Bases and Salts

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Welcome to class! 

In today’s class, we will be talking about bases and salts. Enjoy the class!

BASES AND SALTS

CONTENT

• Bases
• Salts
• Efflorescence, deliquescence and hygroscopic

A base is a substance which will neutralize an acid to yield a salt and water only. Most oxide and hydroxide of metals are bases e.g. Na₂O, K₂O, MgO, NaOH, KOH e.t.c.

An alkali is a basic hydroxide which is soluble in water NaOH, KOH, Ca(OH)₂. A basic oxide (or hydroxide) is a metallic oxide (or hydroxide) which contains ions (O^2‑ or OH^‑) and will react with an acid to form a salt and water only.

Note: An exception to this definition is the reaction of lead (IV) oxide with hydrochloric acid to produce lead (ii) chloride (a salt), water and chlorine gas.

PbO_2(s) +   4HCl _(aq)   →  PbCl_{2 (aq) +}    2H₂O _(l)   +   Cl_{2 (g)}

From the equation above, PbO₂ is not a base.    The nature of the hydroxides of the metals varies according to the position of the metal in the electrochemical series, as illustrated below.

Metal	Solubility	Decomposition by heat
K Na Ca	The hydroxides of these metals are soluble in water and are alkalis.	The hydroxide of sodium and potassium can not be decomposed by heat.
Mg, Al, Zn Fe, Pb, Cu	These metals form hydroxides which are insoluble in water. They are amphoteric except the three hydroxides of Magnesium, iron, and copper.	Decomposed on heating to form oxide and water.
Hg, Ag, Au	Hydroxides of these metals do not exist.

STRENGTH OF BASE

Like an acid, we have the strength of a base. The strength of a base can either be weak or strong.

Weak base: Weak base is a base that ionizes slightly in aqueous solution to produce positively charged metallic ion and negatively charged hydroxide ion e.g. CaO, NH₃.

NH_{3 (g)}    +   H₂O _(l)     →     NH₄⁺(_aq) OH^–_(aq)

Strong bases: Strong base is a base that ionizes completely in aqueous solution to produce positively charged metallic ion and negatively charged hydroxide ion e.g. Na₂O, K₂O.

Na₂O_(s)   +      H₂O _(l)     →       2NaOH_(aq)

K₂O_(s)      +     H₂O _(l)    →       2KOH_(aq)

NaOH _(aq)    →     Na⁺_(aq)    +       OH^–_(aq)

KOH _(aq)    →       K⁺_(aq)         +       OH^–_(aq)

PHYSICAL PROPERTIES OF ALKALIS

1. Alkalis have a bitter taste.
2. Alkalis are soapy to the touch.
3. Alkalis turn red litmus blue.
4. The concentrated form of the caustic alkalis of NaOH and KOH are corrosive.

CHEMICAL PROPERTIES OF ALKALIS

1. Reaction with acid: All base react with acid to form salt and water only.

NaOH_(aq)   +    HCl_(aq)     →      NaCl_(aq)       +    H₂O_(l)

MgO_(s)      +   2HNO_3(aq)     →      Mg(NO₃)_2(aq)   +   H₂O_(l)

2. Displacement of volatile ammonia from ammonium salt by a non-volatile alkali: if an ammonium salt is warmed with an alkali (in the presence of water) ammonia gas is liberated

NaOH _(aq)   +      NH₄Cl _(aq)      →        NaCl _(aq)    +     H₂O_(l)   +     NH_3(g)

Ca (OH)_2(aq)  +  (NH₄)₂SO_4(aq)       →            CaSO_4(aq)   +    2H₂O_(l)  +   2NH_3(g)

USES OF ALKALIS/BASE

Names	Uses
NaOH	Used in the manufacture of soap, Na salts and plastic Used in petrol refining.
KOH	Used in the manufacture of liquid soap Used in dyeing and electroplating.
Ca(OH)2	Used in the manufacture of  mortar, cement and plaster Used in dissolving acidic soil.
Mg(OH)2	Used in the manufacture of toothpaste Used as a laxative.
Aqueous NH3	Used for bleaching cloth Used as detergent.

 EVALUATION

1. Define alkali giving examples
2. State three physical properties of alkalis
3. Using balanced equations, state two chemical properties of bases
4. State the uses of bases

NEUTRALIZATION REACTION

Neutralization reaction can be defined in three major ways.

1. In terms of acid and the base present.
2. In terms of H⁺ ion and OH^– present in the acid and base.
3. In terms of oxonium ions (H₃O⁺) and hydroxide ion (OH^–).

Neutralization is the process whereby an acid reacts completely with an alkalis/bases to form salt and water.

HCl_(aq)   +   NaOH_(aq)    →          NaCl_(aq)    +    H₂O_(l)

Neutralization is the combination of hydrogen ion (H⁺) and hydroxide ions (OH^–) to form water molecules. Salt is also formed at the same time.

H⁺_(aq)    +     OH^–         →             H₂O_(l)

Neutralization can also be defined as the combination of oxonium ions (H₃O⁺) and hydroxide ions (OH^–) to form a water molecule. Salt is also formed at the same time.

H₃OCl_(aq)        +      KOH_(aq)      →        KCl_(aq)    +    H₂O_(l)

(H₃O)₂SO_4(aq)   +      2NaOH_(aq)       →           Na₂SO_4(aq)    +     4H₂O_(l)

H₃O⁺_(aq)              +      OH^–_(aq)      →         2H₂O_(l)

OXONIUM ION

In aqueous solution, the hydrogen ion becomes associated with a water molecule to form oxonium ion (H₃O⁺). This is an example of a coordinate covalent combination.

H⁺    +   H₂O    →       H₃O⁺

During neutralization, oxonium ion H₃O⁺ behaves as hydrogen ion and thus reacts with hydroxide ion (OH^–) to form water molecules.

EVALUATION

1. Define the term neutralization.
2. Write TWO balanced equations to show neutralization reactions.

SALTS

A salt is referred to as the compound formed when all or part of the ionisable hydrogen ion in an acid is replaced by a metallic or ammonium ion e.g.

HCl_(aq) + NaOH_(aq) → NaCl_(aq) + H₂O_(l)

H₂SO_4(aq) + KOH_(aq) → KHSO_4(aq) + H₂O_(l)

TYPES OF SALTS

There are five main types of salts namely:

1. Normal salt.
2. Acid salts
3. Basic salts
4. Double salts.
5. Complex salts.

 

1. Normal salts: are the salts formed when all the replaceable hydrogen ion in the acid has been completely replaced by a metal ion e.g. NaCl, K₂SO₄, Na₃PO₄, NaNO₃ etc. Normal salts are neutral to litmus

HCl_(aq)   +    NaOH_(aq)       →         NaCl_(aq)  +    H₂O_(l)

H₂SO_4(aq)  +  KOH_(aq)      →            K₂SO_4(aq)  +   H₂O_(aq)

2. Acid salts: Acid salts are formed when the replaceable hydrogen ion in the acids are only partially replaced by a metal e.g. NaHSO₄, Na₂HPO₄, NaH₂PO₄, NaHCO₃. They can be produced from acids which contain more than one replaceable hydrogen ion. Acids with two replaceable hydrogen ions can form only one acid salt while acid with three replaceable hydrogen ions can form two different acid salts

H₂SO_4(aq)    +    NaOH_(aq)       →         NaHSO_4(aq)  +  H₂O_(l)

2H₃PO_4(aq)   +   3NaOH_(aq)     →         NaH₂PO_4(aq)  + Na₂HPO_4(aq)  +3H₂O_(l)

Acid salts turn blue litmus red. Acid salts can be converted to normal salt if the remaining replaceable hydrogen ions in the acid salt are replaced in with metallic ions.

KHSO_4(aq)   +    KOH_(aq)     →          K₂SO_4(aq)  +  H₂O_(l)

3. Basic salts: Basic salts are formed when only part of the hydroxide ions of a base is replaced by the negative ions from an acid. It can occur when there is an insufficient supply of acid for complete neutralization of the base e.g Zn(OH)Cl, Mg(OH)Cl, Mg(OH)NO₃, Bi(OH)₂NO₃ e. t .c.

Zn(OH)_2(aq)   +   HCl_(aq)      →          Zn(OH)Cl_(aq)  +   H₂O_(l)

Because of the presence of hydroxide ion in the salt, it has basic properties. Basic salts turn red litmus blue. Basic salts react with excess acid to form a normal salt and water only.

Mg(OH)NO_3(aq)   +  HNO_3(aq)     →           Mg(NO₃)_2(aq)   +  H₂O_(l)

4. Double salts: Double salts are salt which ionize to produce three different types of ions in solution. Usually, two of these are positively charged (metallic or NH4⁺ ion) while the other is negatively charged e.g. (NH₄)₂Fe(SO₄)₂.6H₂O, KAl(SO₄)₂.12H₂O, KCr(SO₄)₂.12H₂O.

(NH₄)₂Fe(SO₄)₂.6H₂O: Ammonium iron (II) tetraoxosulphate (VI) hexahydrate.

KAl (SO₄)₂.12H₂O: Aluminium Potassium tetraoxosulphate (V) dodecahydrate (Potash alum).

KCr (SO₄)₂.12H₂O: Chromium (III) Potassium tetraoxosulphate (VI) dodecahydrate (Chrome alum).

5. Complex salts: Complex salts contains complex ion i.e ion consisting of a charged group of atom e.g. Na₂Zn(OH)₄, K₄Fe(CN)₆, NaAl(OH)₄.

Na₂Zn(OH)₄: Sodium tetrahydroxozincate (ii)

K₄Fe(CN)₆ : Potassium hexacyanoferrate (iii)

NaAl(OH)₄: Sodium tetrahydroxoaluminate (iii)

Na₂Zn(OH)₄      →          2Na⁺   +   [Zn(OH)₄]^2-

K₄Fe(CN)₆        →           4K⁺    +     [Fe(CN)₆]^4-.

 

In our next class, we will be talking about the Hydrolysis of Salt.  We hope you enjoyed the class.

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