# Homework Help: Particle Model of Thermal Energy questions

1. Feb 23, 2015

### romantichero7

1. The problem statement, all variables and given/known data
a) Assume that a substance in a closed container is at thermal equilibrium with about half of its
molecules in the gas phase and half of its molecules in the liquid phase. How could you use the
Particle Model of Thermal Energy to explain which molecules are moving faster, on
average-those in the gas phase, those in the liquid phase, or neither?

b) Now assume that liquid water (H20) and gaseous oxygen (O2) are at thermal equilibrium in a
closed container at room temperature. How would your explanation for which molecules are
moving faster (the H2O molecules in the liquid phase, the O2 molecules in the gas phase, or
neither) be the same as your explanation in part a), and how would it differ?

2. Relevant equations

3. The attempt at a solution
What puzzles me about this stuff is that we are supposed to be using an equation for thermal energy, where Eth = # of modes x kB/2 x T, where kB is a constant. So at thermal equilibrium, does a monatomic solid (6 modes) have greater Eth than a monatomic gas (3 modes)? I have been primed to think gases have higher Eth... I am just puzzled as to how to think of comparing Eth of different states within one substance, and then going beyond that to compare different substances.

2. Feb 25, 2015

### Simon Bridge

Back up: what is thermal energy?
How would it be related to heat capacity?