Approximately how much more energy is released in a magnitude 7 earthquake compared to a magnitude 5 earthquake?

The Richter Scale used for earthquake magnitude is a logarithmic scale.

That means each step change indicates an increase in magnitude of 10 times. From 5 to 7 is three step changes (5, 6, 7), so the energy difference is 10 * 10 * 10 = 1000. Figure from http://www.seismo.ethz.ch/eq_swiss/Ursache_Erdbeben/index