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.
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.
So, let's go back to our caterpillar chemistry. There would be a reaction rate constant for the forward reaction here:
caterpillar + cocoon --> butterfly
This reaction rate constant would be represented by a k subscript f (f stands for forward).
There would also be a reaction rate constant for the reverse reaction:
caterpillar + cocoon <-- butterfly
This reaction rate constant would be called k subscript r (r for reverse).
When you divide these reaction rate constants, you can create an equilibrium constant, which is represented by a k subscript eq (eq for equilibrium). This looks like this:
Kf/Kr = Keq
You might be thinking, 'Cathy Caterpillar, uh, so what?' And I get you, it doesn't sound particularly exciting. Heck, on a typical day I'm happily eating leaf matter and I'm not impressed by too much... but this equilibrium constant is pretty cool.
Remember our caterpillar + cocoon <--> butterfly chemical equation was in equilibrium, right? Well we can use the equilibrium constant to tell us if there are more caterpillars and cocoons or butterflies when the reaction is at equilibrium.
- If the equilibrium constant is 1, then the products equal the reactants, meaning the number of caterpillars and cocoons equals the number of butterflies.
- If the equilibrium constant is less than 1, the reactants are greater than the products, meaning there are more caterpillars and cocoons than butterflies.
- If the equilibrium constant is greater than 1, there are more products than reactants, meaning more butterflies than caterpillars and cocoons.
I feel a change coming on! Before I completely transform, let's review this lesson.
- In a chemical reaction, there are reactants and products, and the rate at which reactants turn into products is called the reaction rate.
- Chemical kinetics studies reaction rates in chemical reactions.
- Some chemical reactions can go forward and reverse.
- Equilibrium is when the rate of the forward reaction is equal to the rate of the reverse reaction; this doesn't mean the reactants equal the products.
- k is the reaction rate constant and is used to determine how fast chemical reactions take place.
- k is not actually constant; it can change depending on the reaction as well as changes to the conditions, like a temperature increase.
- k for forward and reverse reactions can be used to determine the equilibrium constant. This number tells you if there are more reactants or products when a chemical reaction reaches equilibrium.
Well, I think that about covers it, and, speaking of transformations, check this out! I guess you could say my reaction rate was pretty fast! From now on you can call me Cathy Butterfly, but I guess that doesn't have the same ring to it.
Once you've completed this lesson, you'll be able to:
- Define chemical kinetics and reaction rates
- Explain what equilibrium in a chemical reaction means
- Identify what factors affect reaction rates and equilibrium constants
- Describe how to find the equilibrium constant of a reaction and what different equilibrium constants mean
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