Chemistry Help (1 Viewer)

[Farhanthestudent005]

Will appreciate if I am assisted with this question. Thank you

 

[jazz519]

deltaH =-ve as the reaction is exothermic and releases heat to the surroundings. Entropy is a measure of the disorder in a system. Here the reactants have more entropy because there are more particles than the products (2:1). This means entropy is decreasing, so deltaS = -ve

deltaG = deltaH - TdeltaS
deltaG = -ve - (+ve)(-ve)
deltaG = -ve +ve

As the temperature increases, the 2nd term (TdeltaS) increases in magnitude and so the deltaG is becoming more positive. As a result the spontaneity of the reaction is decreasing as the temperature goes up.

 

[CM_Tutor]

While I agree with @jazz519's reasoning, I think the conclusion drawn in an answer regarding spontaneity depends on perspective.

If you take the view that a reaction is spontaneous if and non-spontaneous if , then you might answer that:

• as temperature increases, the value of will increase as and
• as , there must be a critical temperature at which
• the reaction will be spontaneous so long as , and will be non-spontaneous when

An example of this perspective is to consider the freezing of water, H₂O(l) ---> H₂O(s), which is exothermic () and entropically disfavoured (), like the above example. For pure water under standard conditions, freezing is spontaneous at any temperature below 0 ^oC and non-spontaneous (i.e. the reverse process, melting, is spontaneous) at any temperature above 0 ^oC. Is it really meaningful to say that water freezes "more spontaneously" at (say) -5 ^oC than at -4 ^oC, or that spontaneity is greater?

Certainly, there are books / teachers / chemists who use language like "greater spontaneity" to mean that an already-negative is decreasing,. Describing this as a becoming "more negative" is linguistically debatable, as is the view of spontaneity as a relative concept that can be present in greater or lesser amounts, rather than as an absolute that is either present or absent under a given set of conditions. I favour the latter perspective and so would not say that " the deltaG is becoming more positive" when I mean that is negative but increasing towards zero as temperature increases because I don't see why -5 kJ mol^-1 is "more positive" than -10 kJ mol^-1.

I would answer:

As temperature increases, Gibbs Free Energy increases but the reaction remains spontaneous until the temperature passes the threshold at which becomes positive, where the reaction becomes non-spontaneous. We are told that the reaction is exothermic () and that the reaction proceeds (thus, ), but the reaction is entropically disfavoured () as order increases when two species combine to form one in the same state. Since , increasing temperatures will increase the value of the positive term () and so cause to increase from its initial negative value, ultimately to become positive at some critical temperature, at which point the reaction ceases to be spontaneous.

 

[Farhanthestudent005]

Thank you both of you! God bless you both