Chemical equation and reaction 1

1. Which among the following are physical or chemical changes?[NCERT Exemplar]

Evaporation of petrol

Physical change

Burning of Liquefied Petroleum Gas (LPG)

Chemical change

Heating of an iron rod to red hot

Curdling of milk

Sublimation of solid ammonium chloride

2. How do we come to know that a chemical reaction has taken place?

The presence of any of the following changes helps us to determine that a chemical reaction has taken place.

Formation of new substance(s)                   
Change in state
Change in colour 
Change in temperature
Formation of a precipitate                           
Evolution of a gas
For example, if on mixing two substances a gas is evolved, then we can say that a chemical reaction has taken place.

3. Which among the following changes are exothermic or endothermic in nature ? [NCERT Exemplar]

Decomposition of ferrous sulphate

Ans: endothermic as heat is absorbed in these changes.

Dilution of sulphuric acid

is exothermic as heat is released in these changes.

Dissolution of sodium hydroxide in water

Dissolution of ammonium chloride in water

4. What is an oxidation reaction? Give an example of oxidation reaction. Is oxidation an exothermic or an endothermic reaction?

The reaction in which oxygen combines with other elements or compounds is known as an oxidation reaction. For example, burning of hydrogen is an oxidation process in which hydrogen combines with oxygen to form water.

2H2(g) + O2(g) →  2H2O(l)]

Oxidation reactions are exothermic.

5. Why do fire flies glow at night?[NCERT Exemplar]

Fire flies have a protein which in the presence of enzyme undergoes aerial oxidation. This is a chemical reaction which involves emission of visible light. Therefore, fire flies glow at night.

6. Aluminium is a reactive metal but is still used for packing food articles.

On exposure to air, aluminium forms a hard protective layer of aluminium oxide (Al2O3) which prevent further oxidation.

7. Red litmus paper turns blue when touched with aqueous solution of magnesium oxide.

Magnesium oxide is an oxide of a metal, so, it is basic in nature. Due to its basic character it turns red litmus paper blue when touched with its aqueous solution.

8. Why do we store silver chloride in dark coloured bottles?[NCERT Exemplar]

9. Giving an example, list two informations which make a chemical reaction more useful?[CBSE Delhi 2015]

(i) Indicating physical state of reactants and products.

(ii) Indicating energy change in terms of heat.

N2(g)+3H2(g)⇌2NH3(g)+heat

10. Complete the missing components/variables given as x and y in the following reactions:[NCERT Exemplar]

Pb(NO3)2(aq) + 2KI(aq)   →   PbI2(x) + 2KNO3(y)

x — (s); y — (aq)

Cu(s) + 2AgNO3(aq) →  Cu(NO3)2(aq) + x(s)

x — 2Ag

Zn(s) + H2SO4(aq)  →  ZnSO4(x) + H2(y)

x — (aq); y — (g)

CaCO3(s) →x CaO(s) + CO2(g)

x — Heat

11. Zinc liberates hydrogen gas when reacted with dilute hydrochloric acid, whereas copper does not. Explain why.[NCERT Exemplar]

Zinc is above hydrogen whereas copper is below hydrogen in the activity series of metals. That is why zinc displaces hydrogen from dilute hydrochloric acid, while copper does not.

Zn + HCl   →  ZnCl2 + H2

Cu + HCl   →  No reaction

12. On adding dilute HCl to copper oxide powder, the solution formed is blue-green. Predict the new compound formed which imparts a blue-green colour to the solution.

The new compound formed is copper(II) chloride (CuCl2), which imparts blue-green colour to the solution.

CuOCοpperoxide+2HCl→CuCl2Cοpper(II)chloride(Blue−green)+H2O

13. A solution of potassium chloride when mixed with silver nitrate solution, an insoluble white substance is formed. Write the chemical reaction involved and also mention the type of the chemical reaction.
[NCERT Exemplar]

KCl(aq) + AgNO3(aq) → AgCl(s) + KNO3(aq)

It is a double displacement and precipitation reaction.

14. Ferrous sulphate decomposes with the evolution of a gas having a characteristic odour of burning sulphur. Write the chemical reaction involved and identify the type of reaction.[NCERT Exemplar]

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

It is a thermal decomposition reaction.

15. Identify the substance oxidised, substance reduced, oxidising agent and reducing agent:

MnO2 + 4HCl → MnCl2 + 2H2O + Cl2
[CCE 2016]

Substance oxidised :  HCl

Substance reduced :  MnO2

Oxidising agent :  MnO2

Reducing agent :  HCl

16. Grapes hanging on the plant do not ferment but after being plucked from the plant can be fermented. Under what conditions do these grapes ferment? Is it a chemical or a physical change?[NCERT Exemplar]

Grapes when attached to the plants are living and therefore, their own immune system prevents fermentation. The microbes can grow in the plucked grapes and under anaerobic conditions these can be fermented. This is a chemical change.

17. Identify the reducing agent in the following reactions.[NCERT Exemplar]

4NH3 + 5O2 → 4NO + 6H2O

Ammonia (NH3)

H2O + F2  →    HF + HOF

Water (H2O) as F2 is getting reduced to HF

Fe2O3 + 3CO →  2Fe + 3CO2

Carbon monoxide (CO)

2H2 + O2 → 2H2O

Hydrogen (H2)

18. What is the role of a catalyst in a chemical reaction?

Catalyst changes (usually increases but sometimes decreases) the rate of a chemical reaction without itself being consumed in the reaction.