Sometimes, it is easy to think that energy gets completely used; however, in truth, while one form of energy may disappear, the amount of energy in the universe stays constant. The "vanished" energy has converted into a new energy form.
The concept of entropy can be pretty confusing. Try thinking about entropy as a measurement, like when you measure distance or size. Your pile of laundry in the corner, for example, has more entropy than the clothes folded nicely in your drawers. What's that? Your drawers are missing, off their dresser tracks, or full of comic books? Nevermind. Go clean your room.
The formula for the first law of thermodynamics is useful, but don’t over-interpret the value of ΔE. Knowing whether ΔE is positive or negative does not tell you how likely a reaction is to occur.
Looking at Cells Under the Energy Microscope
Do not confuse enthalpy with entropy. Entropy is the measurement of disorder in a system while enthalpy is, for our purposes, basically the total energy content of a system, including the energy that can be used as work + the heat energy that increases the disorder of the universe.
If you are having a hard time figuring out whether a reaction will proceed spontaneously or not, plug the values into the Gibbs equation. If ΔG is negative, the reaction will occur spontaneously.
We often get caught in the trap of thinking that if ΔG of a reaction is very negative, the reaction will proceed rapidly. Do not fall for this nonsense! ΔG tells you nothing about how fast a reaction will occur. Nothing.
Enzymes in Depth
Often, people think that enzymes change a reaction that normally wouldn’t occur (positive ΔG) to a spontaneous reaction (negative ΔG). This isn’t the case. Enzymes lower the activation energy of the same reaction such that the reaction will have a negative ΔG and occurs more quickly.
Remember that enzymes are like the fast-forward button on your remote. They don’t alter the content of the movie; they increase the speed of the playback.