What triggers a chemical reaction? - Kareem Jarrah

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Chemicals are in everything we see, and the reactions between them can look like anything from rust on a spoon to an explosion on your stovetop. But why do these reactions happen in the first place? Kareem Jarrah answers this question by examining the two underlying forces that drive both endothermic and exothermic chemical reactions: enthalpy and entropy.

Lesson by Kareem Jarrah, animation by Flaming Medusa Studios Inc.

You know how sometimes
you go to bake a cake

but your bananas have all gone rotten,

your utensils have rusted,

you trip and pour all of your baking soda
into the vinegar jug,

and then your oven explodes?

My friend, you and your chemical reactions
have fallen victim to enthalpy and entropy

and, boy, are they forces
to be reckoned with.

Now, your reactants are all products.

So, what are these "E" words,
and what's their big idea?

Let's start with enthalpy,

an increase or decrease of energy
during a chemical reaction.

Every molecule has a certain amount
of chemical potential energy

stored within the bonds between its atoms.

Chemicals with more energy
are less stable,

and thus, more likely to react.

Let's visualize the energy flow
in a reaction,

the combustion of hydrogen and oxygen,
by playing a round of crazy golf.

Our goal is to get a ball, the reactant,
up a small rise

and down the other much steeper slope.

Where the hill goes up,
we need to add energy to the ball,

and where it goes down, the ball releases
energy into its surroundings.

The hole represents the product,
or result of the reaction.

When the reaction period ends,
the ball is inside the hole,

and we have our product: water.

This, like when our oven exploded,
is an exothermic reaction,

meaning that the chemical's final energy
is less than its starting energy,

and the difference has been added
to the surrounding environment

as light and heat.

We can also play out
the opposite type of reaction,

an endothermic reaction,

where the final energy is greater
than the starting energy.

That's what we were trying
to achieve by baking our cake.

The added heat from the oven would
change the chemical structure

of the proteins in the eggs
and various compounds in the butter.

So that's enthalpy.

As you might suspect,

exothermic reactions are more likely
to happen than endothermic ones

because they require less energy to occur.

But there's another independent factor
that can make reactions happen:

entropy.

Entropy measures a chemical's randomness.

Here's an enormous pyramid of golf balls.

Its ordered structure
means it has low entropy.

However, when it collapses,
we have chaos everywhere,

balls bouncing high and wide.

So much so that some
even go over the hill.

This shift to instability,
or higher entropy,

can allow reactions to happen.

As with the golf balls,
in actual chemicals

this transition from structure to disorder
gets some reactants past the hump

and lets them start a reaction.

You can see both enthalpy
and entropy at play

when you go to light
a campfire to cook dinner.

Your match adds enough energy

to activate the exothermic reaction
of combustion,

converting the high-energy
combustible material in the wood

to lower energy carbon dioxide and water.

Entropy also increases
and helps the reaction along

because the neat, organized log of wood

is now converted into randomly moving
water vapor and carbon dioxide.

The energy shed by this
exothermic reaction

powers the endothermic reaction
of cooking your dinner.

Bon appétit!