Tag: what are enthalpy changes

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Multiple choice chemistry endothermic and exothermic reactions endothermic reactions what are enthalpy changes energy change in chemical reactions

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.
Reveal answer Fill a bubble to check yourself
E Correct answer
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.

Multiple choice chemistry endothermic and exothermic reactions endothermic reactions what are enthalpy changes energy change in chemical reactions

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) $
Reveal answer Fill a bubble to check yourself
B Correct answer
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.

Multiple choice chemistry endothermic and exothermic reactions endothermic reactions what are enthalpy changes energy change in chemical reactions

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
Reveal answer Fill a bubble to check yourself
B Correct answer
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.

Multiple choice chemistry endothermic and exothermic reactions endothermic reactions what are enthalpy changes energy change in chemical reactions

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
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation
  • All chemical reactions, including exothermic reactions, need 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.
Multiple choice chemistry endothermic and exothermic reactions endothermic reactions what are enthalpy changes energy change in chemical reactions

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
Reveal answer Fill a bubble to check yourself
B Correct answer
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.

Multiple choice chemistry endothermic and exothermic reactions endothermic reactions what are enthalpy changes energy change in chemical reactions

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
Reveal answer Fill a bubble to check yourself
B Correct answer
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.