Delta H and Delta S

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tabishis

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I am trying to solve a few questions and one of those is :


For the process, H
2O(g) 2 H(g) + O(g), one would expect


H to be negative and S to be positive.



H to be positive and S to be negative.



H to be negative and S to be negative.



H to be positive and S to be positive.


Where H=Delta H and S= Delta S.




How do you think we'd be able to determine the Delta H and S values for the reaction above?


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To give you hints about enthalpy (delta H) and entropy (delta S):

Enthalpy is the heat of reaction. A positive value means that the surroundings are doing work on the system, and a negative value means the opposite, that the system is doing work on the surroundings. So if we're turning gaseous water into oxygen and hydrogen gas, would that release energy and have a negative delta H or require an overall input of energy and have a positive delta H?

For entropy, delta S is basically a measure of the change in order/disorder of the reaction. Increasing order gives us a negative delta S value, and decreasing order gives a positive delta S value. Easy ways to detect a change in entropy are phase changes (a solid has less entropy than a liquid, which has less entropy than a gas) and an overall change in moles of gas from reactants to products (an increase in moles of gas would indicate disorder over a wider area, which is an increase in entropy)
 
2H2O(g) ----------> 2H2(g) + O2 (g)

so here 2 moles of water gives two moles of H2 and one mole of O2
so no of moles are increasing
reactant has only 2 moles and products have three moles .
It means randomness is increasing from reactant to product
S(product) > S(reactant)
deta S = S(product) - S(reactant) = +ve
Reaction is decomposition reaction and needs energy to break H2O .(endothermic)
Hence H(product) > H reactant
delta H = H(product) - H reactant = +ve value
 
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