First Law Of Thermodynamics And Hess law MCQ Quiz in मल्याळम - Objective Question with Answer for First Law Of Thermodynamics And Hess law - സൗജന്യ PDF ഡൗൺലോഡ് ചെയ്യുക

Concept :

• A reaction is called exothermic if heat is evolved for the reaction

• So ∆r H = (energy of the product )- (energy of reactant)
• Now from thermodynamics, we know

∆r U = q + w = q - P ∆r V

Again H= U + PV

So ∆r H =∆r U + P ∆r V + V ∆r P

∆r H = q - P ∆r V +P ∆r V - V ∆r P

∆r H = q + V ∆r P

So at constant pressure ∆r P =0 

∆r H = qp

Explanation :

So from the graph, it is clear that 

(energy of the product ) < (energy of reactant)

The value of [ (energy of the product )- (energy of reactant) ] is negative

Then ∆r H = (energy of the product )- (energy of reactant) 

Thus  the value of ∆r H is also negative 

So, ∆r H < 0

Again  ∆r H = qp

As the value of  ∆r H  is negative then the value of  qp is also negative 

qp < 0

Conclusion :

So both the value of  ∆r H and qp is negative 

The correct options are 1 and 2

 Additional Information

Endothermic reaction: In a reaction called endothermic reaction the heat is absorbed.

For this type of reaction, the value of [ (energy of the product )- (energy of reactant) ] is positive

Thus both values of ∆r H and qp is positive 

Concept :

The relation between ∆f UΘ and  ∆f HΘ is 

∆f HΘ =  ∆f UΘ +  ∆ngRT 

Where R is the universal gas constant and T is the temperature 

∆ng  = (moles of product in the gaseous state ) - (moles of reactants in the gaseous state )

Explanation :

CH4 is formed from Carbon and hydrogen 

the equation of the formation of CH4 is 

 C(s) + 2H2(g)  CH4(g)

So in reactant H₂ is in a gaseous state and used 2 mole 

In the product, CH₄ is in a Gaseous state and 1 mole is produced 

Thus for the reaction   = 1-2 = -1 

Now  ∆f HΘ =  ∆f UΘ +  ∆ngRT 

∆f HΘ =  ∆f UΘ + (-1)RT 

∆f HΘ =  ∆f UΘ -  RT 

value of R and T is always positive

So  ∆f HΘ  <  ∆f UΘ   

Conclusion : 

the value of  ∆f HΘ for the formation of CH4 is less than the value of ∆f UΘ 

i.e.  ∆f HΘ  <  ∆f UΘ  

So, the correct option is 2