The net cell reaction is obtained by adding the two half-reactions. The total standard cell potential $E^o_{cell} = E^o_{cathode} + E^o_{anode} = 1.51 V + (-1.23 V) = 0.28 V$. Since $E^o_{cell} > 0$, the reaction is spontaneous, meaning $MnO_4^-$ will liberate $O_2$ from water in the presence of an acid.