Factors Influencing Rates - Online Tutor, Practice Problems & Exam Prep

Chemical reactions are influenced by four key factors: reactant concentration, reactant surface area, temperature, and catalysts. Higher concentrations of reactants increase collision frequency, thus accelerating reaction rates. Larger surface areas of reactants also enhance collision frequency, boosting reaction rates. Elevating temperature raises collision energy, causing molecules to move faster and collide more forcefully, which can double the reaction rate for every 10°C increase. The addition of a catalyst lowers activation energy, leading to more successful collisions and a faster reaction rate. These factors are essential for understanding how to control and predict the speed of chemical reactions.

Factors Influencing Rates Concept 1

There are four major factors that directly influence how fast or slow a reaction proceeds. Now here we're going to say for a reaction to occur, reactive molecules must collide with enough energy, and with this whole idea of energy and collisions, we have collision frequency and successful collisions.

Now, collision frequency is the number of molecule collisions per unit of time, and successful collisions are just those energetic collisions resulting in product formation, because sometimes molecules may collide together, but they may not do it with the right amount of energy or force, and therefore products will not be made.

So if we take a look here, we have molecules that are energized, they're energized and hit each other with enough force, and this results in a successful collision where they're stuck together. If they don't have enough energy, then they're not going to lead to a successful collision. They will not stick together and eventually they will just bounce off one another.

Keep this in mind. These are the fundamental things we need for any successful collision for a chemical reaction to proceed. Click on the next video and let's go look at more in depth in terms of these four factors that have a direct impact on the successful collisions.

Factors Influencing Rates Concept 2

The four factors that have a direct impact on influencing reaction rates include concentration of reactants, surface area of reactants, temperature, and catalysts. Now here if we take a look at concentration of reactants, first we're going to say the condition is if we increase the reactant concentrations, what effect does this have on collisions? Well, increasing the amount of reactants within a mixture means they're going to be more reactants floating around. This is going to result in an increase in collision frequency because since there's more molecules floating around, you have a higher likelihood of them bouncing into each other. This results in an increase in your rate.

Now surface area of reactants. Here you want to have a larger surface area, so we want to increase our surface area. Usually when we talk about surface area for compounds, we can look at compounds as either being linear or branched. When we say linear, linear compounds have higher surface areas. Here we have hydrocarbon. It's made up of CH₃, CH₂, CH₂, CH₃. Branched means that whereas in linear we just have a straight line of elements together, something that's branched has a portion of it that's sticking out. Here, this CH₃ part is a branching group. Branching decreases our surface area. So just remember, if we can increase the surface area of our reactants, that's going to cause an increase in our collision frequency, which in itself again will cause an increase in our rate.

Now temperature, we're going to say here that increasing, increasing our temperature is going to cause an increase in our collision frequencies. If you think of it in terms of your heating up, your mixture, your solution which has reacted molecules floating around, if you're heating it up, those molecules will absorb that extra external heat source and basically use it to power themselves up. This will cause them to move around with greater energy. If they're moving faster, they can bump into each other even faster. So this is going to cause an increase in collision frequency because there's an increase in collision energy from absorbing the thermal energy. Now we're going to say the rule of thumb is if we increase the temperature by just 10°C, this causes a doubling of our rate, so our rate would increase by 2X. So increasing temperature increases collisions, increases the force of the collisions because the molecules are moving faster. All of this causes an increase in our rate.

Finally, a catalyst, we're going to say the addition of a catalyst. Remember, addition of a catalyst causes a decrease in your activation energy. And that's a good thing, because the lower your activation energy is, the more likely that your reactants can become products. So decreasing our activation energy will cause an increase in successful collisions, which in turn will cause an increase in your rate. So these are the four factors you need to keep in mind when dealing with reaction rates. Affecting them can have a big impact on how fast or slow your chemical reaction will proceed.

Factors Influencing Rates Example 1

Which of the following would cause the rate of a chemical reaction to slow a increase in temperature of the reactants? Remember, the rule of thumb is if you're increasing the temperature of a reactants, that's always going to cause an increase in your rate, because the molecules will absorb this thermal energy and use it to energize themselves, but move faster, hitting each other more often and with greater force.

B increase in concentration of the reactants. Well, if you're increasing the concentration of reactants, there's more reactants floating around, so they're more likely to hit each other, which increases their chances of successfully colliding together. So this would only increase your rate, not decrease it.

C increased surface area of the reactants. Increasing the surface of your area of your reactants always causes an increase in your rate because there's just more spots where things can connect together. O here. This always makes the rate faster.

The answer here is increase in volume that the reactants take up. This is the answer if we think about it. Let's say we have a container that's this big and in it we have some floating molecules. They have to collide in order to successfully combine. But what would happen if I increase the volume of the container while keeping the number of molecules the same, right? And I'm not. I didn't actually count each dot from the first container, but just think of this visualization here.

The container's bigger are the reactant molecules more likely or less likely to collide together. Now they're less likely because there's so much more space between them. There's a lower chance of them finding each other colliding. And then hopefully some of those collisions lead to sticking together. Because remember, you can collide with each other. That doesn't guarantee you'll stick together.

All right, So here, increasing the volume that the reactants increase in, volume that the reactants take up means the increase in the container volume. This does not help reaction rate, it lowers it because reactive molecules are less likely to find each other, right? So here only option D would potentially slow down a chemical reaction.

Which of the following reacting molecules are able to produce products the fastest?

• A.
• B.
• C.
• D.

A chemist tries to burn sulfur in air and notices that it burns very slowly. Chemist then burns sulfur in pure oxygen and observes a brilliant blue flame. Which factor is affecting the reaction rate?

• A. increase in temperature
• B. the presence of a catalyst
• C. increase in reactant concentration
• D. physical state of reactants

Here’s what students ask on this topic:

How does temperature affect reaction rates?

Temperature significantly influences the rate at which chemical reactions occur. As temperature increases, the kinetic energy of the molecules involved in the reaction also increases. This heightened kinetic energy results in molecules moving faster and colliding more frequently with greater force.

These more frequent and energetic collisions increase the likelihood that the molecules will overcome the activation energy barrier, which is the minimum energy required for a reaction to occur. When the activation energy barrier is surpassed, reactant molecules can break their bonds and form new ones, leading to the products of the reaction.

Therefore, higher temperatures typically lead to faster reaction rates because a greater proportion of the reacting molecules have enough energy to undergo the transformation. Conversely, lower temperatures result in slower reaction rates due to reduced molecular movement and fewer effective collisions. This relationship is often quantified in the form of the Arrhenius equation, which mathematically relates the rate of a reaction to temperature.

How do catalysts affect reaction rates?

Catalysts are substances that increase the rate of a chemical reaction without being consumed in the process. They work by providing an alternative reaction pathway with a lower activation energy compared to the uncatalyzed reaction. Activation energy is the minimum energy that reacting molecules must possess for a reaction to occur.

By lowering the activation energy, catalysts enable more reactant molecules to have enough energy to reach the transition state, which is a high-energy, unstable state necessary for the reaction to proceed. This results in an increased number of successful collisions per unit time, thereby speeding up the reaction rate.

It's important to note that while catalysts speed up reactions, they do not alter the equilibrium position of the reaction; they simply help the system reach equilibrium faster. Catalysts are specific to particular reactions and do not change the final products of the reaction. They can be used in very small amounts relative to the reactants, and can often be recovered and reused.