Execution by gas has been accomplished by mixing acid with sodium cyanide. HCN kills
by prenenting oxygen transport in blood. It is a stupid choice. Carbon monoxide would
be much more humane. Remember how often it kills people because they cannot detect it.

As for lethat injection:
Sodium Thiopental causes unconsciousness.
Pancuronium Bromide stops respiration.
Potassium Chloride stops heart.