give two reasons why a burette is used for the acid in a titration.

It can add the acid in small increments (dropwise) and can measure variable volumes accurately — more accurate than a measuring cylinder.

What are concordant titres?

Repeat titres that agree within 0.10 cm$^3$ of each other; only these are used to calculate the mean titre.

What are the steps of the titration required practical for a strong acid and strong alkali?

e.g. finding the volume of hydrochloric acid that neutralises 25.0 cm$^3$ of sodium hydroxide

1. Use a pipette to add 25.0 cm$^3$ of sodium hydroxide to a conical flask and add a few drops of indicator.
2. Fill a burette with the acid and take the initial reading.
3. Add acid, swirling, until the indicator changes colour permanently, then record the final reading.

What is the end point in an acid–alkali titration?

The point at which the indicator changes colour permanently, showing the acid has exactly neutralised the alkali.

Give three things that improve the accuracy of a titration.

Swirl the flask, use a white tile under it, add the acid dropwise near the end point, and repeat to calculate a mean.

How is the titre obtained from the burette in a titration?

Titre = final burette reading − initial burette reading.

Which acids and alkalis are used in the titration required practical?

Strong acids (hydrochloric, nitric or sulfuric) with a strong alkali such as sodium hydroxide solution.

What is the formula linking moles, concentration in mol/dm$^3$ and volume?

moles = concentration (mol/dm$^3$) × volume (dm$^3$)

How many cm$^3$ are there in 1 dm$^3$?

1000 cm$^3$ — divide a volume in cm$^3$ by 1000 to get dm$^3$.

What are the steps to calculate the concentration of an alkali from titration results?

e.g. 25.0 cm$^3$ of NaOH neutralised by 20.0 cm$^3$ of 0.100 mol/dm$^3$ HCl

1. Moles of acid = $\frac{20.0}{1000} \times 0.100 = 0.00200$ mol
2. Use the equation ratio (1 : 1): moles of NaOH = 0.00200 mol
3. Concentration = $0.00200 \div \frac{25.0}{1000} = 0.0800$ mol/dm$^3$

How do you convert a concentration in mol/dm$^3$ into g/dm$^3$?

e.g. 0.0800 mol/dm$^3$ NaOH ($M_r = 40$)

1. Multiply the concentration by the relative formula mass: $0.0800 \times 40 = 3.2$ g/dm$^3$

True or false? In every acid–alkali titration, the number of moles of acid equals the number of moles of alkali.

False. They are equal only if the balanced equation has a 1 : 1 ratio.

True or false? Diluting a strong acid with water can raise its pH above 7.

False. Adding water lowers the hydrogen ion concentration so the pH increases towards 7, but the solution stays acidic.

True or false? The rough (first) titre should be included when calculating the mean titre.

False. Only the concordant titres are used; the rough result is left out.