Thermodynamics REACTION SPONTANEITY “stuff happens”

Stuff happens – or does it? An important area of science is determining: will a process happen spontaneously (or not) and why?

For example: Will a car rust? Will a house burn? Will a chocolate melt? Will a drug inhibit a cancerous tumor growing in a patient’s body?

We can start this topic of reaction spontaneity by thinking about Humpty Dumpty.

Why does Humpty fall, once he has slipped off the wall?

You drop an egg.

The egg falling and breaking is an example of a ___________________________ PROCESS This is a physical or chemical change that occurs with no outside _________________________.

Can you imagine the egg spontaneously reassembling into an unbroken egg, and jumping back into your hand? This would be an example of a

Can non-spontaneous processes happen? YES NO Explain.

Lighting a match and letting it burn is a SPONTANEOUS NONSPONTANEOUS process

What would the enthalpy diagram for this process look like?

Notes: Is this process (with the match) endothermic or exothermic?

Processes that are EXOTHERMIC ENDOTHERMIC are more likely to be spontaneous. Explain.

EXPLAIN:

Rusting is a SPONTANEOUS NONSPONTANEOUS process. It is also EXOTHERMIC ENDOTHERMIC

___________________________ processes involve a substance gaining energy and going to a higher energy state. Does this mean that they are always non-spontaneous?

To answer this question, consider this: WHY DO ICE CUBES MELT?

Draw the enthalpy diagram for an ice cube melting.

An ice cube melting is ENDOTHERMIC EXOTHERMIC An ice cube melting (at room temperature) is a SPONTANEOUS NONSPONTANEOUS process

If all things want to be in their lowest possible energy state, how can an endothermic process (like an ice cube melting) be spontaneous?

There must be another factor - in addition to a lower energy state - that explains why anything happens in the physical universe.

The other factor is that all things want to be: The natural “way of the universe” is that things are constantly – spontaneously –

You must constantly _____________________________ into repairing things – i.e., to maintain __________________ Think about your room at home.

ORDER REQUIRES:

Another way to state this alternative factor: all things want to attain maximum ________________________

______________ is defined as “a measure of the disorder or randomness of the particles that make up a system.”

This is a part of the Second Law of Thermodynamics. “Spontaneous processes always proceed in such a way

Another way of saying this is: ALL THINGS TEND TO

What does this mean in our lives?

An example: you are lying down – not thinking – not moving….

Let’s say you’re in a coma. If they don’t feed you intravenously, what will happen to you? (Do you remember the Terry Schaivo case in 2005?)

You’ll _______________ Why? You eat to get _______________. = ___________ Why would you need more of this if you are doing absolutely nothing?

You need ___________ to constantly repair your body – to maintain its high degree of ____________. Living systems are systems of extremely high ___________!

A lot of what you eat (i.e., a lot of the energy that you get from your food) goes simply into maintaining that high degree of order – i.e., to constantly fix things in your body that are breaking down - TO FIGHT _________________!

Back to the ice cube – why does an ice cube melt – even though it has to go to a higher energy state to do so?

Solid water (ice) is extremely _____________________. Models of ice crystals are shown below. When ice melts, the water molecules become more ______________________________ This increase in entropy makes the process happen spontaneously – even though it is endothermic.

Therefore – a reaction in which entropy (disorder) is ______________________ is more likely to be spontaneous. How can you tell if the entropy is increasing or decreasing in a reaction?

Any process that gives the particles more _____________________________ is likely to have an increased entropy.

As a substance goes from a solid to a liquid to a gas, entropy is INCREASED DECREASED (and vice versa)

Usually if you dissolve a solid or a liquid to form a solution, entropy INCREASES DECREASES

Dissolving a gas into a liquid INCREASES DECREASES the entropy of the gas.

If there are fewer moles of gases in the products than the reactants, the entropy INCREASES DECREASES (and vice versa). Example: CO(g) + 3H 2 (g) → CH 4 (g) + H 2 O(g)

Will the entropy increase or decrease? Explain. a.NH 4 NO 3 (s) → NO 3 1- (aq) + NH 4 1+ (aq) b.N 2 (g) + 3H 2 (g) → 2NH 3 (g)

Will the entropy increase or decrease? Explain. c.H 2 O (l) → H 2 O (s) d. O 2 (g) → O 2 (aq)

Next class, we’ll look at another way to state the Laws of Thermodynamics: You can’t win. You can’t even break even. You can’t get out of the game!