10th Science Unit 10 Types of Chemical Reactions Book Back Answers

Two reactants, hydrogen and chlorine, join to give a single product, hydrogen chloride.

When two or more reactants combine to form one compound, it is a combination (synthesis) reaction.

In photo decomposition, a compound splits when it is exposed to light, e.g. $2AgBr_{(s)} \xrightarrow{Light} 2Ag_{(s)} + Br_{2(g)}$.

Decomposition by heat is thermolysis and by electricity is electrolysis; photolysis is caused by light.

Carbon and oxygen (two elements) combine to give one compound $CO_2$ — a combination reaction (i).

The reactant burns in oxygen giving an oxide and heat — a combustion reaction (ii).

The gaseous product escapes and cannot be changed back to solid carbon, so it is irreversible (iv).

It is not a decomposition reaction, since no single compound splits up.

Two aqueous solutions exchange their ions (double displacement).

One product, $BaSO_4$, is insoluble and settles as a precipitate (shown by $\downarrow$).

A double displacement giving an insoluble product is a precipitation reaction.

(i) Chemical equilibrium is dynamic — forward and backward reactions keep going even though it looks static.

(ii) At equilibrium the rate of forward reaction = rate of backward reaction.

(iv) The amounts of reactants and products remain constant but need not be equal, so their concentrations may be different.

Hence statements i, ii and iv describe chemical equilibrium correctly.

Only metals placed above hydrogen in the activity series can displace hydrogen from an acid.

Zn and Mg lie above hydrogen, so they liberate $H_2$ from HCl.

Ag and Cu lie below hydrogen and cannot displace it.

In options a, b and d, two elements (C+O₂, K+Br₂, Fe+O₂) combine to form a compound.

In option c, carbon monoxide is a compound and oxygen is an element, so it is a 'compound + element → compound' reaction.

In a precipitation reaction two aqueous solutions react to give an insoluble solid and a soluble product.

Only option c has both reactants as (aq) and one product as a solid (s).

pH = 3 means $[H^+] = 1 \times 10^{-3}$ M.

Ionic product of water: $K_w = [H^+][OH^-] = 1 \times 10^{-14}$.

So $[OH^-] = \dfrac{1 \times 10^{-14}}{1 \times 10^{-3}} = 1 \times 10^{-11}$ M.

Powdering a solid reactant increases its surface area.

More surface area means more collisions and more energy available on collision, so the reaction rate increases.

An acid reacts with a base to form salt and water, and both neutralise each other.

Example: $NaOH_{(aq)} + HCl_{(aq)} \rightarrow NaCl_{(aq)} + H_2O_{(l)}$

Lithium stands above hydrogen in the activity series, so it displaces hydrogen from the acid.

$2Li_{(s)} + 2HCl_{(aq)} \rightarrow 2LiCl_{(aq)} + H_2{(g)}$ — a single displacement reaction.

Melting of ice is a physical change (ice ⇌ water).

When the rate of melting equals the rate of freezing, the volume of both phases stays constant — this is physical equilibrium.

Fruit juice with pH 5.6 is acidic.

Slaked lime, $Ca(OH)_2$, is a base and neutralises the acid, lowering the $H^+$ concentration.

Lower $[H^+]$ means a higher pH value.

Ionic product of water, $K_w = [H^+][OH^-]$.

At $25^\circ$C its value is $1.00 \times 10^{-14}\ mol^2\ dm^{-6}$.

The pH of human blood ranges from 7.35 to 7.45.

The ideal (normal) value is 7.4, which is slightly basic.

In electrolysis, electrical energy splits a single compound into simpler substances.

Example: $2NaCl_{(aq)} \xrightarrow{electricity} 2Na_{(s)} + Cl_2{(g)}$ — so it is an electrolytic decomposition reaction.

A synthesis (combination) reaction is $A + B \rightarrow AB$.

Two or more reactants join to give only a single product.

In the chemical volcano, ammonium dichromate on heating splits into simpler substances with sparks and ash.

A single compound breaking into two or more products is a decomposition (thermal decomposition) reaction.

In self-ionisation of water a proton is transferred from one water molecule to another.

The $H^+$ so released dissolves in water as $H_3O^+$: $H_2O + H_2O \rightleftharpoons H_3O^+ + OH^-$.

$\mathrm{NH_4OH + CH_3COOH \rightarrow CH_3COONH_4 + H_2O}$ — a base reacts with an acid to give a salt and water, so it is a Neutralisation reaction.

$\mathrm{Zn + CuSO_4 \rightarrow ZnSO_4 + Cu}$ — the element zinc displaces copper from its compound, so it is a Single Displacement reaction.

$\mathrm{ZnCO_3 \xrightarrow{Heat} ZnO + CO_2}$ — a single compound splits into simpler substances on heating, so it is Thermal decomposition.

$\mathrm{C_2H_4 + 4O_2 \rightarrow 2CO_2 + 2H_2O + Heat}$ — the reactant burns rapidly in oxygen giving oxides and heat, so it is a Combustion reaction.

In the activity series, silver (Ag) is placed below hydrogen, so it is less reactive than hydrogen.

Only metals above hydrogen (like zinc) can displace hydrogen from acids: $Zn + 2HCl \rightarrow ZnCl_2 + H_2$.

Hence silver cannot displace hydrogen gas from an acid.

Pure rain water has a pH of about 7 (neutral).

When polluted air contains oxide gases of sulphur and nitrogen, they dissolve in rain water and form acids.

This lowers the pH below 7, and such rain is called acid rain.

At equilibrium, the rate of the forward reaction becomes equal to the rate of the backward reaction.

What remains constant is the amount (concentration) of reactants and products – they do not change with time.

Their concentrations need not be equal to each other; for example in $PCl_5 \rightleftharpoons PCl_3 + Cl_2$ the amount of $PCl_5$ is more than that of the products at equilibrium.

In a reversible reaction the products combine back to form the reactants.

If one of the products is removed periodically, the backward reaction is reduced.

So the forward reaction proceeds more and more product is obtained, as in the decomposition of $H_2O_2$ where oxygen bubbles away.

A pH paper contains a mixture of indicators and shows a specific colour for each pH value.

A yellow colour on the pH paper corresponds to a pH less than 7.

A solution with pH less than 7 is acidic, not basic (basic solutions give green to blue/violet shades).

Both are aqueous solutions of two compounds, so their ions are interchanged — it is a double displacement (precipitation) reaction.

Silver combines with chloride to give silver chloride, which is insoluble in water and settles as a white precipitate; potassium nitrate stays in solution.

Balanced equation: $KCl_{(aq)} + AgNO_{3(aq)} \rightarrow AgCl_{(s)}\downarrow + KNO_{3(aq)}$

The white precipitate is silver chloride ($AgCl$).

Temperature is one of the factors that influence the rate of a reaction.

Adding heat supplies energy to the reactant molecules, which helps to break more bonds in the reactants.

With more bonds broken and more energetic collisions, the products are formed faster.

Example: calcium carbonate reacts slowly with hydrochloric acid at room temperature, but the rate increases when the mixture is heated. (That is why food kept in a refrigerator spoils more slowly.)

Definition: A combination reaction is a reaction in which two or more reactants combine to form a single compound. It is also called a synthesis or composition reaction.

General scheme: $A + B \rightarrow AB$.

Most combination reactions are exothermic, because new bonds are formed and bond formation releases a large amount of energy as heat.

Example of an exothermic combination reaction: quick lime (calcium oxide) reacting with water, $CaO_{(s)} + H_2O_{(l)} \rightarrow Ca(OH)_{2(aq)} + Heat$. (Another example: $2Na_{(s)} + Cl_{2(g)} \rightarrow 2NaCl_{(s)}$.)

Reversibility: A reversible reaction can be reversed under suitable conditions; an irreversible reaction cannot be reversed.

Direction: In a reversible reaction both forward and backward reactions take place simultaneously; an irreversible reaction is unidirectional and proceeds only in the forward direction.

Equilibrium: A reversible reaction attains equilibrium; an irreversible reaction never attains equilibrium.

Conversion: In a reversible reaction the reactants cannot be completely converted into products, whereas in an irreversible reaction they can be completely converted.

Speed: Reversible reactions are relatively slow; irreversible reactions are fast. Example: $PCl_{5(g)} \rightleftharpoons PCl_{3(g)} + Cl_{2(g)}$ is reversible, while $C_{(s)} + O_{2(g)} \rightarrow CO_{2(g)} + Heat$ is irreversible.

A solid that splits into a solid and a gas on heating is a thermal decomposition reaction.

The gas C dissolves in water and makes it acidic, so C is carbon dioxide ($CO_2$), which forms carbonic acid: $CO_2 + H_2O \rightarrow H_2CO_3$.

Hence A is calcium carbonate ($CaCO_3$) and B is calcium oxide, quick lime ($CaO$).

The reaction is $CaCO_3(s) \xrightarrow{Heat} CaO(s) + CO_2(g)$, an endothermic thermal decomposition.

A more active metal displaces a less active metal from its salt solution (single displacement reaction).

Nickel lies above copper in the activity series, so nickel is more reactive than copper.

So nickel would displace copper from copper sulphate solution: $Ni(s) + CuSO_4(aq) \rightarrow NiSO_4(aq) + Cu(s)$.

The blue solution changes colour, copper gets deposited on the spatula and the spatula is eaten away, spoiling the solution.

A spatula of a less reactive metal (like silver or platinum) or a glass rod should be used instead.

Given: pH of lemon juice = 2.

Formula: $\mathrm{pH} = -\log_{10}[\mathrm{H^+}]$, so $[\mathrm{H^+}] = 10^{-\mathrm{pH}}$.

Substitution: $[\mathrm{H^+}] = 10^{-2}$ mol litre$^{-1}$.

Result: $[\mathrm{H^+}] = 1 \times 10^{-2}$ mol litre$^{-1}$ = 0.01 M, so lemon juice is acidic.

Given: $[\mathrm{OH^-}] = 1 \times 10^{-11}$ M.

Formula: $\mathrm{pOH} = -\log_{10}[\mathrm{OH^-}]$.

Substitution: $\mathrm{pOH} = -\log_{10}(10^{-11}) = -(-11 \times \log_{10}10) = 11$.

Relation: $\mathrm{pH} + \mathrm{pOH} = 14$, so $\mathrm{pH} = 14 - 11$.

Result: $\mathrm{pH} = 3$ (strongly acidic solution).