Topic 8.4: The pH scale

8.4.1 Distinguish between aqueous solutions that are acidic, neutral or alkaline using the pH scale

The pH scale is created through this equation

On the pH scale, 7 is neutral, Anything below 7 is acidic and anything above 7 is alkaline.


8.4.2 Identify which of two or more aqueous solutions is more acidic or alkaline using pH values

To identify which of two solution is more acidic or alkaline, we have to understand that the lower the pH value, the more acidic it is. The high the pH value, the more alkaline it is.

8.4.3 State that each change of one pH unit represents a 10-fold change in the hydrogen ion concentration [H+(aq)]

A change of one pH unit represents a 10-fold change in the concentration of hydrogen ions.

This means that increasing the pH by one unit represents a decrease in the concentration of hydrogen by 10 times; decreasing by one pH unit represents an increase in the concentration of hydrogen by 10 times.

8.4.4 Deduce changes in [H+(aq)] when the pH of a solution changes by more than one pH unit

If the pH of a solution is changed from 3 to 5, deduce how the hydrogen ion concentration changes.

pH = 3, so the concentration of hydrogen is 10^-3 mol dm^-3

pH = 5, so the concentration of hydrogen is 10^-5 mol dm^-5

Therefore the concentration of hydrogen has decreased by 100.

Topic 8.3: Strong and weak acids and bases

8.3.1 Distinguish between strong and weak acids and bases in terms of the extent of dissociation, reaction with water and electrical conductivity

Acids produce H+ ions and bases produce OH- ions in aqueous solution.

Strong acid and bases have complete dissociation. Weak acid and bases have partial dissociation, often forms an equilibrium between the OH- ions and the base itself.

The reaction with water causes the final result to change the pH value. This is due to release of Hydrogen ion and hydroxide ion from the acid and base respectively.

Electricity conductivity of a solution depends on the concentration of mobile ions. Strong acids and strong bases will therefore show higher conductivity than weak acid and bases. This can be measured using a conductivity probe.

8.3.2 State whether a given acid or base is strong or weak

Strong acids include Hydrochloric acid, Nitric acid and Sulphuric acid

Strong bases include Lithium hydroxide, Sodium hydroxide, Potassium hydroxide and Barium hydroxide

Weak acids include Ethanoic acid, Carbonic acid and Phosphoric acid

Weak bases include Ammonia and Ethylamine

8.3.3 Distinguish between strong and weak acids and bases, and determine the relative strengths of acids and bases, using experimental data

Electrical conductivity of a solution as mentioned above depends on the availability of hydrogen ions. This can be measured using a conductivity meter.

The reactions of acids described depends on the concentration of hydrogen ions. They will therefore happen at a faster rate with stronger acids. This may be an important consideration, for example, regarding safety in the labouratory, but usually does not provide an easy means of quantifying data to distinguish between weak and strong acids.

As we will learn in the next section, the pH scale is a measure of the hydrogen ions concentration and so can be used directly to compare the strengths of acids. It is a scale in which the higher the hydrogen ion concentration, the lower the pH value.

Topic 8.2: Properties of acids and bases

8.2.1 Outline the characteristic properties of acids and bases in aqueous solution

We will look here at some typical reactions of acids and bases in aqueous solutions where H+ is the ion common to all acids. The bases considered here are those that neutralize acids to produce water and these include metal oxides and hydroxides, ammonia, soluble carbonates and hydrogencarbonates.

Acids react with metal, bases and carbonates to form salts.

Acid + Metal -> Salt + Hydrogen

Acid + Base -> Salt + Water

Acid + Carbonate -> Salt + Water + Carbon Dioxide

Topic 8.1: Theories of acids and bases

8.1.1 Define acids and bases according to the Bronsted-Lowry and Lewis theories

A Bronsted-Lowry acid is a proton donor

A Bronsted-Lowry base is a proton acceptor

A Lewis acid is an electron pair acceptor

A Lewis base is an electron pair donor.

8.1.2 Deduce whether or not a species could act as a Bronsted-Lowry and/or a Lewis acid or base

All Bronsted-Lowry acids are Lewis acids

Not all Lewis acids are Bronsted-Lowry acids

The term Lewis acid is usually reserved for those species which can only be described by Lewis theory, that is those that do not release H+.

8.1.3 Deduce the formula of the conjugate acid (or base) of any Bronsted-Lowry base (or acid)

Topic 8: Acids and bases

Topic 8 of the IB HL Chemistry syllabus is the Acids and bases. IBO recommends to spend 6 hours on this topic.

This topic has 4 sub-chapters: "Theories of acids and bases", "Properties of acids and bases", "Strong and weak acids and bases" and "The pH scale". Each are separated with numerical values in order of mentioned.

These are advanced HL syllabus statements, it is recommended to bring a Casio Graphical Calculator instead of Texas. Casio Calculators have the periodic table installed already.