Strong and Weak Acids

Some molecules which contain Hydrogen can have this Hydrogen removed by adding water. A covalent bond in the molecule breaks, making a H⁺ ion and a negative ion.
Some molecules which contain Hydrogen, do not produce H⁺ in an aqueous solution; they are not proton donors; they are not acids.

e.g. CH₄, NH₃, C₂H₆

When a molecule splits up into ions, we say that is Dissociates.
STRONG ACIDS are full dissociated in aqueous solutions. 100% of the molecule breaks up into ions.
HCl à H⁺ + Cl^-
H₂SO₄ à H⁺ + HSO₄^- _(aq)
WEAK ACIDS are only partially dissociated in aqueous solutions.

CH₃COOH_(aq) ó CH₃COO^- + H⁺
ethanoic acid ethanoate ion

The symbol ó means the reaction does is reversible: it does not go to completion.
Ethanoic acid molecule has the structural formula
__i_H_i__O
_i__|_i_//
H - C - C
__i_|_i_\
__i_H__iO - H (Potentially Removable)

The words STRONG and WEAK have nothing to do with the concentration. It is quite possible to have a concentrated solution of a weak acid.

The effect of strong and weak acids on the rate of reaction

Equal volumes of Ethanoic acid and Hydrochloric acid were placed in individual test-tubes. Magnesium ribbon of equal length were then placed in each tube.

Hydrochloric Acid - More vigorous and finished quicker.
Both resulted in a colourless solution and equal volumes of Hydrogen gas.

Ionic - Mg + 2H⁺ à Mg²⁺ + H₂
Mg + 2HCl à MgCl₂ + H₂
Mg + 2CH₃COOH à Mg(CH₃COO)₂ + H₂

We can show these reactions on a sketch graph

Both curves become less steep with time because the reactions are slowing down. As the acids are reacting, their acceleration is falling.
N.B.2, The ethanoic acid reaction is slower because the same concentration of molecules produces a much lower concentration of H⁺
N.B.3 The ethanoic acid eventually produces the same volume of hydrogen because the magnesium removes H⁺ from the equilibrium below :

CH₃COOH_(aq) ó H⁺_(aq) + CH₃COO^-_(aq)

So the backward reaction can’t happen. Eventually, all the removable hydrogen on the CH₃COOH has been converted to hydrogen gas.
N.B.4 Similar results are obtained if marble chips are used on the acids instead of magnesium ribbon.

Salt + water + carbon dioxide

Neutralisation reactions in terms of ions

Acid + Alkali à Salt + Water
H⁺ + OH^- à H₂0

Acid + Metal Oxide à Salt + Water
2H⁺ + O^2- à H₂0

Acid + Metal Carbonate à Salt + Water + Carbon Dioxide
2H⁺ + CO₃^2- à H₂0 + CO₂

Metal + Acid à Salt + Hydrogen
Metal + 2H⁺ à M²⁺ + H₂

Common Lab Acids

Hydrochloric Acid HCl H⁺Cl^- Chloride
Sulphuric Acid H₂SO₄ H⁺So₄^2- Sulphide
Nitric Acid HN0₃ H⁺NO₃^- Nitrate

A Salt is a compound in which the Hydrogen ion part of an acid has been completely or partly replaced by another positive ion. i.e. a metal ion or an ammonium ion

e.g.
Zinc + Hydrochloric Acid à Zinc Chloride + Hydrogen
Zn + 2HCl à ZnCl₂ + H₂

Copper Oxide + Nitric Acid à Copper Nitrate + Water
CuO + 2HNO₃ à Cu(NO₃)₂ + H₂O

Magnesium Carbonate + Sulphuric Acid à Magnesium Sulphate + Water + Carbon Dioxide
MgCO₃ + H₂So₄ à MgSO₄ +H₂0 + CO₂

Most metal oxides and metal hydroxides don't dissolve in water, for instance, if Iron (iii) Nitrate is mixed with Sodium Hydroxide solution, a red brown solid of Iron (iii) Hydroxide is formed. This is known as a precipitate meaning "It happens".

Fe(NO₃)₃ + NaOH à Fe(OH)₃ +3NaNo₃

The Sodium ions and Nitrate ions don't take part in this precipitation reaction: they are "Spectator ions" and so they are omitted from the ionic equation.

Fe³⁺ + 3OH^- à Fe(OH)₃

This solid, like all other insoluble metal hydroxides, is not an alkali because, although it contains hydroxide ion, it is not an aqueous solution. It is however a base, it is a proton acceptor and will neutralise an acid to make a salt.

2Fe(OH)₃ + 3H₂SO₄ à Fe₂(SO₄)₃ + 6H₂O

Metal oxides are also basic.

O^2- + 2H⁺ à H₂O

Most metal oxides don't dissolve in water, but those that do, react to form alkaline solution: oxide ions don't exist in solution.

0^2- + H₂0 à 2OH^-
Na₂O + H₂O à 2NaOH

Both metal oxides and metal hydroxides are basic. They contain negative ions which attract the H⁺ causing a reaction.
Non-metal oxides are covalently bonded with either simple or macro-molecular structures. They lack the negative ions that attract protons. They are in fact acidic.

They either....
dissolve in water to produce strong acidic solutions,
dissolve in water to produce a weak solution,
or they can be insoluble in water, but react with alkalis to make salts.

SiO_2(s) + 2NaOH_(aq) à Na₂SiO_3(aq) + H₂O

Some oxides and hydroxides are Amphoteric: they react with acids and alkalis to form salts.
Water is a NEUTRAL oxide.

Titration

When an alkali neutralises an acid, both reactants are colourless solution, and the product is also a colourless solution, so it is hard to tell when the reaction is over, i.e. it is hard to prevent one of the reactants being in excess. Titration uses precisely calibrate glassware to find out the exact volume of an acid, which is required to neutralise a fixed volume of alkali.

The apparatus is rinsed with the solution which is going to contain. This prevents dilution.
A pipette is used to transfer exactly 25 cm of alkali into a conical flask.
A few drops of the suitable indicator were then added to the solution in the conical flask, and acid was added from the burette until the indicator changed colour. This is a rough result. The volume of the burette was then noted.

Methyl orange Acid = Red Alkali = Yellow
Phenolphthalein Acid = Colourless Alkali = Pink-purple

The process was repeated, but this time the acid was dropped drop by drop near the neutralisation point. The conical flask is constantly swirled at this time. When enough acid has been added, we say that the end point or equivalence point of the titration is reached.

Finally the process was performed the third time, without any indicator so that the salt is not contaminated by any dye.
This resulting solution is poured into an evaporation dish and heated until crystallisation point is reached. It is then left to cool to room temperature and then evaporates slowly, leaving large crystals of the salt.

Acid + Alkali à Salt + Water

Potassium Hydroxide + Nitric Acid à Potassium Nitrate + Water

Precipitation Reactions

Sometimes when two solutions are mixed, a solid forms. This solid is known as a precipitate. We can use the solubility rules to predict if a solid is formed when two solutions are mixed, and if we wish to prepare an insoluble solid, we can choose two solutions and mix them together.

e.g. Silver Nitrate Sol. + Sodium Chloride à Silver Chloride + Sodium Nitrate Sol.
MMMMAgNO₃ MMMMI+MMMM NaClMMI à MMMAgClMMII + MMMMNaNO₃

We remember that spectator ions are omitted from ionic equations. So here we have:

e.g.1 If we want to prepare a sample, we choose a solution of a soluble barium compound e.g. Barium chloride and a solution of a soluble sulphate, e.g. sodium sulphate. When these solutions are mixed, a white precipitate will form.

Barium Chloride Sol. + Sodium Sulphate Sol. à Barium Sulphate + Aqueous Sodium Chloride
MMMMBaCl₂ MMMM+MMMMM Na₂SO₄ MMIà MMMMBaSO₄ MM+ MMMMMM2NaCl

If we wanted to prepare a sample of Copper Carbonate, we would choose a solution of a soluble Copper Compound.

Copper Chloride + Sodium Carbonate à Copper Carbonate + Sodium Chloride
MMMCaCl₂ MMI+ MMMNa₂CO₃ MMIà MMMMCuCO₃ MM+ MMM2NaCl

Precipitation reactions are commonly used to test whether experiments to see if solutions contain particular ions.

Using precipitation reactions to prepare pure samples of insoluble salts

1. If the solubility rules say that a compound is insoluble, then it can be prepared by a precipitation reaction.
2. Solutions of soluble compounds containing the required ions are then mixed.
3. This solution should be filtered, and the residue in the filter paper should be washed of unwanted ions. Distilled water should be used. This solid should be left to dry, and will now be pure.

Testing for Cations

Most metal Hydroxides are insoluble. Many have characteristic colours which can be used to identify ions present.

Fe²⁺ + 2OH^- à Fe(OH)₂ Dark Green

This precipitate can be oxidised by oxygen in the air. It turns brown at the surface as Iron (iii) Hydroxide is formed.

Cu²⁺ + 2OH^- à Cu(OH)₂ Pale Blue
Al³⁺ + 3OH^- à Al (OH)₃ White

All these insoluble metal Hydroxides are basic: they will react with acids to produce solutions of salt. However, we remember that Aluminium Hydroxide is Amphoteric. Adding excess NaOH therefore causes this white precipitate to dissolve to form a colourless solution.