Lesson Video: Rates of Reactions Science

In this video, we will learn how to describe the rate of a chemical reaction and explain the effect the type of reagent and the surface area have on it.

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Video Transcript

In this video, we’ll learn about the speed or rate of a chemical reaction. We will describe the rate of reaction, learn how it’s measured, and understand what factors affect the rate of a reaction.

During a chemical reaction, a substance or substances, which are called the reactants, are converted into new substances, which are called the products. For example, hydrogen and oxygen gas react to form a brand new substance, which is water. Chemical reactions occur all around us all the time. There’s a combustion reaction that occurs in our cars. Fuel is burned to power the vehicle. There’s lots of chemical reactions involved in making food, such as the Maillard reaction, which is responsible for the browning of bread and other foods.

Chemical reactions are involved when something gets bleached, when a metal rusts, and when diamond is formed. Chemical reactions occur all the time inside our body. Our cells are constantly undergoing cellular respiration, which converts glucose into energy. Not all of these chemical reactions occur at the same speed. Some occur faster than we could time with a stopwatch. Some reactions take minutes to complete or hours. Some reactions take days or even weeks to complete. Some reactions are even slower and take hundreds of years to finish. As chemical reactions take place on a range of time scales, we’d like to be able to measure their speed. To do this, we’ll have to understand what’s going on to the reactants and products during a chemical reaction.

Say we have a chemical reaction where two reactants combine to form a product. We place our reactants in a beaker. We can express the initial amount of our reactant in terms of a concentration, usually given in units of moles per liter. Initially, we won’t have any product because none has been formed yet. But the reactant particles will begin to collide with each other, which forms the product. The concentration of the product increases because products are being formed. And the concentration of the reactant decreases as it gets used up. Eventually, the concentration of the products stop changing over time. When this happens, the reaction has finished. We can see this took about 50 seconds for this reaction.

We’d like some way to measure the speed of this chemical reaction. We’ll measure the speed of our reaction similarly to how we’d measure the speed of a car. We determine the speed of a car by measuring the change in distance per unit time. And the speed of our chemical reaction will also be a change in something per unit of time. The thing that’s changing during a chemical reaction isn’t the distance but the amount of the products and reactants. This amount can be a mass, volume, or some other measure. But the most common measure is probably concentration like we saw in this graph. So the speed of a chemical reaction, which we call the rate of reaction, is the change in the concentration of the reactants and products per unit time.

We can measure the rate of reaction by measuring the disappearance of a reactant or the formation of a product over time. Let’s say we have a copper sulfate solution, which is a bright blue. We add some sodium hydroxide to the copper sulfate. After some time, the solution will become colorless and a solid will form on the bottom of the beaker, which is copper hydroxide. There are two ways we could measure the rate of this reaction because the reactant, copper sulfate, is colored but the product isn’t. We can easily measure the disappearance of the reactant by measuring how long it took the solution to become colorless. We can also measure the rate of reaction by determining how much copper hydroxide formed after a certain period of time.

There are several factors that affect the rate of reaction, such as the nature of the reactants, the surface area of the reactants, the concentration of the reactants, the temperature of the reaction, and the use of catalysts. In this video, we’ll focus on how the nature of the reactants and the surface area affect the rate of reaction. We’ll start off by looking at the nature of the reactants. Specifically, it’s the type of bonding in a compound that affects the rate of reaction. Compounds with ionic bonding, such as sodium chloride, can disassociate into positive and negatively charged ions. These ions can react quickly. We can compare this to reactions between compounds with covalent bonding, like hydrogen and oxygen. Here, the bonds between the atoms and the reactant molecules must break so that bonds can form between the atoms and the products.

The making and breaking of bonds takes time. So reactions between covalently bonded compounds are often much slower than reactions between compounds with ionic bonding. The surface area also affects the rate of reaction. In a solid, only particles on the surface can react. Particles in the middle of the solid can’t react, until after the particles on the surface have reacted. Say that we have two samples that contain the same amount of a solid. In one sample, the solid is in one piece, but in the other, it’s broken up into smaller pieces. When the solid is in smaller pieces, it has a larger surface area and more of the particles are exposed and available to react than when the solid is in a large piece. So the reaction will occur more quickly when the solid is in smaller pieces. In other words, the larger the surface area, the faster the reaction.

We can see this in the iron filing experiment. In this experiment, we have two flasks. In one, we put a piece of iron ribbon. In the other, we put the same mass of iron, but in the form of iron filings. We put some hydrochloric acid in each flask and stopper them. Iron and hydrochloric acid react to form iron chloride and hydrogen. Hydrogen is a gas. As it’s formed, it will produce bubbles in the flask. We can collect this hydrogen gas in a gas syringe. And we can plot the volume of hydrogen gas produced over time. For both the iron filings and the iron ribbon, we can see that both flasks produce the same amount of hydrogen gas. This makes sense since we started with the same mass of iron in each flask.

Comparing the two lines, we see that they reach the maximum amount of hydrogen gas at different times. The flask with the filings reached the maximum amount much faster than the flask with the ribbon. So the reaction occurred faster in the flask with the filings than the flask with the ribbon. Now we’ve learned about the rate of reaction. So let’s work some problems before we conclude this video.

Which of the following statements best defines the rate of a chemical reaction? (A) The measure of change in the concentration of the reactants or products per unit of time. (B) The difference in mass between the reactants and the products. (C) The final concentration of the products following a chemical reaction. (D) The speed at which particles need to move in order to successfully collide. (E) Time at which the concentration of the products and reactants are equal.

The rate of a chemical reaction tells us its speed, in other words whether the reaction will be fast or slow. To define this quantity, we should know that a rate in general is a change in some quantity per unit time. For example, the rate or speed of a car is the change in distance per unit time. But during a chemical reaction, it’s not the distance that’s changing. It’s the amount of the reactants and products. Over time, the amount of reactants decrease as they’re used up. And the amount of the products increases as they’re formed. We can measure this amount as a volume, a mass, or a concentration. But the most common measure is probably the concentration of the reactants and products. Given what we’ve talked about, answer choice (A) seems to best define the rate of a chemical reaction: the measure of change in the concentration of the reactants or products per unit of time.

The graph below shows the concentration of oxygen gas produced during the following chemical reaction: two H2O2 reacts to form two H2O plus O2. How long does this reaction take to reach completion? (A) 90 seconds, (B) 120 seconds, (C) 80 seconds, (D) 100 seconds, (E) 20 seconds.

In the reaction in this question, hydrogen peroxide decomposes to form water and oxygen gas. The concentration of oxygen gas has been graphed over time. We need to determine how long it takes this reaction to reach completion. At the start of the reaction, the concentration of oxygen is zero because no oxygen gas has been produced yet. Then, the concentration gradually increases until the concentration stops changing when it reaches a value of one mole per liter. When the concentration of the reactants and products stop changing, it means the reaction has reached completion. The graph shows that the concentration of oxygen stopped changing after 90 seconds. So answer choice (A) is the correct answer. It took 90 seconds for this reaction to reach completion.

In an experiment, a student added a sample of a solid into a solution, resulting in a chemical reaction. The student repeated the experiment five times but changed the surface area of the solid. The graph showing how the concentration of one of the products changes over time for each experiment is shown below. In which experiment was the surface area of the solid the greatest?

In the experiment in this question, a solid was added to a solution, and the solid reacted. The concentration of a product of this reaction was graphed over time. Each of the lines on this graph represents the same reaction with the same mass of solid. The only difference is the surface area of the solid. We need to figure out which of these lines represents the experiment where the surface area of the solid was the greatest. In a solid, only particles on the surface can react. If we increase the surface area, more solid particles are exposed and available to react. This means the reaction will occur more quickly if the surface area is larger. So the experiment where the solid had the largest surface area will also be the experiment where the reaction occurs the fastest.

Looking at this graph, we can see that each line reaches the same maximum value for the concentration of the product, about 1.5 moles per liter. But each line took a different amount of time to reach 1.5 moles per liter. The red line got there first. The pink line took the longest to get there. In other words, the red line was the experiment where the reaction happened the fastest and the pink line was the experiment where the reaction happened the slowest. As we said, the fastest reaction will correspond to the greatest surface area. So the red line represents the experiment where the surface area of the solid was the greatest, making answer choice (B) the correct answer.

Now, let’s conclude this video with the most important points. Chemical reactions occur on a range of time scales. During a chemical reaction, the concentrations of the reactants decrease over time, but the concentrations of the products increase over time. The rate of a chemical reaction is the change in the concentration of the reactants and products per unit of time. The rate of a chemical reaction is affected by the nature of the reactants, the surface area, the concentration of the reactants, the temperature of the reaction, and catalysts.

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