# Homework Help: Criteria for spontaniety

1. Oct 16, 2012

### utkarshakash

1. The problem statement, all variables and given/known data
Why a spontaneous process is accompanied by a decrease in internal energy?

2. Relevant equations

3. The attempt at a solution
I know that ΔH and ΔG will be negative for a spontaneous process

2. Oct 16, 2012

### JohnRC

ΔH does not have to be negative for a spontaneous process. Solution of ammonium nitrate is spontaneous, and that is a process that takes in a lot of heat from its surroundings, Many processes are endothermic and spontaneous.

3. Oct 16, 2012

### utkarshakash

I know exceptions exist but in most of the cases it is negative. But I don't know why internal energy has to be negative

4. Oct 16, 2012

### AGNuke

Remembered that Stability is inversely proportional to Internal Energy, if you asked that question.

As JohnRC pointed, ΔH need not be negative. It's only ΔG = ΔH - TΔS. See, even if ΔH is positive, a sufficient temperature of the system, accompanied by increase in entropy, can easily make ΔG negative.

5. Oct 16, 2012

### utkarshakash

Why is it so?

6. Oct 17, 2012

### AGNuke

Its a generally observed trend. Everything is stable when they are relaxed (Low energy state).

You are not comfortable standing while lifting a water filled bucket, are you? (High energy state as you need energy to keep the bucket lifted).

7. Oct 17, 2012

### Ygggdrasil

The spontaneity of a reaction is related to whether the reaction satisfies the second law of thermodynamics; that is, whether the reaction increases the total entropy of the universe. Obviously, the change of entropy of the system is an important component of the spontaneity. The change of entropy of the surroundings is the other important component. For an isothermal reaction, any reaction that transfers heat from the system to the surroundings will increase the entropy of the surroundings (you can think of this as energy confined to the system being liberated to the larger surroundings). Thus, a decrease in internal energy of the system, which for isobaric processes means that heat is transferred to the surroundings, will favor spontaneity.