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Endothermic reactions - class-X

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Statement I: An exothermic reaction has a positive $\Delta H$.
Statement II: Heat must be added to the system for the reaction to occur.

  1. Statement 1 and Statement 2 are correct and Statement 2 is the correct explanation of Statement 1

  2. Both the Statement 1 and Statement 2 are correct and Statement 2 is NOT the correct explanation of Statement 1

  3. Statement 1 is correct but Statement 2 is not correct

  4. Statement 1 is not correct but Statement 2 is correct.

  5. Both the Statement 1 and Statement 2 are not correct.


Correct Option: E
Explanation:

Option $E$ is the correct answer. Both the statements are incorrect.

In an exothermic reaction, heat is produced or given out when the reaction occurs.
And exothermic reaction has a negative delta $H$ value.

Which of the following is the most exothermic reaction?

  1. $\displaystyle { Ag }^{ + }\left( aq \right) +{ Cl }^{ - }\rightarrow AgCl(aq) $

  2. $\displaystyle { C } _{ 5 }{ H } _{ 12 }\left( s \right) +8{ O } _{ 2 }\left( g \right) \rightarrow 5{ CO } _{ 2 }\left( g \right) +6{ H } _{ 2 }O\left( l \right) $

  3. $\displaystyle { NH } _{ 3 }\left( s \right) +{ H } _{ 2 }O\left( s \right) \leftrightharpoons { NH } _{ 4 }^{ + }\left( s \right) +{ OH }^{ - }\left( s \right) $

  4. $\displaystyle H{ C } _{ 2 }{ H } _{ 3 }{ O } _{ 2 }\left( l \right) +{ OH }^{ - }\left( aq \right) \rightarrow { H } _{ 2 }O\left( O \right) +{ C } _{ 2 }{ H } _{ 3 }{ O } _{ 2 }^{ - }\left( aq \right) $

  5. $\displaystyle Ca{ CO } _{ 3 }\left( s \right) \rightarrow CaO\left( s \right) +C{ O } _{ 2 }\left( g \right) $


Correct Option: B
Explanation:

Option B is the correct answer.

The reaction written in option B is a combustion reaction. It is combustion of Propane. Combustion reactions are exothermic in nature.
Thus, combustion of propane is an exothermic reaction and a lot of heat is generated, which causes more propane to combine with Oxygen.

Reactions, involving liberation of heat, are called ___________ .

  1. decomposition

  2. endothermic

  3. exothermic

  4. combination


Correct Option: C
Explanation:

In exothermic reactions heat is liberated.

A chemical reaction in which heat is evolved is called a/an:

  1. exothermic reaction

  2. endothermic reaction

  3. thermite reaction

  4. catalytic reaction


Correct Option: A
Explanation:

A reaction in which heat is released is called an exothermic reaction.

The reaction of nitrogen with oxygen to form nitric oxide is :

  1. Exothermic reaction

  2. Endothermic reaction

  3. Thermal balance reaction

  4. None of these


Correct Option: B
Explanation:

Oxygen and nitrogen do not react at ambient temperatures. But at high temperatures, they undergo an endothermic reaction producing various oxides of nitrogen. Such temperatures arise inside an internal combustion engine or a power station boiler, during the combustion of a mixture of air and fuel, and naturally in a lightning flash.

Which may be correct for exothermic reaction?

  1. $\Delta H > {E _a}$

  2. $\Delta H < {E _a}$

  3. $\Delta H \ne {E _a}$

  4. All of these


Correct Option: A
Explanation:
  • All chemical reactions, including exothermic reactionsneed activation energy to get started. Activation energy is needed so reactants can move together, overcome forces of repulsion, and start breaking bonds.
  • If the molecules in the reactants collide with enough kinetic energy and this energy is higher than the transition state energy, then the reaction occurs and products form. In other words, the higher the activation energy, the harder it is for a reaction to occur and vice versa.
  • Hence by the above points we can conclude that $\Delta H>E _a$ for exothermic reaction.

Identify the following reactions as endothermic or exothermic.
(i) $2NaHCO _3+  {energy}\rightarrow Na _2CO _3+H _2O+CO _2$
(ii) $2Mg+O _2\rightarrow 2MgO+energy$

  1. (i) Exothermic (ii) Endothermic

  2. (i) Endothermic (ii) Exothermic

  3. (i) Exothermic (ii) Exothermic

  4. (i) Endothermic (ii) Endothermic


Correct Option: B
Explanation:

Reaction with the absorption of energy is called an endothermic reaction.
In the reaction 

(i), Sodium Bicarbonate needs the energy to form sodium carbonate, water and carbon dioxide. 
Hence it is an endothermic reaction.
Reaction with the release of energy is called an exothermic reaction.
In the reaction 

(ii) on the burning of magnesium wire, magnesium oxide is formed by the release of energy in the form of heat and light.
Hence it is an exothermic reaction.

Which of the following process is exothemic in nature?

  1. Melting

  2. Boiling

  3. condensation

  4. Evaporation


Correct Option: A

When ammonium chloride is dissolved in water, the solution becomes cold. 


The process is :

  1. exothermic

  2. endothermic

  3. super cooling

  4. None of these


Correct Option: A

Limestone can be changed into slaked lime in two chemical reactions.
1 When limestone, $CaCO _{3}$, is heated it decomposes into lime, $CaO$.
2 Water is slowly dripped onto the cooled lime. The lime appears to expand and
steam is produced. Slaked lime, $Ca(OH) _{2}$, is formed.
Which row shows the correct description of each of the chemical reactions?

  1. Reaction $1$ - enodthermic, reaction $2$ - endothermic

  2. Reaction $1$ - enodthermic, reaction $2$ - exothermic

  3. Reaction $1$ - exothermic, reaction $2$ - endothermic

  4. Reaction $1$ - exothermic, reaction $2$ - exothermic


Correct Option: B
Explanation:

Solution:- (B) Reaction $1$- endothermic, Reaction $2$- exothermic

When limestone is heated, the heat is absorbed and thus the reaction is endothermic and when steam is produced, the heat is released and thus the reaction is exothermic.

Differentiate between an exothermic and an endothermic reaction.

  1. Endothermic releases heat and exothermic absorbs energy

  2. Exothermic releases heat and endothermic absorbs energy

  3. Hydrogen gas is evolved in exothermic but not in endothermic

  4. Hydrogen gas is evolved in endothermic but not in exothermic


Correct Option: B
Explanation:

Exothermic reactions are reactions or processes that release energy, usually in the form of heat or light. In an exothermic reaction, energy is released because the total energy of the products is less than the total energy of the reactants.
Endothermic reactions are reactions that require external energy, usually in the form of heat, for the reaction to proceed. Since endothermic reactions draw in heat from their surroundings, they tend to cause their environments to cool down. They are also generally non-spontaneous since endothermic reactions yield products that are higher in energy than the reactants. As such, the change in enthalpy for an endothermic reaction is always positive. 


Hence, the correct option is $B$

In which of the following process heat is required?

  1. Mixing water and strong acids

  2. Some polymerisation reactions such as the setting of epoxy resin

  3. The setting of cement and concrete

  4. Combustion of fuels such as wood


Correct Option: D
Explanation:

In combustion of fuels such as woods.

$C+O _2\longrightarrow CO _2+Heat$

Give an example of each of the following chemical changes: An endothermic reaction 

  1. making ice cubes

  2. condensation of rain from water vapor

  3. baking bread

  4. mixing sodium sulfite and bleach


Correct Option: C
Explanation:

Exothermic reactions are reactions or processes that release energy, usually in the form of heat or light. In an exothermic reaction, energy is released because the total energy of the products is less than the total energy of the reactants.
When quick lime reacts with water to form slaked lime , it releases a lot of energy . 
Endothermic reactions are reactions that require external energy, usually in the form of heat, for the reaction to proceed. Since endothermic reactions draw in heat from their surroundings, they tend to cause their environments to cool down. They are also generally non-spontaneous, since endothermic reactions yield products that are higher in energy than the reactants. 
When hydrogen reacts with iodine to form hydrogen iodide , it absorbs a lot of energy .

If you, pour a few drops of petrol on your palm, it will feel cool as the petrol evaporates. This change is a/an:

  1. endothermic reaction

  2. exothermic reaction

  3. chemical reaction

  4. slow reaction


Correct Option: A

The process(es) requiring the absorption of energy is/are:

  1. $F \rightarrow F^-$

  2. $Cl \rightarrow Cl^-$

  3. $O \rightarrow O^{2-}$

  4. $H \rightarrow H^-$


Correct Option: C
Explanation:

All the elements can easily gain an electron to form ions like $H^-$, $F^-$, $Cl^-$, and $O^{2-}$ atoms.

But, all the rest atoms except oxygen accepts only one electron, so they release energy and the value of electron gain enthalpy is negative.
But, oxygen first adds one electron where its first electron gain enthalpy is negative, but now the second electron is added to an anion, so that will require energy to add and hence the electron gain enthalpy becomes positive.
The answer is option C.

In which of the following process energy is liberated:

  1. $ Cl \rightarrow Cl^+ + e^-$

  2. $ HCl \rightarrow H^+ + Cl^-$

  3. $ H^++e^- \rightarrow H$

  4. $ O + 2e^-\rightarrow  O^{2-}$


Correct Option: C

Select endothermic reaction:

  1. $S^{-}(g) + e^{-}(g) \rightarrow S^{2-}(g)$

  2. $Ne (g) + e^{-} (g) \rightarrow Ne^{-} (g)$

  3. $N(g) + e^{-} (g) \rightarrow N^{-}(g)$

  4. none of the above


Correct Option: B,C
Explanation:

The following are examples of endothermic reactions. They have a positive value of electron gain enthalpy and energy is absorbed when an electron is added to the atom.

$\displaystyle  Ne (g) + e^{-} (g) \rightarrow Ne^{-} (g)$

$\displaystyle N(g) + e^{-} (g) \rightarrow N^{-}(g)$

The following is an example of an exothermic reaction. They have a negative value of electron gain enthalpy and energy is released when an electron is added to the atom.

$\displaystyle S^{-}(g) + e^{-}(g) \rightarrow S^{2-}(g)$.

So, the correct options are $B$ and $C$

Which of the following are exothermic processes?
(A) Reaction of water with quick lime
(B) Dilution of an acid
(C) Evaporation of water
(D) Sublimation of Camphor (Crystals)

  1. (a) and (b)

  2. (b) and (c)

  3. (a) and (d)

  4. (c) and (d)


Correct Option: A
Explanation:

Addition of quick lime to water and dilution of acid are examples of exothermic reactions.
The addition of quick lime (CaO) to water gives a hissing sound and is known as slaking of lime. It is an exothermic reaction with release of 15000 cal/mol
$CaO + H _2O \rightarrow Ca(OH) _2 + 15000  : cal$
Conc sulfuric acid has very high affinity for water. When sulfuric acid is mixed with water, a large amount of hat is released. The temperature may rise to as high as $120^o  : C$.
Hence, sulfuric acid is always diluted by adding acid to water.
On the other hand, evaporation of water and sublimation of Camphor are endothermic processes.

Which of the following is not an endothermic reaction?

  1. $CaCO _3\rightarrow CaO+CO _2$

  2. $2H _2O\rightarrow 2H _2+O _2$

  3. $6CO _2+6H _2O\rightarrow C _6H _{12}O _6+6O _2$

  4. $C _6H _{12}O _6+6O _2\rightarrow 6CO _2+6H _2O$


Correct Option: D
Explanation:

$C _6H _{12}O _6+6O _2\rightarrow 6CO _2+6H _2O$  is not endothermic reaction.
It is a synthesis reaction . the reaction is called cellular respiration as it produces energy for the body to use for activity. n oxidizing the glucose, the cell takes the energy from this chemical process and uses it for ATP synthesis. The ATP synthase is considered to be the most important carrier of energy in a cell. It captures energy, then uses that energy to fuel other processes within the cell. While most are formed during respiration, some are also made during photosynthesis.
Heat is not absorbed so it is not an endothermic reaction

The gas which is produced when sodium reacts with water:

  1. hydrogen

  2. oxygen

  3. water gas

  4. soda gas


Correct Option: A
Explanation:

When sodium reacts with water sodium hydroxide is formed and hydrogen gas is evolved.$2Na+2H _2O\rightarrow2NaOH+H _2$

The process of respiration is:

  1. an oxidation reaction which is endothermic

  2. a reduction reaction which is exothermic

  3. a combination reaction which is endothermic

  4. an oxidation reaction which is exothermic


Correct Option: D
Explanation:

The process of respiration is an oxidation reaction which is exothermic.
Respiration is considered as an exothermic reaction because in respiration oxidation of glucose takes place which produces large amount of heat energy. 

The reaction between carbon and oxygen can be represented as:
$C(g)\ + \ O _2(g)\rightarrow CO _{2}(g) + heat$
In which of the following type(s), the above reaction can be classified?
I. Combustion reaction
II. Displacement reaction 
Ill. Endothermic reaction
IV. Combination reaction

  1. (I) and (III)

  2. (I), (III) and (IV)

  3. (I) and (IV)

  4. (I) only


Correct Option: C
Explanation:

$C(g)+O _2(g)\rightarrow CO _2(g)+heat$

As, the carbon is combining with oxygen to form $CO _2$, it is a combination reaction and the oxidation of carbon is also taking place in the presence of oxygen, it is also a combustion reaction.

When quickline is reacted with water, the reaction is:

  1. an endothermic reaction

  2. a reaction involving reduction

  3. an exothermic reaction

  4. a reaction involving oxidation


Correct Option: C
Explanation:

Quicklime is reacted with water to form calcium hydroxide.

$CaO + H _2O\rightarrow Ca(OH) _2$
This reaction is exothermic reaction because some amount of heat evolved in the reaction.
So answer is C.

Which of the following reaction is not endothermic in nature?

  1. breaking of bonds

  2. digestion of food

  3. combustion of Carbon

  4. evaporation of water


Correct Option: C
Explanation:

Combustion or burning is a high temperature exothermic redox chemical reaction between fuel and an oxidant oxygen. Evaporation and sublimation are phase change physical process of water. Digestion of food is breakdown of larger molecules into small ones by bong breaking which is an endothermic process.

Select the correct alternative.

Which of the following reactions are exothermic in nature?

  1. Combustion of Carbon

  2. Bond breaking

  3. Bond formation

  4. Both (a) and (c)


Correct Option: D
Explanation:

Combustion or burning is a high temperature exothermic redox chemical reaction between fuel and an oxidant oxygen. Bond breaking requires energy to be supplied for bond dissociation so it is an endothermic process while bond formation releases energy so that the product becomes stable which is an exothermic process.

Which of the following is an exothermic process?

  1. Dilution of sulphuric acid

  2. Sublimation of dry ice

  3. Evaporation of water

  4. Melting of ice


Correct Option: A
Explanation:

A large amount of heat is released because the hydration reaction of sulfuric acid is highly exothermic and dilution should always be performed by adding the acid to the water rather than the water to acid. As hydration of sulfuric acid is thermodynamically favorable and the affinity of it for water is sufficiently strong.

If you pour a few drops of petrol on your palm, it will feel cool as the petrol evaporates. This change a/an:

  1. endothermic reaction

  2. exothermic reaction

  3. chemical reaction

  4. slow reaction


Correct Option: A
Explanation:

If you pour a few drops of petrol on your palm.it will feel cool as the petrol evaporates this is a endothermic reaction.Because petrol is very volatile in nature, so when it evaporates.it absorb heat energy from the surroundings.

A chemical reaction in which energy is liberated is called:

  1. exothermic reaction

  2. endothermic reaction

  3. isothermal reaction

  4. physical change


Correct Option: A
Explanation:

Exothermic Reaction: This is a chemical reaction that releases energy through light or heat.

$CH _4+2O _2\rightarrow CO _2+2H _2O+\triangle$
Burning of $CH _4$ gas its produces $CO _2$ and water vapour and releases heat.
Endothermic Reaction: This is a chemical reaction which requires or absorbs energy from its surroundings usually in the form of heat.
$N _2+O _2\rightarrow 2NO$
$\triangle0=+180.5$kg

During the dissolution of urea, heat:

  1. is absorbed

  2. is evolved

  3. does not change

  4. none of the above


Correct Option: A
Explanation:

During the dissolution of urea, heat is absorbed the term,dissolution,refers to a solute dissolving in a solvent to form a solution.In this experiment the solute is  solid urea.Urea being a molecular substance,does not dissociate into ions.The dissolution process involves adding energy to break the attractive forces between the molecules in the solid structure and releasing energy as the molecules form new attractive forces with the water molecules sourrounding it.

One of the following is an exothermic reaction. This is:

  1. electrolysis of water.

  2. conversion of limestone into quicklime.

  3. process of respiration.

  4. process of photosynthesis.


Correct Option: C
Explanation:

(A)In electrolysis of water heat energy is used to convert water into hydrogen and oxygen.

$2H _2O\rightarrow2H _2+O _2$
 it is an endothermic reaction.

(B)conversion of limestone into quicklime can be done by heating limestone.
$CaCO _3\rightarrow CaO+CO _2$ it is an endothermic reaction.


(C)During respiration breakdown of glucose in presence of oxygen releases energy, therefore it is an exothermic process.
$C _6H _{12}O _6+6O _2\rightarrow6H _2O+6CO _2+energy$

(D)In process of photosynthesis- light energy is used to convert carbon dioxide and water into glucose and oxygen
$6CO _2+6H _2O\rightarrow C _6H _{12}O _6+6O _2$

So the answer is (c)






.

One of the following is an endothermic reaction. This is:

  1. combination of carbon and oxygen to form carbon monoxide.

  2. combination of nitrogen and oxygen to form nitrogen monoxide

  3. combination of glucose and oxygen to form carbon dioxide and water

  4. combination of zinc and hydrochloric acid to form zinc chloride and hydrogen.


Correct Option: B
Explanation:
(A) This is an incomplete combustion reaction so energy will be released.
(B)Combination of nitrogen and oxygen to form Nitrogen Monoxide-
Heat is required for this process.
So $N _2 + O _2\rightarrow2NO-180.6KJ$ is an endothermic reaction.
(C) combination of glucose and oxygen to form carbon dioxide and water is a combustion reaction.So energy will be released.
(D)in a combination of zinc and hydrochloric acid to form zinc chloride and hydrogen heat is released.

so correct answer is  (B).

The formation of a larger chemical from the smaller components is known as 

  1. an endothermic reaction that requires energy

  2. an endothermic reaction that releases energy

  3. an exothermic reaction that requires energy

  4. an exothermic reaction that releases energy

  5. hydrolysis


Correct Option: C
Explanation:

The internal energy of a larger chemical is smaller than that of the constituent ones. Hence energy is liberated when smaller components combine to form a large molecule. Hence the process is exothermic.

The correct answer is option D.

Which is a reaction that absorbs energy?

  1. Exothermic

  2. Particle Size

  3. Endothermic

  4. Catalyst

  5. Concentration


Correct Option: C
Explanation:

The term endothermic process describes the process or reaction in which the system absorbs energy from its surroundings; usually, in the form of heat. The opposite of an endothermic process is an exothermic process, one that releases, "gives out" energy in the form of heat.

Which term means a reaction which releases energy?

  1. Exothermic

  2. Particle Size

  3. Endothermic

  4. Catalyst


Correct Option: A
Explanation:

An exothermic reaction is a chemical reaction that releases energy by light or heat. It is the opposite of an endothermic reaction. Expressed in a chemical equation: reactants → products + energy.

The dissolution in water of which of the following substances is endothermic?

  1. Hydrochloric acid

  2. Sodium hydroxide

  3. Pure sodium

  4. Ammonium chloride


Correct Option: D
Explanation:

(A) $HCl _{(l)}+H _2O _{(l)}\rightarrow HCl _{(aq)}+$ Heat (exothermic)

(B) $NaOH+H _2O\rightarrow Na^++OH^-+H _2O+$ Heat (exo)
(C) $Na _{(s)}+H _2O\rightarrow NaOH+$ Heat (exo)
(D) $AlCl _3+H _2O\rightarrow Al(OH) _3+3HCl$ (endo)

(D) is endothermic reaction.

The decomposition of vegetable into compost is an example of __________ reaction.

  1. exothermic

  2. endothermic

  3. displacement

  4. none of these


Correct Option: A
Explanation:

The decomposition of vegetable into compost is an example of  exothermic reaction.

The decomposition of vegetable matter into useful compost is carried out by microbes.These reactions involve break down of bonds.The overall reaction is exothermic.

$Si + C \rightarrow$ SiC. This chemical reaction is an example for:

  1. exothermic reaction

  2. endothermic reaction

  3. dissociation reaction

  4. displacement reaction


Correct Option: A
Explanation:

$Si + C \rightarrow SiC$.

This chemical reaction is an example of an exothermic reaction as heat is liberated during the reaction. It is also an example of a combination reaction. Two elements combine to form a single compound.

To start, every biochemical reaction is:

  1. exergonic

  2. endergonic

  3. both $A$ and $B$

  4. none of the above.


Correct Option: B
Explanation:

Since any chemical reaction requires some energy of activation to start with, so this step is endergonic or since biochemical reaction is also a type of chemical reaction, so its first step is endothermic or endergonic.

A solution of 500 ml of 0.2 M KOH and 500 ml of 0.2 M HCl is mixed and stirred; the rise in temperature is $T _1$. The experiment is repeated using 250 ml of each solution, the temperature raised is $T _2$. Which of the following is true :

  1. $T _1=T _2$

  2. $T _1=2T _2$

  3. $T _1=4T _2$

  4. $T _2=9 T _1$


Correct Option: A

The following is (are) endothermic reaction(s) :

  1. Combustion of methane.

  2. Decomposition of water

  3. Dehydrogenation of ethane to ethylene.

  4. Conversion of graphite to diamond.


Correct Option: B,C,D
Explanation:

Combustion reaction are exothermic, hydrogenation of alkenes are exothermic and graphite is thermodynamically more stable.

During the reaction of some metals with dilute hydrochloric acid, the following observations were made. The temperature of the reaction mixture rises when aluminium (Al) is added mainly because :

                       

  1. Heat is evolved

  2. Due to the catalyst

  3. It is an endothermic reaction.

  4. It is an exothermic reaction.


Correct Option: A,D
Explanation:

The temperature of the reaction mixture rises beacause, the following exothermic reaction takes place:

$2Al(s)  +  6HCl(aq)  \rightarrow   2AlCl _{3}(aq)  +  3H _{2}(g)  +  heat$

Decomposition of ferrous sulphate is a/an:

  1. precipitation reactions

  2. exothermic reactions

  3. endothermic reactions

  4. Synthesis reactions


Correct Option: C
Explanation:

Decomposition of ferrous sulphate is an endothermic reaction as it involves absorption of heat to form ferric oxide ($Fe _{2}O _{3}$), sulphur dioxide ($SO _{2}$) and sulphur trioxide ($SO _{3}$). The equation for this reaction can be written as:
$ 2FeSO _{4}(s)  \overset{heat}{\rightarrow}  Fe _{2}O _{3}(s)  +  SO _{2}(g)  +  SO _{3}(g)$

During the reaction of some metals with dilute hydrochloric acid,
following observations were made. 
 
The reaction of sodium metal is found to be highly explosive because:

  1. Heat is evolved

  2. Due to the catalyst

  3. It is an endothermic reaction.

  4. It is an exothermic reaction.


Correct Option: A,D
Explanation:

Reaction of sodium with hydrochloric acid is highly exothermic one and therefore explosive in nature.
$2Na(s)  +  2HCl(aq)  \rightarrow   2NaCl(aq)  +  H _{2}(g)$  +  heat

Dilution of sulphuric acid is a/an:

  1. synthesis reaction

  2. exothermic reaction

  3. endothermic reaction

  4. none of these


Correct Option: B
Explanation:

Dilution of sulphuric acid with water is a highly exothermic process as it occurs with the evolution of heat. The equation for this reaction  can be wriiten as:
$H _{2}SO _{4}  +  2H _{2}O  \rightarrow   2H _{3}O^{+}  +  SO _{4}^{2-}  +  heat$

Which of the following are endothermic process?

  1. boiling of water

  2. dissolving salts in water

  3. dissolving conc. $H _2SO _4$ in water

  4. evaporation of water


Correct Option: A,B,D
Explanation:
  1. Boiling of water would be an endothermic process because the system (water) is absorbing heat to turn into vapour. Hence both option 1 $ 3 are endothermic.
     And the reverse process would therefore be exothermic.

    2. Addition of Conc. acids to water is an exothermic reaction.

Chemical reactions occur with absorption of heat energy are called:

  1. precipitation reactions

  2. exothermic reactions

  3. endothermic reactions

  4. synthesis reactions


Correct Option: C
Explanation:

The term endothermic process describes a process or reaction in which the system absorbs energy from its surroundings in the form of heat where in case of exothermic it releases energy to the surroundings. 

Which of the following is/are endothermic reactions?

  1. $CaCO _{3}\rightarrow CaO+CO _{2}$

  2. $CH _{4}+2O _{2}\rightarrow CO _{2}+2H _{2}O$

  3. $2HgO\rightarrow 2Hg+O _{2}$

  4. $2H _{2}+O _{2}\rightarrow 2H _{2}O$


Correct Option: A,C
Explanation:
$CaCO _{3}\rightarrow CaO+CO _{2}$
is a decomposition reaction and requires energy for breaking bonds. Its an endothermic reaction.
$CH _{4}+2O _{2}\rightarrow CO _{2}+2H _{2}O$
Its a combustion reaction hence releases energy and is an exothermic reaction
$2HgO\rightarrow 2Hg+O _{2}$
is a decomposition reaction and requires energy for breaking bonds. Its an endothermic reaction.
$2H _{2}+O _{2}\rightarrow 2H _{2}O$
Its a combustion reaction hence releases energy and is an exothermic reaction
Thus A and D are endothermic reactions. Option A and D are correct

In endothermic reactions:

  1. reactants have more energy than products

  2. reactants have less energy than products

  3. reactants and products have same energy

  4. reactants have lower temperature than products


Correct Option: B
Explanation:

An endothermic occurs if the products have a higher energy level than the reactants and $\Delta H$ will be positive. In other words, reactants have lesser energy than products.

In an exothermic reaction, the reacting mixture absorbs heat from the surroundings. Therefore, the reactants will have higher energy than the product here.

So the answer is option B.

For per gram reactant, the maximum quantity of ${ N } _{ 2 }$ gas is produced in which of the following thermal decomposition reaction? (Given: Atomic wt. Cr=52 u, Ba=137 u)
  1. $(NH _4) _2Cr _2O _7(s) \rightarrow N _2(g) + 4H _2O(g)+Cr _2O _3(s)$
  2. $2NH _4NO _3(s) \rightarrow 2N _2(g) + 4H _2O(g)+O _2(g)$
  3. $Ba(N _3) _2(s) \rightarrow Ba(s) + 3N _2(g)$
  4. $2NH _3(g) \rightarrow N _2(g) + 3H _2(g)$

Correct Option: B
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