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X(g) + 3Y(g) ---- 2z(g) H = +ve. if the reaction above takes place at...

Chemistry
JAMB 2017

X(g) + 3Y(g) ---- 2z(g) H = +ve. if the reaction above takes place at room temperature, the G will be

  • A. negative
  • B. zero
  • C. positive
  • D. indeterminate
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Correct Answer: Option C
Explanation

ΔG= ΔH − TΔS.
To determine whether ΔG will be positive or negative, the value of ΔH(change in enthalpy) and ΔS (change in entropy) must be given. Likewise the temperature.

Entropy represents the degree of disorder in a system. In a gaseous reaction, this is determined by the change in the number of moles of gas.

  • Reactants: 1 mole of X + 3 moles of Y = 4 moles of gas
  • Products:  2 moles of  Z = 2 moles of gas.
  • The spontaneity of a reaction is determined by the equation: ΔG= ΔH − TΔS.
  • ΔH is positive , T = is always positive, ΔS = is negative.  Substituting the signs into the equation:  ΔG= ΔH − TΔS = (+) - (T x (-)) = + 
  • Since both terms are positive, ΔG must be positive regardless of the specific value of T as long as T is expressed in Kelvin (which is always > 0). A positive ΔG indicates the reaction is non-spontaneous at room temperature. 

There is an explanation video available below.


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Explanation Video

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