Kinetic molecular theory

Kinetic molecular theory

In order for molecules to react they must collide (bang into) each other.

They must collide and collide in the exact right way (orientation)

And with enough speed

Imagine trying to get a key into the lock with your eyes closed.

You need to get it in the exact right orientation for it to go in the hole.

The more collisions you have the higher the odds that the collisions will occur with the right orientation.

Factors determing reaction rates

The more chemicals you have in a place the greater the chance they will collide.

So as Concentration increases so do the amounts of reactions.

Surface area

As the amount of surface area increases so do the amount of collisions

Think of a candy, which get dissolved faster, before or after you chew it?

Temperature

As the temp goes up the molecules move faster and are more likely to collide.

pressure

The higher the pressure the closer together the molecules are….this makes them more likely to collide and react.

Catalysts

A protein that speeds up a reaction is called a catalysts.

More catalyst = more reactions.

It grabs the molecules and holds them together for them to react

Nature of reactants

Generally the more bonds that need to be broken the slower it will occur.

Covalent substances usually have more bonds then ionic so they react slooowwweeerrr.

Energy of reactions

All chemical bonds contain energy, and in order to break them energy must go in.

Even when something gives off energy we must still put in energy…

The amount of energy we need to invest in something to make it react is called the activation energy

The higher the activation energy the longer something takes to react.

If the amount of activation energy is lowered the reaction will happen faster.

The way a catalyst works is by lowering the activation energy

Heat of reaction

The difference in energy the reactants and products is called the heat of reaction.

If something ended with more energy it is endothermic.

If something ended with less energy it is exothermic.

Catalysts

Catalysts lower the activation energy by making the reaction happen in

an alternate route

Equilibrium

Until now we have only spoke about a reaction going in 1 direction…

But really reactions can go both ways…

If A& B collide to make C & D….Then C & D can collide to make A & B

When a reaction occurs at equal rates in both the forwards and backwards directions it is at equilibrium.

The quantities are not equal!!!

Just because something is in equilibrium does not mean there are equal amounts of reactants and products…

Just that the reaction is occurring at equal rates.

If in this class we have 31 students and there is always 1 student in the bathroom

Class Bathroom

As soon as 1 person comes in, 1 goes out… There are always more people in the class, but the rates of in and out are equal.

3 types of equilibrium

There are 3 types of equilibreum

PhaseSolutionChemical

When a substance is changing phases both backwards and forwards at equal rates it is in phase equilibrium

Solution equilibrium

When the first drop of sugar is put into water it dissolves into the water…After more sugar is dissolved the water becomes full ‘saturated’ with sugar…

After the water is saturated with sugar for every drop of sugar you dissolve one drop has to ‘un-dissolve’ from the water.

Chemical equilibrium

When reactants turn into products at the same rate that the products turn back into reactants chemical equilibrium has been reached

Equilibriums can change

Even if this class has an equilibreum between the bathroom and in class it can be distrubed.If I get really boring the equilibreum will shift to the bathroom (3 people out at a time)If it’s the week before the regents it will shift to the class room (no 1 out)

This is called Le Chatliers principle

When a stress is added to a system the reaction will respond to remove the stress

If we add chemicals on 1 side it will need to balance out on the other.

If we remove chemicals from 1 side it will need to balance out on the other

(Pen demonstration)

Heat is a reactent

Just like changing the amount of chemicals on 1 side can affect the equilibrium….So can changing the tempN2(g) +3H2(g)

2NH3(g) + Heat

If heat is added we shift leftIf heat is taken out we shift right

Pressure

N2(g) +3H2(g) 2NH3(g) + Heat

Look at this equation…Which side (reactant or product) seems to take up more room?

N2(g) +3H2(g) 2NH3(g) + Heat

If we put this reaction under pressure which way would the reaction shift to ‘release’ the pressure?

If we gave it more pressure which way would it need to shift to ‘release’ the pressure

The rule is:

Increasing pressure pushes the reaction towards the side with less molecules.

Decreasing pressure pushes the reaction to the side with more molecules.

Enthalpy--- a fancy word for energy

Entropy--- a fancy word for messiness

Enthalpy changes

In nature things naturally lose their energy

Things fallThings break

Natural reaction favor lower enthalpy (energy)

Entropy

Entropy is how disordered something is..When you wake up and shower, brush teeth, and get dressed you have low entropyBy the end of the day when you look like a slob you have high entropy

High entropy Low entropy

In nature things naturally gain entropy…They become more random with time.

Natural reactions favor more entropy..

Which has more entropy

Between solid liquid & gas which has the most entropy, which has the least?