CHEMISTRY 10CHEMISTRY 10thth CLASS CLASS

INTRODUCTION We commonly observe many

chemical reactions in our surroundings but we rarely notice them. We are so much used to them. Infact millions of chemical reactions take place in our own body.

BRAIN EXERCISE

• What happens to milk when it is left at room temperature for long time?

•What happens to iron tawa/pan/nail, when they are left exposed to humid atmosphere?

BRAIN EXERCISE

•grapes get fermented.

food is cooked

Burning of a magnesium ribbon in air and collection

of magnesiumoxide

• In all the above situations, the nature and the identity of the initial

• substance have somewhat changed.

Whenever a chemicalchange occurs, we can say that a chemical reaction has taken place

CHEMICAR REACTION IS ACCOMPANIED WITH

• change in state• change in color• evolution of a gas• change in temperature

CHEMICAL EQUATIONS• when a magnesium ribbon is burnt in oxygen, it gets converted to

magnesium oxide.This statement can be written as a

word equation

Magnesium + Oxygen → Magnesium oxide (Reactants) (Product)

• A word-equation shows change of reactants to products through an

• arrow placed between them.

• The reactants are written on the left-hand side (LHS) with a plus sign (+) between them.

• Similarly, products are written on the right-hand side (RHS) with a plus sign (+) between them.

• The arrowhead points towards the products, and shows the direction of the reaction.

WRITING A CHEMICAL EQUATION

• There is still a shorter way to represent a chemical reaction by the use of chemical formulae.

• It can be written as shown below:

Mg + O2 → MgO

Skeletal chemical equation

• Count and compare the number of atoms of each element on the LHS and RHS of the arrow in the previous equation.

• If the number of atoms of each element on both the sides is not equal, then the equation is unbalanced because the mass is not the same on both sides of the equation.

• Such a chemical equation is a skeletal chemical equation for a reaction.

Balanced chemical equation

• the number of atoms of each element remains the same, before and after a chemical reaction.

• Hence, we need to balance a skeletal chemical equation.

Balancing a chemical equation

• Zinc + Sulphuric acid → Zinc sulphate + Hydrogen

• The above word-equation may be represented by the following

chemical equation –

• Zn + H2SO4 → ZnSO4 + H2

TYPES OF CHEMICAL REACTIONS

• atoms of one element do not change into those of another element. Nor do atoms disappear from the mixture or appear from elsewhere.

• chemical reactions involve the breaking and making of bonds between atoms to produce new substances.

COMBINATION REACTION

Calcium oxide reacts vigorously with water to produce slaked lime(calcium hydroxide) releasing a large amount of heat. CaO(s) + H2O(l) → Ca(OH)2(aq) (Quick lime) (Slaked lime)

• In this reaction, calcium oxide and water combine to form a single product, calcium hydroxide.

• Such a reaction in which a single product is formed from two or more reactants is known as a combination reaction.

• Other examples of combination reactions

• (i) Burning of coal

• C(s) + O2(g) → CO2(g)

• (ii) Formation of water from H2(g) and O2(g)

• 2H2(g) + O2(g) → 2H2O(l)

• large amount of heat is evolved in these reactions.

• This makes the reaction mixture warm.

• Reactions in which heat is released along with the formation of products are called exothermic chemical reactions.

Other examples of exothermic reactions

• Burning of natural gas

• CH4(g) + 2O2 (g) → CO2 (g) + 2H2O (g)

• respiration is an exothermic process

• We need energy to stay alive. We get this energy from the food we eat. During digestion, food is broken down into simpler substances. These carbohydrates are broken down to form glucose. This glucose combines with oxygen in the cells of our body and provides energy.

• The special name of this reaction is respiration,

• C6H12O6(aq) + 6O2(aq) + 6H2O(l) → 6CO2(aq) + 12H2O(l) + energy

DECOMPOSITION REACTION

a single reactant breaks downto give simpler products. This is a decomposition reaction.

• Ferrous sulphate crystals (FeSO4, 7H2O) lose water when heated and the color of the crystals changes.

• It then decomposes to ferric oxide (Fe2O3), sulphur dioxide (SO2) and sulphur trioxide (SO3).

• Ferric oxide is a solid, while SO2

and SO3 are gases.

•2FeSO4(s) → Fe2O3(s) + SO2(g) + SO3(g)

ELECTROLYSIS OF WATER

Other examples are

• 2Pb(NO3)2(s) →2PbO(s) + 4NO2(g) + O2(g)

2AgCl(s) →2Ag(s) + Cl2(g)

2AgBr(s) →2Ag(s) + Br2(g)

CaCO3(s) → CaO(s) + CO2(g)

DISPLACEMENT REACTION

Chemical reaction in which a less reactive element is replaced in a compound by a more reactive one.

• When iron nail is kept in copper sulphate solution iron nail becomes brownish in color and the blue color of the solution fades.

• This is so because of the reaction taking place both of them.

• This is an example of displacement reaction.

Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s)

EXAMPLES

• Zn(s) + CuSO4(aq)→ ZnSO4(aq) + Cu(s)

• Pb(s) + CuCl2(aq) → PbCl2(aq) + Cu(s)

DOUBLE DISPLACEMENT

REACTION

• The reaction in which two compounds react by an exchange of ions to form two new compounds.

OXIDATION • If a substance gains oxygen during

a reaction, it is said to be oxidised.Then this process is called oxidation

• The surface of copper powder becomes coated with black copper oxide. Why has this black substance formed?

BRAIN EXERCISE

REDUCTION REACTION

• If a substance loses oxygen during a reaction, it is said to be reduced .During this reaction ,the copper oxide is losing oxygen and is being reduced. The process is called reduction.

ZnO + C → Zn +CO

EFFCTS OF OXIDATION REACTIONSEVERYDAY LIFE?

E

• CORROSION• RANCIDITY

CORROSION• Iron articles are shiny when new, but

get coated with a reddish brown powder when left for some time. This process is commonly known as rusting of iron. When a metal is attacked by substances around it such as moisture, acids, etc., it is said to corrode and this process is called corrosion.

•General Rules For Preventing Galvanic Corrosion:

• Use metals below the water that are as close to each other as possible on the galvanic scale. Don't mix metals.

• Fasteners must always be more noble than the fitting on which they're used.

• With a simple boat, or a wood boat of any kind, strictly for the sake of corrosion protection it will be best to electrically isolate all underwater metal fittings from each other, and then.....

• Put a zinc on it...!  But don't put too much zinc on it! Weld the zinc on if possible, or bolt the zinc directly to the piece.

• Painting the cathodic metals is beneficial.

• Never paint a zinc anode! • Never use graphite-bearing

lubricants. Graphite is noble to almost everything!

• When bringing AC shore power aboard, it should always first pass through a true marine grade isolation transformer.

• Battery chargers must be a marine quality isolation transformer type.

• Use bonding if necessary, if you have a highly complicated electrical system, in order to reduce the electrical shock hazard.

• Do everything possible to avoid stray currents in the water, and to prevent them aboard.

BRAIN EXERCISE

Have you noticed the color Have you noticed the color of the coating formed on of the coating formed on

copper and silver?copper and silver?