Standard 10 Chapter 2 Chemical Reactions2.1 Chemical equation2.1.1 Writing chemical equations2.1.2 Balancing the chemical equation2.1.3 Steps involved in balancing2.2 Types of chemical reactions2.2.1 Combination (synthesis) reaction2.2.2 Decomposition reactions2.2.3 Displacement reactions2.2.4 Double displacement reactions2.3 Oxidation and reduction2.3.1 Oxidation in and around you2.3.2 Rancidity2.4 Neutralization

Standard 10 Chapter 2

Chemical Reactions

When a chemical change (permanent) occurs a chemical reaction takes place.

Temporary changes - physical

Permanent changes – chemical

2.1 Chemical equation

Word equation Simple form of representation of chemical reaction using wordse.g. copper + oxygen → copper oxide

Chemical equation Representation of chemical reaction using chemical formulaee.g. Cu + 02 → CuO

2.1.1 Writing chemical equations

Reactants → Productsleft hand side (LHS) right hand side (RHS)

The arrow•points towards the products and tail towards the reactants •represents the direction of the reaction Conditions of the reaction are to be indicated above and below the arrow

A plus (+) sign indicates two or more reactants are involved, or products are formed

Physical states of the reactants and products make it more informative. The gaseous, liquid and solid states are symbolized as (g), (l) and (s)

Mix vegetable oil with nickel powder as a catalyst and heat it with hydrogen gas, fats are obtained on cooling at high temperatureEdible oil (l) + Hydrogen (g) nickel Δ Fats (s)

2.1.2 Balancing the chemical equation

The chemical equation

Iron sulphide + Sulphuric acid Ferrous sulphate + Hydrogen sulphide

can be represented asFeS + H2S04 FeS04 + H2S

Balanced equation The number of atoms is same on the LHS and RHS of the chemical equation

2.1.3 Steps involved in balancing

Step 1 Rewrite the given equation as it isSO2 + H2S → S + H2O

Step 2 Compare the number of atoms of each element in the given equation on both sides of the equation

Step 3 Choose the reactant or product having maximum number of atoms. Change the coefficientSO2 + H2S → S + 2H2O

Step 4 'S' and H atoms are not yet balanced. You may select any one of the two. Select hydrogen atoms for balancing. Equalise the number of hydrogen atoms, exactly as mentioned in Step 3.SO2 + 2H2S → S + 2H2O

Step 5 Select 'S' to be balanced

Step 6 Count the number of atoms of each element on LHS and RHSSO2(g) + 2H2S(g) → S(s) + 2H2O(l)

2.2 Types of chemical reactions

Chemical reaction involves breaking and making of the bonds between the atoms to produce new substances

Reaction Description ExampleCombination (synthesis)A + B → AB

Two or more elements combine to give a compound

2H2 + O2 → 2H2O

DecompositionAB → A + B

Compound is broken down into elements

2H2O → 2H2 + O2

Substitution (single replacement)A + BC → AC + B ORA + BC → BA + C

Atom or group of atoms is replaced by another atom or group

Zn + 2HCl → ZnCl2 + H2

ORCl2 + 2NaBr → 2NaCl + Br2

Types of chemical reactions

Reaction Description ExamplePrecipitation (double replacement)AB + CD → AD + CB

Solutions of two soluble compounds mix to give a solid compound

AgNO3 + NaCl → AgCl + NaNO3

Neutralisation (acid + base) HA + BOH → BA + H2O

Acid and base react to give salt and water

HCl + NaOH → H2O + NaCl

Types of chemical reactions

Reaction Description ExampleReduction-Oxidation (redox)Red. A + ē → A- Ox. B → B+ + ē

Transfer of electrons(ionic bond)

Na → Na+ + ēē + Cl- → Cl

CombustionA + O2 → H2O + CO2

A hydrocarbon reacts with oxygen to produce carbon dioxide and water

C6H12O6(aq) + 6O2(g) → 6CO2(g) + 6 H2O(l) + heatexothermic

Types of chemical reactions

2.2.1 Combination (synthesis) reaction Two or more substances (reactants) combine (elements or compounds) to form single product

A + B → ABFormation of iron sulphide by mixing iron and sulphurFe(s) + S(s) → FeS(s)

Increase in formation of C02 in environment leads to acid rains, when it mixes with water vapourH2O(g) + CO2(g) → H2CO3(l)

At construction sites the wet cement with sand and gravel sets into concrete which imparts strength to the building 3Ca0.Al203(s) + 6H2O → 3Ca0.Al203.6H20(s) + Heat Tricalcium aluminate + Water → Concrete

Plaster of Paris (POP) when mixed with water sets quickly into hard mass known as gypsum which is the raw material in manufacturing cement.2CaS04.H20 + 3H20 → 2CaS04.2H20 + HeatPOP Gypsum

Plaster of Paris is used in surgical bandages, casting and moulding in dentistry, in making statues, decoration of roofs, crayons manufacturing etc.

During combination of two or more reactants, the reactants may require or release (liberate) heat with formation of products

Take 1OO ml of distilled water in two polythene bottles to prevent heat loss. Note temperature of water in both bottles. Add about 5 gm of potassium nitrate (KN03) to one bottle. Stir well. Note temperature of the solution. Add 5 gm of NaOH to the other bottle. Note the temperature.

The above reactions can be represented as:KNO3(s) + H20(l) + Heat → 4KN03(aq)NaOH(s) + H20 (l) → 4NaOH(aq) + Heat

In the case of KN03 there is absorption of heat during the reaction. Hence the temperature of the solution (product) falls

Endothermic reactions Absorption of heat. The reactants require (absorb) heat to form products

When NaOH(s) dissolves in water, there is evolution of heat leading to a rise in temperature of the product

Exothermic reactions Heat is evolved

Carbohydrates such as rice, potato, sago etc. are major sources of energy in our diet. During digestion carbohydrates are broken down into glucose. Glucose combines with oxygen in our body and provides energy.

C6H12O6(aq) + 6O2(g) → 6CO2(g) + 6 H2O(l) + energy

It is an exothermic reaction

2.2.2 Decomposition reactions (require heat and light energy)

AB → A + BA single reactant (sugar) has broken down to give a simple product (C + H20)C12H22O11(s) heat (Δ) 12C(s) + 11H2O(g)

Decomposition reactions carried out by heating (thermal decomposition) During manufacturing of cement, at a temperature above 1000°C (1273K) calcium carbonate decomposesCaCO3(s) → CaO(s) + CO2(g)

The pale yellow silver bromide turns grey when exposed to sunlight.2AgBr(s) sunlight 2Ag(s) + Br2(g)

A similar reaction is given by silver chloride 2AgCl(s) sunlight 2Ag(s) + Cl2(g)

Both are used in photography during the process of developing.

Some decomposition reactions are brought about by acids

CaS(s) + 2HCl(l) → CaCl2(s) + H2S(g)Calcium sulphide + Hydrochloric acid → Calcium chloride + Hydrogen sulphide

2.2.3 Displacement reactions (substitution/ single replacement)

A + BC → AC + BA more reactive element removes the element having less reactivity from its compound

Zinc is more reactive than copper. It removes copper from copper sulphate.

CuS04(aq) + Zn(s) → ZnS04(aq) + Cu(s)

2.2.4 Double displacement reactions(precipitation)

AB + CD → AD + CB

A brown insoluble substance, known as precipitate (ppt), is formed.

CuCl2 + 2KI → Cul2↓ + 2KClcopper chloride + potassium iodide → cupric iodide (ppt) + potassium chloride

Precipitation reactions Precipitates are formed

AgNO3 + NaCl → AgCl(s) ↓ + NaNO3silver nitrate + sodium chloride → silver chloride (ppt) + sodium nitrate

White precipitate of AgCl is formed by exchange of ions Ag+ and Cl- between the reactants

2.3 Oxidation and Reduction (redox)Reduction A + ē → A- Oxidation B → B+ + ēGain of ē Loss of ēWhen aluminium burns in presence of oxygen, oxide of aluminium (known as alumina) is formed.4Al + 302(g) → 2Al203

aluminium + oxygen → alumina

Reaction of metallic sodium with alcohol2C2H5OH(l) + 2Na → 2C2H5ONa + H2↑ethyl alcohol + sodium → sodium ethoxide + hydrogen

Na atomic number 11 (2,8,1), 1s2 2s2 2p6 3s1

Na (atom; loss of ē) → Na+ (ion; 2,8) + ēCl atomic number 17 (2,8,7), 1s2 2s2 2p6 3s2 3p5

ē + Cl (atom) → Cl- (chlorine ion; 2,8,8; gain of ē)

Oxidation reaction •Reactants gain oxygen to form corresponding oxide•Reactants lose hydrogen to form product

C(s) + 2H2(g) → CH4(g) carbon + hydrogen → methane

03 light 02 + [O] ozone → oxygen + nascent oxygen

Oxygen is freshly liberated. This oxygen is often called freshly born or "Nascent" oxygen.

In a chemical equation, nascent oxygen is always denoted by showing symbol of oxygen (O) in square brackets such as [0].

Reduction reaction •Reactants gain hydrogen •Reactants lose oxygen to form product

Redox reaction Oxidation and reduction take place simultaneously in a given chemical reaction

Oxidation ReductionReactants gain oxygen gain

hydrogen lose hydrogen lose oxygen

Comparison

2.3.1 Oxidation in and around youDue to the effect of moisture a layer of reddish brown colour is deposited over the surface of iron, called rust. Its chemical formula is Fe203.H20.

Corrosion

•Is a slow process of decay or destruction of metal due to the effect of air, moisture and acids

•Can be prevented by using antirust solution, coating surface by paint, galvanising and electroplating with other metals

2.3.2 Rancidity

Spoilage of food in such a way that it becomes undesirable (and usually unsafe) for consumption

When oil and fats are oxidized or even allowed to stand for a long time, they become 'rancid'

Antioxidants are used to prevent oxidation of food containing fats and oils. Storage of food in air tight containers also retards oxidation

2.4 Neutralization

HA + BOH → BA + H2O

Acid + Alkali (Base) → Salt + Water

• When used plates of food are cleaned with soap/ detergent, we observe changes in color

• The yellow oily left over stains turn red/ orange because of neutralization

• Compounds in edible oil are neutralized by alkaline soap/ detergent

• Edible oils are organic compounds of alcohols and organic acids (carboxylic acids)

• The compounds formed are known as esters of carboxylic acids

• This neutralization reaction is indicated by turmeric (yellow) which turns red

• Acid + Alkali (Base) → Salt + Water

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