Chemical Kinetics, Reaction Rate Constant & Equilibrium Constant

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  • 0:03 Chemical Reactions and…
  • 1:40 Equilibrium
  • 4:52 Rate Constants and…
  • 6:38 Equilibrium Constant
  • 8:26 Lesson Summary
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Lesson Transcript
Instructor: Julie Zundel

Julie has taught high school Zoology, Biology, Physical Science and Chem Tech. She has a Bachelor of Science in Biology and a Master of Education.

Are formulas, rate constants, and chemical reactions getting you down? Don't worry, this lesson will explain how reaction rate and equilibrium constants are related to chemical reactions.

Chemical Reactions and Reaction Rates

Hi there, my name is Cathy Caterpillar, and since I know so much about transformations, I'm here to tell you all about chemical kinetics, which we might as well rename caterpillar kinetics for this lesson. I mean, who knows, I might even transform into a butterfly before this lesson is over!

Chemical kinetics studies the reaction rates of chemical reactions, or how fast one group of substances transforms into another group. Let's use caterpillar chemistry to illustrate this idea. I know, I know, there's no such thing, but stick with me! Okay, so chemical reactions have reactants and products, kind of like this:

Caterpillar + cocoon --> butterfly

The caterpillar and cocoon are reactants and the butterfly is a product. The arrow denotes a chemical reaction took place. This is called a forward reaction: we are going from caterpillar and cocoon to butterfly, or left to right.

I know I prefer caterpillar chemistry, but maybe that's because I'm a caterpillar. But for those of you who want to try out the real thing, take a look at this chemical reaction where nitrogen and hydrogen gas combine to form ammonia gas:

N2 + 3H2 --> 2NH3

So, which molecules are the reactants, and which are the products? If you said nitrogen and hydrogen are reactants and ammonia is the product, you are correct!

Now, different chemical reactions occur at different speeds, and the reaction rate measures how fast reactants turn into products. Some chemical reactions can occur within seconds, whereas some can take hundreds or even thousands of years.


So, we know different chemical reactions occur at different rates, but did you know some chemical reactions can go in reverse? But what if the caterpillar reaction went in reverse? We'd have a butterfly breaking down into a caterpillar and cocoon. Or if we look at our other chemical reaction example, we would have ammonia breaking down into nitrogen and hydrogen gas.

Seems silly, right? But in the real world of chemistry, and not the caterpillar world, it can happen. So now, you'd read the chemical reaction from right to left.

When a chemical reaction can go forwards and backwards, chemists show that by using a double arrow, like this: <-->.

So, if some reactions can go both ways, what ends up happening? I mean if caterpillars and cocoons become butterflies, and butterflies become caterpillars and cocoons, and if nitrogen and hydrogen undergo a chemical reaction to become ammonia, and ammonia breaks down into nitrogen and hydrogen, what in the world do we end up with? Butterflies? Cocoons and caterpillars? Ammonia? Nitrogen and hydrogen? I don't know about you, but my head is about to explode!

Well don't worry -- it's not as confusing at it seems. Let's use our caterpillar chemistry to illustrate what happens. So, let's say you start out with a room full of caterpillars and cocoons. Remember, these are our reactants.

Once the chemical reaction begins, the caterpillars and cocoons come together to form our products, or the butterflies. Everything seems pretty straightforward in this forward reaction, right?

But now that the butterflies are piling up, they are going to start undergoing a reverse reaction. So, butterflies are breaking down into caterpillars and cocoons.

Now we have both happening, butterflies being produced and then butterflies being broken down and becoming caterpillars and cocoons. Eventually, this chemical reaction will reach equilibrium. This doesn't mean reactions stop happening, but it does mean for every butterfly produced, a butterfly is broken down into a caterpillar and cocoon. So, equilibrium just means the rate of the forward reaction is the same as the rate of the reverse reaction.

Before we go on, let's look at equilibrium in a real chemical reaction. Remember, nitrogen gas and hydrogen gas combine to form ammonia gas. At equilibrium, the number of hydrogen and nitrogen molecules coming together to form ammonia equals the number of ammonia molecules breaking down to form hydrogen and nitrogen gas.

But equilibrium doesn't necessarily mean that the number of caterpillars and cocoons equals the number of butterflies or the number of nitrogen and hydrogen molecules equals the number of ammonia molecules. Using caterpillar chemistry, let's say the reaction happened really fast: caterpillars and cocoons quickly became butterflies. This means there were a lot of products (or butterflies) when equilibrium was finally reached. Or, let's say the reaction was really slow and it took forever for the caterpillars and cocoons to become butterflies. In this case, there would be a lot more reactants (or caterpillars and cocoons) than products by the time equilibrium occurred.

Rate Constants and Chemical Reactions

Now that we understand reaction rates, forward and reverse reactions and equilibrium, it's time to add one more piece to the puzzle. That is the reaction rate constant, which is sometimes just called a rate constant. There are many things you should know about reaction rate constants, so here you go:

  • Reaction rate constants are numbers and are usually shown in moles/Liter. I don't want to get into the details of moles in this lesson, but you should know that moles/Liter measures the concentration of something, like how much of a substance is dissolved in water, or, if we are using caterpillar chemistry, we could say how many butterflies are in a certain volume of air.

  • Reaction rate constants are used in rate laws, which are mathematical expressions. Rate laws can help you determine how fast a chemical reaction occurred. Don't worry about the rate laws for this lesson, but realize that the rate constants help determine how fast reactions occur.

  • Reaction rate constants are represented by the variable k.

  • Generally speaking, a large reaction rate constant means a faster reaction rate and a small rate constant means a slower reaction rate. So that means if we had a large reaction rate constant for the reaction caterpillar + cocoon --> butterflies, that reaction would happen really quickly, and the butterflies would start piling up!

  • Although the word 'constant' makes you think it doesn't change, reaction rate constants don't actually remain constant. For example, they change depending on the reactants and products. So our reaction involving nitrogen and hydrogen gas would have a different rate constant than another chemical reaction. And if we changed the temperature, say we heated up the nitrogen and hydrogen, the rate constant would change then, too.

Equilibrium Constant

So, let's go back to our caterpillar chemistry. There would be a reaction rate constant for the forward reaction here:

caterpillar + cocoon --> butterfly

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