HOT Based Questions Chemical Reactions and Equations

 

Take 3 g of barium hydroxide in a test tube, now add about 2 g of ammonium chloride and mix the contents with the help of a glass rod. Now touch the test tube from outside.
(i) What do you feel on touching the test tube?
(ii) State the inference about the type of reaction occurred.
(iii) Write the balanced chemical equation of the reaction involved. (Board Term I, 2017)
Answer:
(i) When barium hydroxide is added into ammonium chloride, the bottom of test tube is found to be cooler.
(ii) It is an endothermic reaction.
(iii) Ba(OH)₂ + 2NH₄Cl → BaCl₂ + 2NH₄OH

 

 

(i) Why is respiration considered as an exothermic reaction?
(ii) Write chemical name and the formula of the brown gas produced during thermal decomposition of lead nitrate.
(iii) Why do chips manufactures flush bags of chips with gas such as nitrogen? (Board Term I, 2015)
Answer:
(i) The glucose produced in our body during digestion combines with oxygen in the cells of our body and provides energy. The special name of this reaction is respiration. Thus respiration is an exothermic process because energy is produced during this process.
C6H₁₂O_6(aq) + 6O_2(g) → 6CO_2(g) + 6H₂O_(l) + Energy

Brown gas evolved is nitrogen dioxide (NO₂).

 

A water insoluble substance X on reaction with dilute hydrochloric acid released a colourless and odourless gas accompanied by brisk effervescence. When the gas passed through water, the solution obtained turns blue litmus red. On bubbling the gas trough lime water, it initially became milky and milkiness disappeared when the gas was passed in excess. Identify the substance X. Write the chemical equation of the reaction involved.

 

Here we will first find out the gas. It is stated that this gas turns lime water milky so it is sure that the gas is carbon dioxide (CO₂). 

When CO₂ reacts with water then carbonic aid is formed (H₂CO₃), which turns blue litmus red. 

CO₂ + H₂O → H₂CO₃ 

When CO₂ is passed through lime water (Ca(OH)₂) it turns it milky and on excess passing of carbon dioxide it forms calcium bicarbonate which forms a colourless solution, so the reaction is: 

Ca(OH)₂ + CO₂→ CaCO₃ + H₂O 

On passing in an excess amount 

CaCO₃ + H₂O + CO₂→ Ca(HCO₃)₂ 

For the identification of substance X, we know that in product carbon dioxide is there so there must be something of carbon must be there in reactant and X is water-insoluble so it should be calcium carbonate that is insoluble. 

So the reaction is: 

CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂ 

X= CaCO₃

 

Compound ‘X’ decomposes to form compound ‘Y’ and CO₂ gas. Compound ‘Y’ is used in the manufacturing of cement.

(i) Name the compounds X and Y.

(ii) Write the chemical equation for this decomposition reaction.

Answer: Calcium carbonate (limestone) decomposes on heating to give calcium oxide and carbon dioxide.

CaCO₃ (s) ⎯→ CaO(s) + CO₂ (g)
calcium calcium   Carbon
Carbonate oxide  dioxide

A substance X, which is an oxide of a group 2 element, is used intensively in the cement industry. This element is present in bones also. On treatment with water it forms a solution which turns red litmus blue. Identify X and write the chemical reactions involved.

Answer: Element ‘X’ is calcium. When calcium oxide reacts with water it forms calcium hydroxide. Since calcium hydroxide is a base, thus it turns red litmus blue. Reaction involved in the reaction of calcium oxide with water can be written as follows:

CaO + H₂O ⇨ Ca(OH)₂ + Heat

Calcium belongs to group 2nd in periodic table. It is used in cement industry and also found in bones.

A magnesium ribbon is burnt in oxygen to give a white compound X accompanied by emission of light. If the burning ribbon is now placed in an atmosphere of nitrogen, it continues to burn and forms a compound Y.

(a) Write the chemical formulae of X and Y.
(b) Write a balanced chemical equation, when X is dissolved in water.

Answer:

2Mg + O₂ ⎯→ 2MgO

3Mg + N₂ ⎯→ Mg₃N₂

(a) X is MgO and Y is Mg₃N₂

(b) MgO + H₂O ⇨ Mg(OH)₂

ou are provided with two containers made up of copper and aluminium. You are also provided with solutions of dilute HCl, dilute HNO₃, ZnCl₂ and H₂O. In which of the above containers these solutions can be kept?

Answer:

With Cu container:
The solution of dilute HCl, ZnCl₂ and H₂O can be kept in the container made of copper.  Since copper is a less reactive metal and is placed below the hydrogen in activity series.  It does not react with HCl, ZnCl2 and H2O.

Cu + HCl(dil)  -----------> No reaction

Cu + ZnCl₂  -------------> No reaction

Cu + H₂O  ----------------> Nop reaction

Cu reacts with dil HNO₃ and gets oxidised to give nitric oxide. HNO₃(dil) cannot be kept in copper container.

Cu(s)  +  8HNO₃(aq)  ——>  3Cu(NO₃)₂(aq)  +  2NO(g)  +  4H₂O(l)

With Al container:
While aluminium is highly reactive metal and reacts with HCl and ZnCl₂.

2Al + 6 HCl ----------> 2AlCl₃ + 3H₂ (g)

2Al + 3ZnCl₂ ---------> 2AlCl₃  + 3Zn

HNO₃ may be kept in Al container, since it forms a protective layer of Aluminium Oxide (Al₂O₃). It there is no layer of Al₂O₃ is there or breached, dil HNO₃ reacts with Al to form aluminium nitrate.

4Al   +  3O₂  --------> 2Al₂O₃ 
3Al + 4HNO₃ ----------> 3Al(NO₃)₃ + NO + 2H₂O

Aluminum metal rapidly develops a thin layer of aluminum oxide that prevents the metal from reacting with water. When this layer is breached a reaction develops, releasing highly flammable hydrogen gas.

2Al + 6H₂O -------> 2Al(OH)₃ + 3H₂

A solid substance P which is very hard is used in the construction of many buildings, especially flooring. When substance P is heated strongly, it decomposes to form another solid Q and a gas R is given out. Solid Q reacts with water with the release of a lot of heat to form a substance S. When gas R is passed into a clear solution of substance S, then a white precipitate of substance T is formed. The substance T has the same chemical composition as starting substance.
(i) What is substance P? Write its common name as well as chemical formula.
(ii) What is substance Q?
(iii) What is gas R?
(iv) What is substance S? What is its clear solution known as?
(v) What is substance T? Name any two natural forms in which substance occurs in nature.

(a) Calcium carbonate, limestone, CaCO3
(b) Calcium oxide, CaO
(c) Carbon dioxide, CO2
(d) Calcium hydroxide, Ca(OH)2 ;Lime water
(e) Calcium carbonate ; Limestone and Marble

 

 

A metal forms a salt XSO4. The salt XSO4 forms a clear solution in water which reacts with sodium hydroxide solution to form a blue precipitate Y. Metal X is used in making electric wires and alloys like brass.
(a) What do you think metal X could be?
(b) Write the name, formula and colour of salt XSO4.
(c) What is the blue precipitate Y?
(d) Write a chemical equation of the reaction which takes place when salt XSO4 reacts with sodium hydroxide solution. Give the state symbols of all the reactants and products which occur in the above equation.

(a) Metal X is Cu (Copper).

(b) Copper sulphate, CuSO4, Blue colour.

(c) Copper hydroxide, Cu(OH)2 it is a blue precipitate.

(d) CuSO4(aq)+2NaOH(aq)⟶Cu(OH)2(s)+Na2S04(aq)

 

A metal X forms a water soluble salt XNO3. When an aqueous solution of XNO3 is added to common salt solution, then a white precipitate of compound Y is formed along with sodium nitrate solution. Metal X is said to be the best conductor of electricity and it does not evolve hydrogen when put in dilute hydrochloric acid.
(a) What is metal X?
(b) What is salt XNO3?
(c) Name the compound Y.
(d) Write the chemical equation of the reaction which takes place on reacting XNO3 solution and common salt solution giving the physical states of all the reactants and products.
(e) What type of chemical reaction is illustrated by the above equation?

(a) Silver (Ag)
(b) Silver nitrate ( AgNO3)
(c) Silver chloride ( AgCl)
(d) AgNO3(aq)+NaCl(aq)⟶AgCl(s)+NaNO3(aq)
(e) Double displacement reaction.