## Quiz 3 Prep Fall 2012 #5

K Honeychurch 1K
Posts: 27
Joined: Fri Sep 26, 2014 2:02 pm

### Quiz 3 Prep Fall 2012 #5

#5 reads "At high temperature NaAl4 decomposes to give NaH, Al, and Hydrogen gas. Given the following balanced reaction,
NaAl4(s)->NaH(s) + Al(s) + 3/2H2(g), write the equilibrium expression.

The given answer is Kp= PH23/2 or Kc= [H2]3/2 OR Kp= PH23 or Kc= [H2]3.

I don't understand why you can either use the coefficient 3/2 or 3 when finding the equilibrium constant since both would yield different K values and K should remain constant for the same reaction (at the same temperature of course). Isn't only the answer using 3 valid, since you are supposed to use stoichiometric coefficients which must be whole numbers? Also, must you give Kp and Kc or only one?

LaurenDinh_3L
Posts: 23
Joined: Fri Sep 26, 2014 2:02 pm

### Re: Quiz 3 Prep Fall 2012 #5

Stoichiometric coefficients can be fractions as long as the equation is balanced correctly. For gases, Kp and Kc can both be applied since gases can be measured using pressure units (atm, bar etc..) or concentration (moles/L).

K Honeychurch 1K
Posts: 27
Joined: Fri Sep 26, 2014 2:02 pm

### Re: Quiz 3 Prep Fall 2012 #5

But, won't the equilibrium constant be different depending on which coefficient you used? In other words, Kp= PH23/2 will not be equal to Kp= PH23? But both are right?

Justin Le 2I
Posts: 142
Joined: Fri Sep 26, 2014 2:02 pm

### Re: Quiz 3 Prep Fall 2012 #5

As the answer key says, H2^3 is only correct if you change the balanced equation so every stoichiometric equation is a whole number. So you would have to specify that in your answer if you use H2^3