Blank screen when plotting a graph and how to partial differentiate?

p_RK = @(v,T) (R*T*(1/(v-b) - a/(R*T^1.5*v*(v+b)))) % The redlich kwong equation

How can I partiall differentiate this function with respect to v?

Also plotting this seems to work for the pressure coefficient

alphav = @(v) ((R/(v-b)+a/(2*T^1.5*v*(v+b)))/p_RK_T(v))
% plot alpha_v over the required range
figure
fplot(alphav, [v1 v2])
hold on

but when I try plotting this I get a blank screen:

diffp_volume=@(v) (((-R*T)/(v^2-b))+(a*(2*v+b)/(v^2*T^1/2*(v+b)^2)))
kt = @(v) -1/diffp_volume*v
% plot alpha_p over the required range
figure
fplot(kt, [v1 v2])
hold on

What can I do? Also I can send the whole code if this doesnt make sense

Thanks

As a note this is about thermodynamics on plotting the coefficient of thermal expansion

[darova]

Attach constant values