Acids and bases

8.1 - Theories of acids and bases

There are three important theories regarding acids and bases.

• Arrhenius
• Bronsted Lowry
• Lewis

Arrhenius refers to hydrogen ions in aqueous solution as being the fundamental cause of acidity. In this theory all acids release free hydrogen ions into the solution by dissociation. These hydrogen ions are solvated by water molecules to form other species such as the hydroxonium ion, H3O⁺. In gerneral the hydrogen ions are more simply referred to as H⁺(aq).

In Arrhenius theory a basic solution is caused by free hydroxide ions, OH-.

Bronsted Lowry looks at acidity from the point of view of hydrogen ions. This theory places no restrictions, such as the need for aqueous solution. In Bronsted Lowry theory an acid is any species that can release a hydrogen ion. Bases become any species that can accept a hydrogen ion. An Arrhenius acid is, of course, also a Bronsted Lowry acid, but the reverse is not necesarily true. Bronsted Lowry theory covers Arrhenius and extends it.

Lewis approaches acidity from the point of view of the electron pairs necessary to bond to the hydrogen ions and extends the concept of acidity to include any species that is able to accept an electron pair. A Lewis base is the species with the available electron pair.

Once again Lewis theory encompasses the previous two theories and extends them. Now there is no need for hydrogen ion and any dative coordinate bonding becomes an acid - base reaction.

8.2 - Properties of acids and bases

Strong and weak acids are defined by their ease of losing (or donating) a proton.

The term "acid" refers to a group of compounds whose chemical properties are similar.

1. They dissolve in water to give a solution with pH less than 7
2. They react with bases to give a salt and water
3. They give distinctive colours when in contact with indicators

The term "base" refers to a group of compounds whose chemical properties are similar.

1. They dissolve in water to give a solution with pH more than 7.
2. They react with acids to give a salt and water
3. They give distictive colours when in contact with indicators

There are several types of compounds which all come under the general heading of bases.

Note: An alkali is simply a soluble metal hydroxide (base)

Indicators are compounds that change colour depending on whether they're in acidic or basic conditions

Each one changes colour at a different pH, and so there will be situations where one is useful and others are not.

Reactions of acids with bases (neutralisation reactions)

Reaction of acids with metal hydroxides or metal oxides : Makes a salt + water,

HCl + NaOH -> H₂O + NaCl.

Reaction of acids with metals: Makes a salt + hydrogen

2HCl + Mg -> MgCl₂ + H₂.

Reaction of acids with carbonates: Makes a salt + water + carbon dioxide

2HCl + CaCO₃ -> CO₂ + H₂O + CaCl₂.

Experimental properties of acids and bases

When acids and bases neutralise, the reaction is noticeably exothermic (ie heat can be felt coming from the reaction). Obviously, they will have an effect on the colour of indicators as described above. The hydrogen produced in the reaction of acids with metal will produce a 'pop' sound if a match is held to it, and the CO₂ from the carbonate reaction will turn limewater a milky white when bubbled through it.

Examples of some common acids : HCl, CH₃COOH, H₂SO₄, HNO₃

Examples of bases: NaOH, NH₃

8.3 - Strong and weak acids and bases

A strong acid, when placed in water, will almost fully ionise/dissociate straight away, producing H₃O⁺ ions from water.

A weak acid will, however, only partially dissociate into ions, leaving a high percentage of unreacted molecules in the solution. An equilibrium is established, and so when some of the H₃O⁺ ions produced by a weak acid react, Le Chatelier's principle means that more of the acid will react to form H₃O⁺ ions. This means that, given an equal number of moles of acid, they will be neutralized by the same amount of strong base, but their solutions will have different pH values.

A weak base is the same as this, only it accepts protons and so produces OH^- ions from water rather than H₃O⁺.

Conductivity

Any solution's ability to conduct electricity is conditioned by the concentration of ions it contains. A strong acid has more ions than a weak one, and so it's solution will be a better electrical conductor than a weak acid. The same goes for strong/weak bases.

• Strong acids : HCl, HNO₃, H₂SO₄.
• Weak acids : CH₃COOH, H₂CO₃.
• Strong bases : group 1 hydroxides (ie NaOH etc), or lower group 2 hydroxides Ba(OH)₂.
• Weak bases : NH₃, CH₃CH₂NH₂.

The strength of an acid or base can be measured with an indicator (universal) or a pH meter. also the rate of reaction...hydrogen production with metals or CO₂ with CaCO₃ will reveal the strength of an acid.

8.4 - The pH scale

pH values ranges from 0 to 14 (7 being the neutral value of pure water at 20c and 1 atm).

Lower pH value are acidic, higher values are basic.

pH can be measured with a pH meter, or with pH paper (paper containing a mixture of indicators to cause a continuous color change).

pH is a measure of the dissociation of an acid or base, and also of the concentration of that acid / base (actually its related to the concentration of H₃O⁺ ions).

If we have two solutions with their pH values, the lower one will be more acidic and the higher one will be more basic (though they could both still be basic/acidic with respect to water -- pH 7).

Relationship between pH and acid concentration

A change of 1 in the pH scale represents a 10 times change in the acidity or basicity of the solution (because it's a log scale).

pH = - log [H+]

Resources

Strong and weak acids

The pH scale

Links