1. Some basic chemical reactions are given below.

III CALCULATIONS ON REACTIONS

(a) Reactive metal + H2O (l) metal hydroxide + H2(g)

(Metals above Mg in reactivity series (Table 1) react with cold water.)

(b) Reactive metal + acid salt + H2(g)

(Metals above H2 in reactivity series (Table 1) react with dilute acid.)

A. Reactions & Equations

(c) Metal hydroxide or metal oxide + acid salt + H2O(l)

(e) NH4+ salt + base salt + NH3(g) + H2O(l)

(f) Pb(NO3)2(aq) + 2 NaCl(aq) PbCl2(s) + 2 NaNO3(aq)

(Precipitation of insoluble compounds (Table 3).)

(g) Hydrocarbons like CxHy or CxHyOz + excess O2(g) CO2(g) + H2O(g) Complete combustion

(d) Metal carbonate or metal hydrogencarbonate + acid

salt + CO2(g) + H2O(l)

(h) Mg(s) + Zn2+(aq) Mg2+(aq) + Zn (s)

(Displacement reaction (Table 1))

(i) Cl2(aq) + 2I -(aq) 2Cl -(aq) + I2(aq)

(Displacement reaction (Table 2))

(j) N2(g) + 3H2(g) 2NH3 (g)

(Haber Process: reversible reactions)

Reducing power of metal decreases from K

NaCaMgAlZnFePb

(H2)CuAgAu

Oxidising power of non-metal decreases from F2

Cl2

Br2

I2

Table 1 Reactivity of Some Metals

Table 2Reactivity of Some Non-metals

Reducing power of metal decreases from K

NaCaMgAlZnFePb

(H2)CuAgAu

Oxidising power of non-metal decreases from F2

Cl2

Br2

I2

Table 1 Reactivity of Some Metals

Table 2Reactivity of Some Non-metals

1. All Group I metal ions like Na+, K+ and all NH4+ are

soluble.

2. All NO3-, HCO3

- and CH3COO- are soluble.

3. All Cl - and Br - are soluble except Ag+, Pb2+.

4. All SO42- are soluble except Ba2+, Pb2+.

5. All CO32-, OH- and O2- are insoluble except Group I

metal ions and NH4+.

Table 3 Solubility of Some Ionic Compounds in Water

Note: Tables 1,2, & 3 are NOT exhaustive; they are just a general guide.

(a) Chemical equations should always be balanced with all state symbols when written.

(b) Ionic equations are chemical equations that show only the reacting species. Ions that are not included in a chemical reaction are called ______________.

2. Recap on Equations:

spectator ions

Eg 3.1 Write a balanced chemical equation for each of the following reactions.

(a) KOH(aq) + H2SO4(aq)

(b) solid sodium hydrogencarbonate + dil. aq. nitric acid

(c) warming a mixture of aq ammonium sulphate and aq sodium hydroxide

K2SO4(aq) + H2O(l)

NaHCO3(s) + HNO3(aq) NaNO3(aq) + CO2(g) + H2O(l)

(NH4)2SO4(aq) + NaOH(aq) Na2SO4(aq) + NH3(g) + H2O(l)

2 2

22 2

Eg 3.2Write a balanced equation (chemical or ionic) for each of the following reactions, if any.

(a) Ca(s) + HCl(aq)

(b) aq silver nitrate + aq calcium bromide

(c) solid calcium carbonate + aq calcium chloride

2 CaCl2(aq) + H2(g)

Ag+(aq) + Br-(aq) AgBr(s)

no reaction

1. For a reaction 2KOH(aq) + H2SO4(aq) K2SO4(aq) + 2H2O(l),

it means “2 mol of KOH(aq) reacts with 1 mol of H2SO4(aq) to give 1 mol of K2SO4(aq) and 2 mol of H2O.”

In calculations, it can be simply expressed as

Hence, if no. of mol of H2SO4 reacted = x

no. of mol of KOH reacted =

no. of mol of K2SO4 formed =

2 mol KOH 1 mol H2SO4

2x

x

B. Calculations

SeeE.g. (a)–(c)

2. Limiting Reactant:

For reactants that are not mixed according to the stoichiometric ratio, the reactant that reacts completely is called the _________________. After the reaction, the other unreacted reactants will be present together with the products of the reaction.

limiting reactant

SeeE.g.(a)-(b)

A) CO2 and H2O are the only products provided complete combustion is ensured, i.e. in the presence of large excess of oxygen.

B) H2O formed immediately after combustion exists as water vapour H2O(g). When the mixture is cooled and all volumes are measured at room temperature, H2O will condense as liquid H2O(l) which occupies negligible (very small) volume compared to the volume of vapour.

3. For combustion problems, note the following points: (E.g. (a) & (b))

D) If insufficient base is used, NaHCO3(aq) may be formed and not all CO2(g) will be absorbed.

NaOH(aq) + CO2(g) NaHCO3(aq)

E) Note that “100 cm3 decreases by 30 cm3” is

equivalent to “100 cm3 decreases to 70 cm3”.

C) Since CO2 is acidic, volume of the gaseous products will decrease when a base like NaOH(aq), KOH(aq) is added. Hence, all CO2 is absorbed only if excess base is used.

2NaOH(aq) + CO2(g) Na2CO3(aq) + H2O(l)

E.g. (a) 6NaOH(aq) + 3Cl2(g) NaClO3(aq) + 5NaCl(aq) + 3H2O(l)

65.0 g of NaOH is reacted completely in the above reaction, cal. the (a) mass of NaClO3 produced, (b) volume (measured at r.t.p.) of Cl2 reacted.

E.g. (b)A sample containing silver has a mass of 1.40 g. When it dissolved in conc. nitric acid, all the Ag in the sample was converted to Ag+ ions. Excess dil. HCl(aq) was then added, & 0.340 g of precipitate were obtained. Calculate the % by mass of silver in the sample. State the assumption(s) in your calculations.

E.g. (c)When 0.100 g of a metal hydride MH(s) reacted with excesswater, the products were the metal hydroxide and hydrogen gas. The vol. of gas evolved at s.t.p. was 56.0 cm3. Calculate the Ar of the metal M.

E.g. (a)Mg(s) + 2AgNO3(aq) Mg(NO3)2(aq) + 2Ag(s)

When 4.56 g of magnesium ribbon is added to 150 cm3 of 0.150 mol dm-3 silver nitrate, what is the mass of Ag obtained?

E.g. (b)4NH3(g) + 5O2(g) 4NO(g) + 6H2O(g)

When 12 dm3 of NH3(g) and 15 dm3 of O2 are mixed,

calculate the total vol. of the mixture after reaction at r.t.p.

E.g. (a)10 cm3 of a hydrocarbon, CxHy, were exploded with 100 cm3 of oxygen. The total vol. after the explosion was 75.0 cm3, which decreased to 25.0 cm3 on shaking with excess sodium hydroxide solution. All volumes were measured at r.t.p. Calculate the molecular formula of the hydrocarbon.(i) Write a balanced equation for the combustion.

(ii) Explain, with aid of balanced equation, the decrease in volume after treatment with NaOH(aq).

(iii) Deduce the molecular formula of the hydrocarbon.

E.g. (b)10.0 cm3 of a hydrocarbon, CaHb, were exploded with an excess of oxygen. A contraction of 25.0 cm3 occurred. On treating the resulting mixture with excess aq. NaOH, a further contraction of 40.0 cm3 occurred. All vol. were measured at r.t.p.

(i) Why was excess oxygen required?

(ii) Deduce the molecular formula of the hydrocarbon.

Before explosion After explosionvol. contracted by 25.0 cm3

After passing through NaOH,contracted by 40.0 cm3

10.0 cm3 of CaHb

(V +x) cm3 of O2

V - Excess O2(g)x - O2(g) reacted.

V cm3 of O2

CO2(g) produced,

V cm3 of O2