Intro to Chemical Kinetics: Videos & Practice Problems

Chemical kinetics is the branch of chemistry that examines how factors influence the rate at which reactants are converted into products. The reaction rate is defined by the change in concentration of reactants or products over time, denoted by square brackets around the chemical species. A faster rate is indicated by the formation of more products in a given time frame, while a slower rate is characterized by the formation of fewer products. Reactions that proceed to completion, where nearly all reactants are transformed into products, are represented by a single arrow in the chemical equation. It's important to note that reaction rates decrease as reactant concentrations diminish over time, ultimately halting when all reactants have been converted to products, thus setting a limit on product formation. This understanding of reaction dynamics is crucial for predicting how chemical processes will progress under various conditions.

Chemical Kinetics

Chemical kinetics deals with the study of factors affecting the reaction rate or speed of motion of reacting molecules. Now when we talk about reaction rate, we're going to say reaction rate is just a measure of speed and change in concentration of reactions or products over time. Now to a shortwave writing concentration is to show it in brackets. So if you see a term in brackets, that means we're talking about its concentration. So if A reactants were in brackets, we'd be talking about reactant concentrations. If products were in brackets, then we're talking about product concentrations.

Now here we have an example of two chemical reactions. In the first one we have reactant A, which is represented by this red rectangle, and we have reactant B mixing with it. Together they give us product C. Now if we were to take a look, we see that in the beginning we have 5A molecules and 5B molecules. And then if we go to the other side, we can see that a good amount of product has been formed. We've formed 4 product Cs and there's still some A and B remaining. All this happens within 55 seconds.

If we take a look at the other reaction, we have AB plus Y gives us W. Here in the beginning we have only AB and Y because at the beginning all we have is reactants. If we look on the other side, we can see not much product is formed. There's only one W product that's formed right here. So if we were to compare these two reactions, we'd say that reaction one within the same amount of time, 55 seconds, we make way more product. And because we're making more product, we can say that this represents a faster rate. If we look at the other reaction, we can see that in the same 55 seconds we only make one product, so not as much. This would be our slower rate. OK, so less reactant is transformed into product.

Now here we're going to say for reactions that come to a completion, that means that almost all the reactants are broken down and products are formed. Remember, we break down reactants in order to form products. Now we're going to say for these reactions that go to completion, we use a single arrow to signify it. So if you have here a single arrow going forward, single arrow going forward. Now here within the given 55 seconds, not much product is forming here. But if we give it enough time, most of the product would be formed. It's just a little bit slower when compared to the first reaction over here.

Now besides that, we can say that reaction rates decrease with time due to reduction in concentrations of reactants. So this reaction can't go on forever. Eventually all your reactant will be gone, and once all your reactant is gone, the reaction stops. The reactant that's broken down has been transformed into product. So there's limits to this. There's a limit because all the reactants will disappear and therefore that sets a limit on how much product can be formed.

Chemical Kinetics Example

Here it says formation of carbon tetrachloride from chloroform is given by the reaction. So here we have one mole of chloroform reacting with one mole of chlorine gas to produce one mole of carbon tetrachloride gas plus one mole of HCl. Now here it says which graph accurately shows the concentration of CCl₄ as a function of time for this reaction.

So remember as time progresses, we expect our reactants to decrease in amount and we breakdown these reactants in order to form product. So we expect these to be increasing over time. Now here we're looking at the concentration of CCl₄, which we'd expect to be increasing. So A can't work. So the answer is either B, C, or D.

Now remember, your reaction does not go on forever. Eventually all your reactants will be broken down and therefore your product reaches a limit of how much of it can be made. Because of this, B & C don't work because in B it's saying that CCl₄ grows exponentially. That can't happen. Eventually you run out of reactants. C is not as steep of an increase, but it is still an increase forever. This doesn't work.

D is the best answer because in D we can see that our product is increasing over time, but eventually it reaches a plateau because we've run out of reactants. Once we've run out of reactants, we can no longer make any more product. So it's going to reach a value or plateau like we see in option D. So out of all the choices, option D is the correct answer.

Here’s what students ask on this topic:

What is chemical kinetics?

Chemical kinetics is the branch of physical chemistry that studies the rates of chemical reactions, the factors that affect these rates, and the mechanisms by which the reactions proceed. It involves the analysis of how different experimental conditions can influence the speed of a chemical reaction and yield information about the reaction's pathway and the transition states through which it occurs.

In chemical kinetics, you'll encounter concepts such as reaction rate, which is the speed at which reactants are converted into products. You'll also learn about rate laws, which express the relationship between the concentration of reactants and the reaction rate. Additionally, kinetic studies can reveal the energy required for a reaction to proceed, known as the activation energy, and the effect of catalysts which can lower this energy barrier and increase reaction rate without being consumed in the process.

Understanding kinetics is crucial for controlling reactions in industrial processes, developing new materials, and studying biological systems, as it helps chemists to design and optimize reactions for desired outcomes.

Which of the following statements correctly describe chemical kinetics? Please select all that apply.

Chemical kinetics is the study of the rates at which chemical reactions occur and the factors that affect these rates. It involves understanding how different conditions such as temperature, pressure, concentration, and the presence of catalysts can influence the speed of a reaction. Kinetics also explores the reaction mechanisms, which are the step-by-step sequences of elementary reactions that lead to the overall chemical change. By analyzing the kinetics of a reaction, one can determine the order of the reaction with respect to each reactant, which indicates how the rate is affected by the concentration of that reactant. Additionally, kinetics is concerned with the energy changes that occur during reactions, particularly the activation energy required to initiate the process. Understanding chemical kinetics is crucial for controlling reactions in industrial processes, developing new materials, and studying biological systems.

Why is chemical kinetics important?

Chemical kinetics is important because it studies the rates of chemical processes and the factors that affect these rates. Understanding kinetics allows chemists to determine how quickly a reaction will occur, which is crucial for a variety of applications. For instance, in the pharmaceutical industry, kinetics informs the development of drugs by helping to predict how the body will metabolize them, thus impacting dosing and timing for maximum effectiveness with minimal side effects. In environmental chemistry, kinetics helps in assessing the fate of pollutants and the efficiency of remediation strategies. Additionally, in industrial processes, knowledge of reaction rates is essential for the design of reactors and for optimizing conditions to maximize product yield while minimizing costs and waste. Kinetics also plays a pivotal role in studying the mechanisms of reactions, providing insights into the step-by-step sequence of events at the molecular level, which is fundamental for advancing chemical theory and innovation.