BIORHYTHMS: April 2020

Thursday, 30 April 2020

Class.X-Oxidation and Reduction Reactions

Oxidation and Reduction Reactions

Text book Activity 1.11

*Heat a china dish containing about 1 g copper powder (Fig. 1.10).

* What do you observe?

The surface of copper powder becomes coated with black copper(II) oxide.

Why has this black substance formed?
This is because oxygen is added to copper and copper oxide is formed.

2Cu + O₂ Heat →2CuO (1.28)
If hydrogen gas is passed over this heated material (CuO), the black coating on the surface turns brown as the reverse reaction takes place and copper is obtained.

CuO +H₂ Heat →Cu+H₂O (1.29)

If a substance gains oxygen during a reaction, it is said to be oxidized.

If a substance loses oxygen during a reaction, it is said to be reduced.

During this reaction (1.29), the copper(II) oxide is losing oxygen and is being reduced. The hydrogen is gaining oxygen and is being oxidized.

In other words, one reactant gets oxidized while the other gets reduced during a reaction. Such reactions are called oxidation-reduction reactions or redox reactions.

Some other examples of redox reactions are:

ZnO + C → Zn +CO (1.31)

MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl₂ (1.32)

In reaction (1.31) carbon is oxidised to CO and ZnO is reduced to Zn.

In reaction (1.32) HCl is oxidised to Cl₂ whereas MnO₂ is reduced to MnCl₂.

From the above examples we can say that if a substance gains oxygen or loses hydrogen during a reaction, it is oxidized. If a substance loses oxygen or gains hydrogen during a reaction, it is reduced.

Class-X-Corrosion & Rancidity

Corrosion & Rancidity

Corrosion

You must have observed that 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. Some other metals also get tarnished in this manner. Have you noticed the colour of the coating formed on copper and silver? 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. The black coating on silver and the green coating on copper are other examples of corrosion.

Corrosion causes damage to car bodies, bridges, iron railings, ships and to all objects made of metals, especially those of iron. Corrosion of iron is a serious problem. Every year an enormous amount of money is spent to replace damaged iron. You will learn more about corrosion in Chapter 3.

Rancidity

Have you ever tasted or smelt the fat/oil containing food materials left for a long time?

When fats and oils are oxidised, they become rancid and their smell and taste change. Usually substances which prevent oxidation (antioxidants) are added to foods containing fats and oil. Keeping food in air tight containers helps to slow down oxidation. Do you know that chips manufacturers usually flush bags of chips with gas such as nitrogen to prevent the chips from getting oxidized?

Class-X-Endothermic & Exothermic Reactions

Endothermic Reactions

An endothermic reaction is any chemical reaction that absorbs heat from its environment. The absorbed energy provides the activation energy for the reaction to occur. A hallmark of this type of reaction is that it feels cold.

Expressed in a chemical equation: Reactants + Energy → Products.

Photosynthesis (chlorophyll is used to react to carbon dioxide plus water plus energy to make glucose and oxygen)
Dissolving ammonium chloride in water
Mixing water and ammonium nitrate
Mixing water with potassium chloride
Reacting ethanoic acid with sodium carbonate

Exothermic Reactions

An exothermic reaction is a chemical reaction that releases energy through light or heat. It is the opposite of an endothermic reaction.

Expressed in a chemical equation: Reactants → Products + Energy.

Examples of Exothermic Reactions

Any combustion reaction.
A neutralization reaction.
Rusting of iron (rust steel wool with vinegar).
The thermit reaction.
Reaction between water and calcium chloride.

Thermit Reaction
The reaction of iron (III) oxide (Fe2O3) with aluminium is used to join railway tracks or cracked machine parts. This reaction is known as the thermit reaction. These displacement reactions are highly exothermic. The amount of heat evolved is so large that the metals are produced in the molten state.

Fe2O3 (s) + 2Al (s) → 2Fe (l) + Al2O3 (s) + Heat

HW-3 Types of chemical equations. 29 APR 2020

HW-3
Types of chemical equations. 29 APR 2020

1. What do you understand by the term “Combination reaction”? Write any two examples.

2. What do you understand by the term “Decomposition reaction”?

3. What do you understand by the term “Electrolysis”? Write an example.

4. What do you understand by the term “Thermal decomposition reaction”? Write an example.

5. What do you understand by the term “Photochemical decomposition reaction”? Write an example.

6. What do you understand by the term “Displacement reaction”? Write any two examples.

7. What do you understand by the term “Double displacement reaction”? Write an example.

8. What do you understand by the term “Precipitation reaction”? Write any two examples.

9. What do you understand by the term “Neutralization reaction”? Give an example and two uses.

10. What do you mean by “REACTIVITY SERIES OF ELEMENTS”?

Note: Please refer NCERT SCIENCE TEXT BOOK OF CLASS.X- Pages 6 to 11.

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Answer Key for HW-3
Types of chemical equations. (29 APR 2020)

1. What do you understand by the term “Combination reaction”? Write any two examples.

Answer: COMBINATION REACTION • In combination reaction, two or more substances combine to form a new substance • For example
• C(s) + O₂(g) CO₂(g)
• 2H₂(g) + O₂(g) 2H₂O (g)

2. What do you understand by the term “Decomposition reaction”? Write any two examples.

Answer: DECOMPOSITION REACTION • In a decomposition reaction, a single compound breaks down to produce two or more similar substances. • The decomposition reactions take place when energy is supplied in the form of heat, electricity or light.

3. What do you understand by the term “Electrolysis”? Write any two examples.

Answer: ELECTROLYSIS • When a substance is decomposed by passing electric current, the process is called electrolysis.
2H₂O (I) electricity 2H₂(g) + O₂(g)

4. What do you understand by the term “Thermal decomposition reaction”? Write an example.

Answer: THERMAL DECOMPOSITION • When a substance decomposes on heating it is called thermal decomposition.
CaCO₃(S) heat CaO (S) + CO2 (g)

5. What do you understand by the term “Photochemical decomposition reaction”? Write an example.

Answer: PHOTOCHEMICAL DECOMPOSITION • When a substance is decomposed in presence of sunlight, it is called a photochemical decomposition.
AgBr sunlght Ag +Br

6. What do you understand by the term “Displacement reaction”? Write any two examples.

Answer: DISPLACEMENT REACTION • In a displacement reaction, a more reactive metal displaces or removes another element from a compound.

• For example • Zn(s) + CuSO4 (aq) ZnSO4(s) +Cu In the above reaction Zn is more reactive Cu, so Zn displaces Cu from CuSO4.
• 2AgNO₃ (aq) + Zn(s) → 2Ag(s) + Zn(NO3)2 (aq) In the above reaction Zn is more reactive than Ag , so Zn displaces Ag from its solution.

7. What do you understand by the term “Double displacement reaction”? Write any two examples.

Answer: DOUBLE DISPLACEMENT REACTION • The reaction in which two different atoms or group of atoms are displaced by other atoms or groups of atoms are double displacement reaction. • For example • BaCl₂(aq) + Na₂SO_4
(aq) BaSO₄(s) + 2NaCl (aq)

8. What do you understand by the term “Precipitation reaction”? Write any two examples.

Answer: PRECIPITATION REACTION • Precipitation reactions occur when cations and anions in aqueous solution combine to form an insoluble ionic solid called a precipitate.
AgNO3 + NaCl AgCl + NaNO3

9. What do you understand by the term “Neutralization reaction”? Give an example and two uses.

Answer: NEUTRALIZATION REACTION • A neutralization is a type of double replacement reaction. • In the reaction, H+ and OH- combine to form HOH or H2O or water molecules. • A salt is the product of an acid-base reaction. • For example
HCl + NaOH NaCl +H₂O

Uses: As an antacid (a base which gives relief from acidity in stomach).
As tooth paste which prevent tooth decay.

10. What do you mean by “REACTIVITY SERIES OF ELEMENTS”?

Answer: REACTIVITY SERIES OF METALS • Reactivity series of metals is a series in which the elements arranged in the decreasing order of their reactivity.

Please Note: Each question carries one mark. If all your Answers are correct you will score 10 out of 10, if nine of your Answers are correct you will score 9 out of 10, if eight of your Answers are correct you will score 8 out of 10. Likewise check your HW with today’s date and award marks accordingly.

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HW-2
Balancing chemical equations. 27 APR 2020

Balance the following chemical equations.

Cu + O₂ CuO

K + H₂O KOH + H₂

H₂S + O₂ 2H₂O + S

Mg + HCl MgCl₂+ H₂

H₂+ Cl₂ HCl

CH₄ + O₂ CO₂ + H₂O

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Answer Key for HW-2
Balancing chemical equations. (27 APR 2020)

Balanced Chemical Equations.

Cu + O₂ CuO

Answer: 2Cu + O₂ 2CuO

K + H₂O KOH + H₂

Answer: 2K + 2H₂O 2KOH + H₂

H₂S + O₂ 2H₂O + S

Answer: 2H₂S + O₂ 2H₂O + 2S

Mg + HCl MgCl₂+ H₂

Answer: Mg + 2HCl MgCl_{2 +} H₂

H₂+ Cl₂ HCl

Answer: H_2
+ Cl₂ 2HCl

CH₄ + O₂ CO₂ + H₂O

Answer: CH₄ + 2O₂ CO₂ + 2H₂O

Monday, 27 April 2020

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Saturday, 25 April 2020

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