RI Y3 Chem Notes Concentration, Strength and Basicity Of Acids.pdf

1 Notes – Concentration, Strength and Basicity of Acids One model of an acid is that it reacts with water to give hydrogen ions (H+), which then combine with water molecules to form hydronium or oxonium ions (H3O+). For an acid ‘HA’ dissolving in water, it may be represented as follows: HA (l) + H2O (l) → H3O+ (aq) + A– (aq) We say that the acid molecules have “ionized”. The A in HA does not stand for a particular element, but for the atom or group of atoms bonded to hydrogen in the molecule. So, for example, in hydrochloric acid ‘HA’ would be HCl, and ‘A –’ would be Cl–, whilst in ethanoic acid ‘HA’ would be CH3COOH, and ‘A–’ would be CH3COO–. Classifying acid solutions 1 Acids (and alkalis) can be described as ‘strong’ or ‘weak’, and as ‘concentrated’ or ‘dilute’. 2 The following diagrams are simplified representations of strong, weak, concentrated or dilute acid solutions. Only four types of particle are shown. (Note that there are other particles present which are not shown). The following key is used to distinguish between the different particles: Diagram 1 - HA A- H2O + H3O+ Raffles Institution Raffles Programme Year Three Chemistry Name: _________________________________ ( ) Class: ________ Date: _______________ (a) What types of particles are shown in the solution represented in this diagram? Mainly H2O molecules with some H3O+ and A–, but no HA. (b) How would you describe this solution? It is a dilute solution of a strong acid.

2 Diagram 2 Diagram 3 Diagram 4 (c) What types of particles are shown in the solution represented in this diagram? Mainly H2O molecules with some acid (HA) molecules and even less H3O+ and A–. (d) How would you describe this solution (compared to diagram 1)? It is a dilute solution of a weak acid. (e) What types of particles are shown in the solution represented in this diagram? Mainly H2O molecules, but more ……......... . molecules than in Diagram 2, and only a few H3O+ and A–. (f) How would you describe this solution (compared to diagrams 1 and 2)? It is a more concentrated solution of a …………………… acid. (g) What types of particles are shown in the solution represented in this diagram? Mainly H2O molecules, but a lot more H3O+ and A– than in Diagram 1 and no …………….. molecules (h) How would you describe this solution (compared to diagrams 1 and 2)? It is a more concentrated solution of a ……………………… acid.

3 3 The diagrams are meant to represent a concentrated solution of a strong acid, a dilute solution of a strong acid, a concentrated solution of a weak acid and a dilute solution of a weak acid. Use the table below to show which diagram is meant to represent each of the four solutions – write the number of the appropriate dia