Name: 【TED-Ed】What triggers a chemical reaction? - Kareem Jarrah
Uploaded: 2015-01-24T02:31:40.000Z
Duration: 3 min 46 s
Description: Thousands of YouTube videos with English-Chinese subtitles! Now you can learn to understand native speakers, expand your vocabulary, and improve your pronunciation...

You know how sometimes you go to bake a cake

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

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

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

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

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,

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

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,

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

We can also play out the opposite type of 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.

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 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,

So much so that some  even go over the hill.

This shift to instability,  or higher entropy,

As with the golf balls,  in actual chemicals

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

You can see both enthalpy and entropy at play

when you go to light  a campfire to cook dinner.

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

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.