Page 52 - Chemistry ICSE Class IX

40 ICSE Chemistry – 9
ƒ Reaction between lead nitrate and hydrogen sulphide
Pb(NO ) (aq) + H S(aq) o PbS(s)p + 2HNO (aq)
3 2
3
2
lead nitrate hydrogen sulphide lead sulphide nitric acid
(black precipitate)
What is an oxidation-reduction (or redox) reaction
A reaction in which oxidation and reduction take place simultaneously
is called an oxidation-reduction (or redox) reaction.
Displacement of copper from copper sulphate solution by zinc is a
redox reaction.
reduction
Zn(s) + CuSO (aq) o Cu(s) + ZnSO (aq)
^ŝůǀĞƌ ŝƐ ƌĞĐŽǀĞƌĞĚ ĨƌŽŵ ƐŝůǀĞƌ ŶŝƚƌĂƚĞ 4 4
ƐŽůƵƟŽŶ ďLJ ŝƚƐ ĚŝƐƉůĂĐĞŵĞŶƚ ďLJ oxidation
ĐŽƉƉĞƌ ŵĞƚĂůͬƐĐƌĂƉ͘ dŚŝƐ ŝƐ ďĞĐĂƵƐĞ In this reaction, Zn metal gets oxidised to Zn ions and Cu ions in
2+
2+
copper is more reacƟǀe than silǀer͘ the solution get reduced to Cu metal.
Ϯ ŐEK ;aqͿ н Ƶ;sͿ
ϯ
ƐŝůǀĞƌ ŶŝƚƌĂƚĞ ĐŽƉƉĞƌ ŵĞƚĂů This metal displacement redox reaction can be described by the ionic
o
equation,
Ϯ Ő ; sͿ н Ƶ;EK Ϳ ;aqͿ reduction
ϯ Ϯ
ƐŝůǀĞƌ ŵĞƚĂů ĐŽƉƉĞƌ;//Ϳ ŶŝƚƌĂƚĞ –
(gain of 2e )
2+
2+
Zn(s) + Cu (aq) o Zn (s) + Cu(s)
^ŽŵĞ ŝŵƉŽƌƚĂŶƚ ĐŽŵŵŽŶ ŽdžŝĚŝƐŝŶŐ oxidation
ĂŐĞŶƚƐ ĂƌĞ KdžLJŐĞŶ ;K Ϳ͕ KnjŽŶĞ ;K Ϳ͕ (loss of 2e)
ϯ
Ϯ
,LJĚƌŽŐĞŶ ƉĞƌŽdžŝĚĞ ;, O Ϳ͕ ŚůŽƌŝŶĞ
Ϯ Ϯ
; ů Ϳ͕ EŝƚƌŝĐ ĂĐŝĚ ;,EK Ϳ͕ ŽŶĐ͘ Some other redox (or oxidation-reduction) reactions are:
Ϯ ϯ
^ƵůƉŚƵƌŝĐ ĂĐŝĚ ;, ^K Ϳ͕ WŽƚĂƐƐŝƵŵ (a) Displacement of silver metal from silver nitrate solution by zinc
ϰ
Ϯ
ƉĞƌŵĂŶŐĂŶĂƚĞ ;<DŶK Ϳ ĂŶĚ scrap
ϰ
WŽƚĂƐƐŝƵŵ ĚŝĐŚƌŽŵĂƚĞ ;< ƌ O Ϳ͘ 2AgNO (aq) + Zn(s) o Zn(NO ) (aq) + 2Ag(s)
Ϯ
Ϯ ϳ
3
3 2
silver nitrate zinc metal zinc nitrate silver metal
&ůƵŽƌŝŶĞ ŝƐ ƚŚĞ ƐƚƌŽŶŐĞƐƚ ŽdžŝĚŝƐŝŶŐ (b) Thermite reaction — reduction of iron oxide with aluminium
ĂŐĞŶƚ ďĞĐĂƵƐĞ ŽĨ ŝƚƐ ŚŝŐŚĞƐƚ powder
ĞůĞĐƚƌŽŶĞŐĂƟǀŝƚLJ͘ heat
ͻ Fe O 3 + 2Al(s) o Al O (s) + 2Fe(l)
2
3
2
^ŽŵĞ ŝŵƉŽƌƚĂŶƚ ƌĞĚƵĐŝŶŐ ĂŐĞŶƚƐ ĂƌĞ iron(III) oxide aluminium aluminium oxide iron
,LJĚƌŽŐĞŶ ;, Ϳ͕ ^ƵůƉŚƵƌ ĚŝŽdžŝĚĞ ;^K Ϳ͕ (c) Reaction between iron and sulphur on heating
Ϯ
Ϯ
,LJĚƌŽŐĞŶ ƐƵůƉŚŝĚĞ ;, ^Ϳ ĂŶĚ ĂƌďŽŶ heat
Ϯ
; Ϳ͘ Fe(s) + S(s) o FeS(s)
iron sulphur iron(II) sulphide
(d) Reaction between moist H S and SO 2
2
2H S + SO o 2H O(l) + 3S(s)
2
2
2
hydrogen sulphide sulphur dioxide water sulphur
Energy Changes in Chemical Reactions
Zinc scrap, when placed in silver During all chemical reactions, energy in the form of heat, light, electricity
nitrate solution, displaces silver or sound is either liberated (given out) or absorbed (taken in). The chemical
from silver nitrate
equation for such a chemical reaction may be written as,
Reactants o Products + Energy (Energy is liberated)
or Reactants + Energy o Products (Energy is absorbed)
At this stage, we would describe only those reactions in which heat
energy is either absorbed or evolved. Thus, there are two types of reactions.
ƒ Reactions in which heat is evolved (given out) - Exothermic reactions
Iron oxide ƒ Reactions in which heat is absorbed (taken in) - Endothermic reactions