Question

Is cellular respiration spontaneous or not? How can you tell?

Answer 1

Respiration is spontaneous because it releases energy (negative enthalpy) and becomes more disordered (positive entropy).

Explanation:

Spontaneous reactions have a decrease in enthalpy (total energy) and an increase in entropy (disorder).

This comes with the law of physics that things in nature, when left to themselves, will become more disordered and will lose energy. This is the Second Law of Thermodynamics. Spontaneous reactions are when a reaction happens naturally, in order to lose energy from a system and become more disordered.

Entropy can also be described as the energy released by a system when something happens.

The purpose of respiration is to release energy from glucose so that a body or organism can do things, like contract muscles and fire neurons. The equation is

`C_6H_12O_6+6O_2->6CO_2+6H_2O`

The fact that respiration releases energy means that it has a positive entropy (`DeltaS>0`). You can also work out that it has a positive entropy because there are more molecules after the reaction than before - `12` afterwards compared to `7` before, adding up the large numbers in the equation. This means it is going from relative order to disorder.

Because the products, `CO_2` and `H_2O` have a lower energy than the reactants, primarily glucose, this means that it is an exothermic reactions and so has a negative enthalpy (`DeltaH<0`). This also makes sense because respiration releases energy from glucose.

As you can see, respiration fits the requirements for a spontaneous reaction. It is positive entropy and negative enthalpy.