1. what is the difference between mechanical advantage and efficiency?

Mechanical advantage is a measure of how much your force is multiplied, efficiency is a measure of how much of the work you do actually goes to moving the thing you're trying to move.

Consider a system of pulleys used to lift a heavy load: the pulleys provide a mechanical advantage to let you lift the load more easily, but in addition to moving the load, you also have to move the rope and the pulleys. 

Adding more pulleys may increase the mechanical advantage, but it also increases the amount of  extra stuff you're moving. 

The more extra stuff you're moving, the less efficient the system.

Heat is involved in chemical change in two ways; chemical changes either result in heat being added to or removed from an object, and chemical changes require a certain amount of heat before they can take place.

In reversible reaction chemistry equilibrium is attained when the forward and reverse reactions are taking place at the same rate, thus, the concentrations of reactants and products remain the same. a reversible reaction is a chemical reaction where the reactants form products that, in turn, react together to give the reactants back. in irreversible reactions on the other hand, the reactants react to form the products, which cannot revert back into reactants.