Acids and pH scale in Chemistry

ACIDS

Definition: An acid is substance which in aqueous solution produces hydroxonium ion (H₃O⁺) or hydrogen ion (H⁺) as the only positive ion. Also, acids can be referred to as proton donor.

CLASSES OF ACIDS

There are two classes of acids:

(1). Organic acids occur as natural products in plants and animal material.

(2). Inorganic acid: Inorganic acid can be prepared from mineral elements or inorganic matter.

An acid is also defined as a substance which produces hydroxonium ion as the only positive ion when dissolved in water.

H⁺_(aq)    +    H₂O_(l)     à  H₃O⁺_(aq)

Acid can be dilute or concentrated depending on the amount of water added. A dilute acid is acid produced when a large amount of water is added to a small amount of acid. A concentrated acid is acid produced when only a little amount of water is added to a relatively large amount of acid.

STRENGTH OF AN ACID

The strength of an acid can either be weak or strong.

(1) Strong acids: are acids which ionize completely in aqueous solution and such acid solution having a high concentration of H⁺. Examples are HCl, H₂SO₄ and HNO₃.

H₂SO₄    à   2H⁺ + SO₄^2-

HNO₃              à        H⁺ + NO₃^–

HCl          à  H⁺ + Cl^–

(2) Weak acids: are acids which ionize or dissociate slightly or partially in aqueous solution and such acid solution have a low concentration of hydrogen ions. Examples are ethanoic acid (CH₃COOH), H₂CO₃, H₃PO₄, H₂SO₃.

H₂CO₃              ↔         2H⁺        +   CO₃^2-

H₃PO₄               ↔        3H⁺        +   PO₄^3-

CH₃COOH      ↔               H⁺         +   CH₃COO^–

H₂SO₃                ↔     2H⁺         +   SO₃^2-

BASICITY OF AN ACID

The basicity of an acid is the number of replaceable hydrogen ions, H⁺, in one molecule of the acid.

EVALUATION

1. Define the term acid

2. Differentiate between strong acid and concentrated acid

3. What is the basicity of the following acids: HCl, HNO₃, H₂SO₄

PHYSICAL PROPERTIES OF ACID

1. They have a sour taste.

2. They turn blue litmus paper to red.

3. They are corrosive in nature especially the strong acid.

4. In aqueous solution, they conduct electricity.

CHEMICAL PROPERTIES OF ACID

Reaction with metals: They react with metals to liberate hydrogen gas and salt of metal i.e

Acid       +          Metal               à     Salt           + Hydrogen gas.

E.g.  2HCl_(aq)   +   Zn_(s)                 à     ZnCl_2(aq)     +    H_2(g)

H₂SO_4(aq)  +   Mg_(s)                  à   MgSO_4(aq)    +    H_2(g)

They react with soluble bases to form salt and water only. This reaction is known as neutralization.

Acid     +         Base           à   salt        +      water

à

1. They react with  trioxocarbonates (iv) salts to liberate carbon (iv) oxide, salt and water  i.e. Acid   +  trioxocarbonate (iv)      à      Salt  +     Water   +    CO₂

PREPARATION OF ACIDS

Acid can be prepared by using the following methods:

1. Dissolving an acid anhydride in water: Acid anhydride is oxides of non-metal that dissolve in water to produce the corresponding acids e.g SO₂, CO2, CO, NO₂, SO₃.

SO_2(g)      +        H₂O_(l)       à       H₂SO_3(aq)

CO_2(g)    +        H₂O_(l)       à       H₂CO_3(aq)

SO_3(g)    +        H₂O_(l)           à   H₂SO_4(aq)

2. Combination of constituent elements.

(a).Burning hydrogen in chlorine, in the presence of activated charcoal as the catalyst, yields HCl gas which dissolves readily in water to give HCl acid.

H_2(g)     +         Cl_2(g)          à  ^{activated charcoal}       2HCl_(g)

(b)Heating hydrogen gas and bromine vapour, in the presence of platinum as the catalyst, produces hydrogen bromide which dissolves readily in water to form hydrobromic acid.

H_2(g)         +          Br_2(g)                à  ^Platinum         2HBr_(g)

(3)By displacement of a weak or more volatile acid from it salt by a stronger or less

volatile acid. For example

(a)Displacement of the more volatile hydrogen chloride from metallic chloride by the less volatile concentrated tetraoxosulphate (vi) acid.

NaCl_(s)   +  H₂SO_4(aq) à    NaHSO_4(aq)        +         HCl_(aq)

(b)Displacement of weaker trioxoborate (iii) acid from ‘borax’ by tetraoxosulphate (vi) acid.

Na₂B₄O_7(s) +  H₂SO_4(aq)    +   5H₂O_(l)     à Na₂SO_4(aq)    +    4H₃BO_3(aq)

Borax                                            Trioxoborate (iii) acid

(4)By precipitating an insoluble sulphide from a metallic salt by hydrogen sulphide

Pb (CH₃COO)_2(aq)    +  H₂S_(g)    à     PbS_(s)    + CH₃COOH_(aq)

Uses of acid

(1) Acids are useful chemicals which are used in many industries to make other consumer chemicals such as fertilizers, detergent and drugs.

(2) They are used in industrial process as drying agents, oxidizing agents and catalysts.

USES OF ORGANIC AND INORGANIC ACID

EVALUATION

1. Mention three physical properties of acids

2. Using balanced equations, state the chemical properties of acids

3. State two methods of preparing acids

4. Outline the uses of acids

pH SCALE

All acidic solution contains H⁺ and all alkaline solution contains OH^– ions. The PH scale measure the concentration of H⁺ ions present in a solution and start from 0 to 14

DEFINITION OF pH

pH is defined as the negative logarithms of the hydrogen ion [H⁺] concentration to the base of 10.

i.e. pH = -log [H⁺].

Thus: If [H⁺] = 0.00001 or 10^-5.

log [H⁺] = log10^-5 = -5

pH= -log [H⁺] = – (-5) = 5.

If [H⁺] =10^-x

Therefore, pH= -log10^-x = – (-x) = x

If [H⁺] = 10^-2, PH = 2

DEFINITION OF pOH

POH is defined as the negative logarithms of the hydroxide ion [OH^–] concentration to the base of 10.

i.e. pOH= -log [OH^–].

PH is the degree of acidity. A solution with PH 7 is neutral. A solution with PH less than 7, i.e. PH 6,5,4, e.t.c, indicate acidity increasing as the numbers decreases. A solution with PH greater than 7, i.e. PH 8,9,10, e.t.c, indicate alkalinity increasing as the numbers increase.

PH 1    2          3          4          5          6          7          8          9          10        11        12        13        14

Increasing acidity       Neutral     Increasing alkalinity

A solution with PH 1 is very acidic [with high concentration of H⁺]. A solution with pH 13 is very alkaline [with low concentration of H⁺, but high concentration of OH^–].

Note that: If pH is 1, it has concentration of H⁺ 10 times greater than pH 2 and 100 times greater than PH 3 e.t.c.

pH 1 > pH 2 > pH 3.

Concentration of H⁺    10^-1      10^-2      10^-3.

                              0.1       0.01     0.001.

Relationship between pH and pOH.

àFrom conductivity measurement, [H⁺]=10^-7moldm^-3, [OH^–]=10^-7moldm^-3.

[H⁺] [OH^–] = K_w=10^-7 x 10^-7=10^-14mol²dm^-6.

Taking logarithm of both sides

log ([H⁺] [OH^–]) = logK_w

log [H⁺] + log[OH^–] =logK_w

Subtracting both sides

-(log[H⁺]  +  [OH^–]) = -logK_w

-log [H⁺] – log[OH^–] = -logK_w

-log [H⁺] + (-log [OH^–]) = -logK_w

pH + pOH = PK_w

pK_w = -log10^-14 = -(-14) = 14

Therefore, pH + pOH = 14.

Worked examples

1. Find the hydrogen and hydroxide ion concentrations in

(a) 0.01moldm^-3 tetraoxosulphate (vi) acid solution.

(b) 0.001moldm^-3 potassium hydroxide solution.

Solution

(a).  H₂SO_4(aq)         à    2H⁺_(aq)  +  SO₄^2-_(aq)

From the equation,  1 moldm^-3 H₂SO₄ ionizes to give 2moldm^-3 H⁺

Therefore, 0.01moldm^-3 H₂SO₄ would ionize to give (2×0.01) moldm^-3 H⁺

[H⁺] = 2×10^-2moldm^-3

[H⁺] [OH^–] = 10^-14

(2×10^-2) [OH^–] = 10^-14

[OH^–] =    10^-14

2x 10^-2

[OH^–] = 0.5x (10^{-14- -2})

[OH^–] =0.5 x10^-14+2

[OH^–] =0.5×10^-12moldm^-3.

(b).     KOH_(aq)                   à           K⁺_(aq)  +     OH^–_(aq)

From the equation,

1moldm^-3 of KOH ionizes to give 1moldm^-3 of OH^–

10^-3moldm^-3 of KOH would ionize to give 10^-3moldm^-3 of OH^‑

[OH^–]=10^-3moldm^-3.

[H⁺] [OH^–]=10^-14

[H⁺] (10^-3) = 10^-14.

[H⁺] = 10^-14

10^-3

[H⁺] = 10^-14+3

[H⁺] = 10^-11moldm^-3

2.  A glass cup of orange juice is found to have a POH of 11.40. Calculate the concentration of the hydrogen ions in the juice.

Solution

pH + pOH = 14.

pH = 14 – 11.4.

pH = 2.6.

pH = -log [H⁺]

2.6 =-log [H⁺].

[H⁺] = Antilog (-2.6)

[H⁺] = 0.0025moldm^-3

[H⁺] = 2.5×10^-3moldm^-3.

Measuring pH of a solution.

We use pH meter and a universal indicator to detect PH of a solution.

Universal indicator is a mixture of indicator and can change to several colours corresponding to a particular PH and compared with the standard colour provided by the manufacturer of the universal indicator. Universal indicator measures PH between 3 and 11.

Process:-

Put 10cm³ of test solution in a test tube, add 2 drops of universal indicator and compare with the colour chart or place 2 drops of test solution on universal indicator paper and compare the colour with the chart.

EVALUATION

1. Define the term pH.

2. What is the pH of a solution having hydrogen ion concentration of 6x 10^-9

mol/dm³

GENERAL EVALUATION/REVISION

1. Give the chemical formula of the following acids (a) Tetraoxosulphate (vi) acid

(b) Trioxonitrate (v) acid (c) Oxochlorate (i) acid

2. What is the IUPAC nomenclature of the following (a) HNO₂ (b) HOBr (c) H₃PO₄ (d) H₂S

3. Determine the oxidation number of Cl and C in each of the following (a) KClO₃

(b) HOCl (c) H₂CO₃ (d) CO₂

4. Mention the laboratory apparatus that are used in for an acid-base titration

What can be used to determine the acidity or alkalinity of a solution?

READING ASSIGNMENT

New School Chemistry for Senior Secondary School by O. S. Ababio, pp97-99, 102-107

WEEKEND ASSIGNMENT

1. The following acids are monobasic except (a) HNO₂ (b) HBr (c) HOCl (d) H₂SO₃

2. Which of the following ions is acidic? (a) K⁺    (b) NO₃^–      (c) S^2-   (d) H₃O⁺.

3. The number of hydroxonium ions produced by one molecule of an acid in aqueous solution is it (a) acidity (b) basicity (c) concentration (d) pH.

4. The basicity of ethanoic acid CH₃COOH is: (a) 0 (b) 1 (c) 2 (d) 3

5. A solution with pH 7 is (a) Acidic  (b) dilute    (c) neutral   (d) saturated

THEORY

1. What is (i) an acid (ii) basicity of an acid?

2. What is the basicity of tetraoxophosphate (V) acid.

3. The concentrations of H⁺ in two solutions are (a) 1 x10^-4moldm^-3and (b) 5 x 10^-9moldm^-3 . What is the PH of each solution?