Chemical Changes and Reactions — ICSE Class 9 Chemistry Important Questions

Answer: (c) Rusting of iron.

Rusting forms a new substance, hydrated iron(III) oxide, and is irreversible, so it is a chemical change. The other three are physical changes forming no new substance.

Answer: (b) Exothermic combination.

Two reactants combine to form a single product and heat is released, so it is an exothermic combination reaction.

Answer: (b) Light (sunlight).

The decomposition of silver chloride into silver and chlorine is a photochemical reaction brought about by sunlight; this is why silver salts are used in photography.

Answer: (d) Double decomposition (precipitation).

$BaCl_2(aq) + Na_2SO_4(aq) \rightarrow BaSO_4(s)\downarrow + 2NaCl(aq)$BaCl_2(aq) + Na_2SO_4(aq) → BaSO_4(s) + 2NaCl(aq). The ions exchange partners and an insoluble white precipitate of $BaSO_4$BaSO_4 forms, so it is a double decomposition (precipitation) reaction.

Answer: (a) $CaCO_3 \xrightarrow{\Delta} CaO + CO_2$CaCO_3 → (Δ) CaO + CO_2 absorbs heat continuously, so it is endothermic and R correctly explains why A is true.

1. In a physical change no new substance is formed, whereas in a chemical change one or more new substances with new properties are formed.

2. A physical change is usually temporary and reversible, while a chemical change is usually permanent and irreversible and is accompanied by an energy change (heat, light, etc.).

The blue colour of the copper sulphate solution slowly fades and a reddish-brown deposit of copper forms on the zinc strip.

$Zn + CuSO_4 \rightarrow ZnSO_4 + Cu$Zn + CuSO_4 → ZnSO_4 + Cu

Zinc, being more reactive, displaces copper, so this is a displacement reaction.

(i) $H_2 + Cl_2 \rightarrow 2HCl$H_2 + Cl_2 → 2HCl — combination reaction.

(ii) $Pb(NO_3)_2 + 2KI \rightarrow PbI_2\downarrow + 2KNO_3$Pb(NO_3)_2 + 2KI → PbI_2 + 2KNO_3 — double decomposition (precipitation); a yellow precipitate of $PbI_2$PbI_2 forms.

(iii) $NH_4NO_2 \xrightarrow{\Delta} N_2\uparrow + 2H_2O$NH_4NO_2 → (Δ) N_2 + 2H_2O — thermal decomposition reaction.

(i) By electricity (electrolysis of water):

$2H_2O \xrightarrow{\text{electricity}} 2H_2\uparrow + O_2\uparrow$2H_2O → (electricity) 2H_2 + O_2

An electric current decomposes acidified water into hydrogen at the cathode and oxygen at the anode.

(ii) By a catalyst (decomposition of hydrogen peroxide):

$2H_2O_2 \xrightarrow{MnO_2} 2H_2O + O_2\uparrow$2H_2O_2 → (MnO_2) 2H_2O + O_2

Manganese dioxide acts as a catalyst, speeding up the decomposition without being used up.

1. Change of colour: e.g. bright silver iron turns to reddish-brown rust on reacting with air and moisture.

2. Evolution of a gas: e.g. zinc with dilute sulphuric acid gives off hydrogen, $Zn + H_2SO_4 \rightarrow ZnSO_4 + H_2\uparrow$Zn + H_2SO_4 → ZnSO_4 + H_2.

3. Formation of a precipitate / change in temperature: e.g. mixing $BaCl_2$BaCl_2 and $Na_2SO_4$Na_2SO_4 gives a white precipitate of $BaSO_4$BaSO_4; and quicklime with water releases heat (temperature rise).

(a)

• Exothermic reaction: releases heat to the surroundings. Example: $C + O_2 \rightarrow CO_2 + \text{heat}$C + O_2 → CO_2 + heat.

• Endothermic reaction: absorbs heat from the surroundings. Example: $N_2 + O_2 \xrightarrow{\text{heat}} 2NO$N_2 + O_2 → (heat) 2NO.

(b)

1. Combination: $A + B \rightarrow AB$A + B → AB

2. Decomposition: $AB \rightarrow A + B$AB → A + B

3. Displacement: $A + BC \rightarrow AC + B$A + BC → AC + B

4. Double decomposition: $AB + CD \rightarrow AD + CB$AB + CD → AD + CB

(c) Respiration in living cells (slow oxidation of glucose) releases heat and is exothermic; another example is the slaking of lime, $CaO + H_2O \rightarrow Ca(OH)_2 + \text{heat}$CaO + H_2O → Ca(OH)_2 + heat.

(a)

(i) Decomposition: on passing electricity, water breaks up: $2H_2O \xrightarrow{\text{electricity}} 2H_2\uparrow + O_2\uparrow$2H_2O → (electricity) 2H_2 + O_2.

(ii) Combination: water is formed when hydrogen burns in oxygen: $2H_2 + O_2 \rightarrow 2H_2O$2H_2 + O_2 → 2H_2O.

(b) It is a highly exothermic combination reaction. Example — the thermite reaction used in welding:

$Fe_2O_3 + 2Al \rightarrow 2Fe + Al_2O_3 + \text{heat}$Fe_2O_3 + 2Al → 2Fe + Al_2O_3 + heat

(c) In respiration glucose is oxidised gradually inside cells at body temperature, releasing energy slowly, so it is a slow reaction. In a burning candle oxidation occurs rapidly at a high temperature with a visible flame, releasing heat and light quickly, so it is a fast reaction. Both consume oxygen and are exothermic, but they differ in rate.

(i) On heating, the green crystals lose their seven molecules of water of crystallisation and turn into white anhydrous ferrous sulphate: $FeSO_4\cdot 7H_2O \xrightarrow{\Delta} FeSO_4 + 7H_2O$FeSO_4· 7H_2O → (Δ) FeSO_4 + 7H_2O.

(ii) The reddish-brown residue is ferric oxide, $Fe_2O_3$Fe_2O_3.

(iii) $2FeSO_4 \xrightarrow{\Delta} Fe_2O_3 + SO_2\uparrow + SO_3\uparrow$2FeSO_4 → (Δ) Fe_2O_3 + SO_2 + SO_3

(iv) The final step is a thermal decomposition reaction (the single compound breaks down on heating into simpler substances).