Chemistry Notes Acid Bases and Salts

Chemistry Notes Acid Bases and Salts

Chemistry Notes Acid Bases and Salts

K.C.S.E Online Revision

A. Acids And Bases

At a school laboratory:

(i)An acid may be defined as a substance that turn litmus red.

(ii)A base may be defined as a substance that turn litmus blue.

Litmus is a lichen found mainly in West Africa. It changes its colour depending on whether the solution it is in, is basic/alkaline or acidic.It is thus able to identify/ show whether

1. An acid is a substance that dissolves in water to form H+/H3O+ as the only positive ion/cation. This is called the Arrhenius definition of an acid. From this definition, an acid dissociate/ionize in water releasing H+ thus:

2.A base is a substance which dissolves in water to form OH‘ as the only negatively charged ion/anion.

This is called Arrhenius definition of a base.

From this definition, a base dissociate/ionize in Water releasing OH’ thus:

 

3. An acid is a proton donor.

A base is a proton acceptor.

This is called Bronsted-Lowry definition of acids and bases.

From this definition, an acid donates H+ .

H+ has no electrons and neutrons .It contains only a proton.

Examples

i. From the equation:

 

(a)(i)For the forward reaction from left to right, H2O gains a proton to form H3O+ and thus H2o is a proton acceptor.It is a Bronsted-Lowry base

(ii) For the backward reaction from right to left, H3O+ donates a proton to form H2o and thus H3O+ is an ,,opposite” proton donor. It is a Bronsted- Lowry conjugate acid

(b)(i)For the forward reaction from left to right, HCl donates a proton to form Cl‘ and thus HCl is a proton donor.

It is a Bronsted-Lowry acid

(ii) For the backward reaction from right to left, Cl” gains a proton to form HCl and thus Cl’ is an ,,opposite” proton acceptor.

It is a Bronsted-Lowry conjugate base.

Every base /acid from Bronsted-Lowry definition thus must have a conjugate product/reactant.

II. From the equation:

 

(a)(i)For the forward reaction from left to right, NH; gains a proton to form NH4 and thus NH3; is a proton acceptor .

It is a Bronsted-Lowry base

(ii) For the backward reaction from right to left, NH4+ donates a proton to form NH; and thus NH4+ is an ,,opposite” proton donor.

It is a Bronsted-Lowry conjugate acid

(b)(i)F or the forward reaction from left to right, HCl donates a proton to form C1‘ and thus HCl is a proton donor .

It is a Bronsted-Lowry acid

(ii) For the backward reaction from right to left, Cl” gains a proton to form HCl and thus Cl’ is an ,,opposite” proton acceptor.

It is a Bronsted-Lowry conjugate base.

4. Acids and bases show acidic and alkaline properties/characteristics only in Water but not in other solvents e.g.

(a)Hydrogen chloride gas dissolves in water to form hydrochloric acid

Hydrochloric acid dissociates/ionizes in water to free

ions. The free

ions are responsible for:

(i)turning blue litmus paper/solution red.

(ii)show pH value 1/2/3/4/5/6

(iii)are good electrolytes/conductors of electricity/undergo electrolysis.

(iv)react with metals to produce /evolve hydrogen gas and a salt. i.e.

Ionically:

 

(v) react with metal carbonates and hydrogen carbonates to produce /evolve carbon(IV)oxide gas ,Water and a salt. i.e.

lonically:

 

(vi)neutralize metal oxides/hydroxides to salt and water only. i.e. Ionically:

 

 

(b)Hydrogen chloride gas dissolves in methylbenzene /benzene but does not dissociate /ionize into free ions.

It exists in molecular state showing none of the above properties.

(c)Ammonia gas dissolves in water to form aqueous ammonia which dissociate/ionize to free NH4+ (aq) and OH'(aq) ions.

This dissociation/ionization makes aqueous ammonia to:

(i)tum litmus paper/solution blue.

(ii)have pH 8/9/ 10/ 11

(iii)be a good electrical conductor

(iv)react with acids to form ammonium salt and water only.

 

(d)Ammonia gas dissolves in methylbenzene/benzene /kerosene but does not dissociate into free ions therefore existing as molecules

6. Solvents are either polar or non-polar.

A polar solvent is one which dissolves ionic compounds and other polar solvents.

Water is polar solvent that dissolves ionic and polar substance by surrounding the free ions as below:

 

Note:Water is polar .It is made up of :

Oxygen atom is partially negative and two hydrogen atoms which are partially positive.

They surround the free Hl and Cl” ions.

A non polar solvent is one which dissolved non-polar substances and covalent

compounds.

If a polar ionic compound is dissolved in non-polar solvent ,it does not ionize/dissociate into free ions as below:

 

7. Some acids and bases are strong while others are Weak.

(a)A strong acid/base is one which is fully/wholly/completely dissociated / ionized into many free H+ /OH’ ions i.e.

i. Strong acids exists more as free H+ ions than molecules. e. g.

 

ii. Strong bases/alkalis exists more as free OH’ ions than molecules. e.g.

(b) A weak base/acid is one which is paitially /partly dissociated /ionized in water into free OH’ (aq) and H+(aq) ions.

i. Weak acids exists more as molecules than as free Hl ions. e.g.

 

ii. Weak bases/alkalis exists more as molecules than free OH’ ions. e. g.

 

8. The concentration of an acid/base/alkali is based on the number of moles of acid/bases dissolved in a decimeter(litre)of the solution.

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