Why does fusing iron in a stellar core use more energy than it releases I know larger stars can fuse heavier and heavier elements up to iron where it stops because fusing iron requires more energy than it releases, causing a collapse and supernova.

Why does fusing iron in a stellar core use more energy than it releases?
An atomic nucleus contains protons and neutrons. There is a balance of forces: the (residual) strong nuclear force attracts protons and neutrons together. The electro-magnetic force pushes them apart.

When nuclei fuse, energy is required to push them together, as you have to push against the electromagnetic repulsion. But when the nuclei come close enough, the nuclei are attracted by the strong force, and energy is released as the nuclei fall into each other. For small nuclei, the energy released in fusion is greater than the energy needed to overcome electromagnetic repulsion. For large nuclei (which have a larger positive charge) the reverse is true, more energy is required to push the nuclei together than is released.

Iron is at the balance point, at which the energy required to overcome electomagnetic repulsion just exceeds what is given out on fusion. So iron is the smallest nuclei to absorb energy when it engages in fusion. And so when substantial amounts of iron form in the core of a star, the core will no longer be able to heat up to maintain the balance with gravity, the core collapses and the star explodes. See Why is iron responsible for causing a supernova?

Iron is therefore one of the most stable nuclei (not the most stable: that is Nickle-62, but for various reasons this is not likely for form inside stars) Iron acts like the "ash" of nuclear fusion. 

A more detailed answer would require understanding the quantum mechanics of how binding energy in nuclei works, which can explain why it is Iron that has close to maximum binding energy.