UNIT A: THERMOCHEMICAL CHANGESCHAPTER 12 (P.522 – 545)

CHEM 30

How does fire work?

What do you need to start a fire?

SeriouslyI know its pretty basic but humour me.

Why doesn’t the gas light on fire?

What needs to happen to get the gas burning?

Some things catch on fire really easily? Why?

Whens This A Good Thing?When is it a bad thing??

A match that lights itself?

Pure oxygen is extremely flammable and can start with a small spark…Continuous

motion machines…..FREE ENERGY!!!

Why did it catch on fire?

Why did it happen so easily?

How much energy did it take to activate and ignite the hydrogen gas?

Is the energy needed to get a reaction started.

A good analogy is pushing a rock over a hill.

You have to invest enough energy to get the rock to the top of the hill then it’ll roll down the other side by itself.

Like lighting gas on fire……you have to invest some energy (the energy from the match) to get it started, then it continues to burn on its own.

Spontaneous Combustion

Activation energy is why things don’t just start on fire by themselves, you need to light them to get them going…….most of the time.

So lets think, things that start on fire really easily like hydrogen for example….all it takes is a little spark and BOOM!

And other things like diesel fuel that don’t burn easily at all.

Activation Energy

So different substances have different activation energies…..some are really high and others are really low…

So low that it almost seems spontaneous and happens on its own.

Activation energy is required for ALL chemical reactions to occur (other than spontaneous reactions….but those would have happened millions of year ago on there own right??)

(Everything on Earth is just waiting to react)

Molecules Need Two Things In Order To React…

Well we already talked about one, activation energy.

1. Molecules need enough energy to overcome the activation energy barrier. (sufficient energy)

2. Molecules need the correct orientation to join together.

It relates to the 3D shapes and bonding electrons from Chem 20…….

But lets just use the lego block or train car analogy…they have to be facing the right way in order to hook together.

Heat (Like a match)Light (Like sunlight)Electricity

Energy Diagrams

Just like Potential Energy Diagrams from last chapter…..still label reactants and products.

Exothermic and Endothermic reactions are still shown the same way with either products or reactants higher or lower.

The only difference is that you need to show the activation energy.

The bonds formed between elements contain a lot of energy.

And in order for bonds to form between elements to form compounds, the bonds in the element itself have to be broken first.

H-H H2(g)

Where does the energy to break these original bonds come from?

Bond Energy and Exothermic/Endothermic Reactions

When initial bonds are broken it takes energy (+).

When new bonds are formed when elements join together to form compounds, energy is released (-).

So when we call a reaction exothermic (- ∆H and releases energy).

What were really saying is that there is more energy released when new bonds are formed than it took to break the original bonds. Or vice versa for endothermic reactions.

A New Way To Look At ∆

So now that we know about bond energy, we can describe ∆H in a new way.

So whether ∆H is (+) or (-) is just a comparison between energy invested to break bonds and the energy released when new bonds form.

New Bond Energy > Old Bond Energy = Exothermic.

Old Bond Energy > New Bond Energy = Endothermic.

A New Way To Look At ∆H

∆H = [Energy Released] – [Energy Invested]

Energy released is the energy released when new bonds are formed to form the product.

Energy invested is the energy needed to break the original bonds of the reactants so that the reaction could occur.

So again, just like before: -∆H is exothermic. +∆H is endothermic.

A catalyst is a substance added to a reaction that speeds up how fast the reaction happens.

***The catalysts is not part of the reaction, it is not changed or used up/consumed during the reaction***

Catalysts decrease the amount of energy needed to get a reaction started.

Because Catalysts decrease the amount of energy a reaction needs to start, catalysts allow reactions to happen at lower temperatures.

Catalysts:Catalysts were discovered early on by accident…..chemists noticed that some substances speeded up reactions chemists didn’t know why

Catalysts were discovered by accident, so once one was discovered chemists tried all sorts of things to see if they would work too.

The discovery of catalysts was purely empirical- trying everything to see what worked.

Biological Catalysts?

Catalysts: How Do They Work?

Speed up the rate of a reaction by decreasing the activation energy of the reaction.

Catalysts reduce the amount of energy needed to do reactions….this is huge in industry (Billions of dollars)

Explain and draw a diagram to show it.