Chemists believe that for compounds, atoms, or ions to react successfully, the particles must collide together. This is the basis for a scientific theory known simply as the collision theory This theory is based on the idea that the rate of a reaction is determined by the number of successful collisions that occur over a given amount of time. A successful collision is one that results in the making of the product(s).
The two factors are sufficient energy and correct orientation. These must be true for a given collsion to successfully result in the production of products.
The collision between reactants must have enough energy for the reactants’ valence energy levels to penetrate each other. This interaction between valence energy levels allows the electrons to rearrange to form new bonds. A Maxwell-Boltzmann distribution graph. The y-axis of the graph is labeled “number of particles,” increasing as you go up the axis, and the x-axis is labeled (energy), increasing left to right.
The shape of the graph is like that of a bell or hill, showing that the majority of the particles have a kinetic energy near the average kinetic energy of the sample, while some particles have more energy (far right of the graph) and some particles have less energy (far left of the graph). A Maxwell-Boltzmann distribution graph. The y-axis of the graph is labeled “number of particles,” increasing as you go up the axis, and the x-axis is labeled (energy), increasing left to right. The shape of the graph is like that of a bell or hill, showing that the particles in this area of the curve do not have enough energy to react when they collide.
However, the particles to the right of the activation energy value are the only particles in the sample with enough energy to possibly react when they collide.
When reactants come in contact with each other during their random movement, the orientation of their collision will determine if the collision is successful. The atoms from each reactant that will bond together to form the new products must come in contact with each other if a bond is going to form between them. The more complex a reaction is, the slower the rate of reaction.
For example, if more than two reactants need to collide together at the same time with correct orientation the reaction time would be slow. For the reaction between ethene (CH2CH2) and hydrogen chloride (HCl) to occur, the double bond between the carbon atoms in ethene must be broken. The double bond is converted to a single bond, allowing the carbon atoms to each bond with one of the atoms from the hydrogen chloride.
The reaction can only happen if the hydrogen end of the hydrogen chloride compound collides with the double bond between the carbon atoms of the ethene. Any other orientation will not be successful in producing the new product; the two reactants will just “bounce” off of each other.
For most reactions, there is no practical way to have an effect on the orientation of particles in collisions. However, there are ways that chemists can increase the number of collisions that have sufficient energy to successfully produce the products. Concentration and Rate For many reactions involving liquids or gases, increasing the concentration of the reactants increases the rate of reaction.
In a few cases, increasing the concentration of one of the reactants may have little noticeable effect of the rate. In reactions that involve two or more reactants, increasing the concentration of the reactants will increase the rate of the reaction. According to the collision theory, a reaction between two different particles can only happen if those particles collide together. If the concentration of reactants is higher, meaning there are more moles of reactants in a given volume, the chances of the particles coming in contact increases.
As the number of collisions between reactants increases, the number of successful collisions in a given amount of time will also increase. The greater the concentration of solute particles in the solution, the greater the chance of the solute particles colliding with the surface of the solid reactant. Be careful: We cannot assume that by doubling the concentration of one of the reactants you will double the rate of the reaction. The amount by which an increase in concentration increases the rate of a reaction depends on many different properties of the reaction. Temperature and Rate
Because temperature is a measure of average kinetic energy, we know that the temperature of a system will affect the number of collisions that have sufficient energy to react. You have experienced the effects of temperature on rate if you have ever cooked something on the stove top or in the oven. You know that increasing the temperature at which you cook your dinner increases the rate of the cooking process. This can be a great time saver, but you need to be careful that you don’t end up burning your food! Increasing the temperature of a system increases the rate of a reaction.
This direct relationship between temperature and rate is due to two reasons, both related to the collision theory.
- Increasing the temperature of a system increases the average kinetic energy of the particles. This means more collisions between the reactants will have enough energy to react successfully.
- Increasing the temperature of a system increases the kinetic energy of the particles, meaning that the particles are moving faster. This means that there are more collisions per minute at a higher temperature than there are in the same system at a lower temperature.
Having more collisions in a given amount of time means there are more opportunities for collisions to meet both requirements for success. As you can see on the Maxwell-Boltzmann distribution curve below, a slight increase of temperature changes the distribution of energy in a sample. Although a slight increase in temperature (T2) may only increase the average kinetic energy by a small amount, there is a significant increase to the number of particles that meet and exceed the activation energy requirement for a given reaction.
Catalysts and Rate We have seen that an increase in temperature increases the rate of a reaction; if a reaction does not happen fast enough at normal temperatures, raising the temperature can help get the job done in less time. However, sometimes it is not possible to raise the temperature when we need a reaction to occur at a faster rate. In industrial settings, scientists need to keep safety and cost in mind, and so sometimes cannot exceed a certain temperature. In your body, living cells can only survive within a narrow temperature range.
Raising the temperature within the cells to speed up the necessary chemical reactions would result in damage to the very cells that the reactions support. So, how do we get the complicated biochemical reactions in your body to occur at a sufficient rate to keep you alive without raising the temperature to a point that would kill the cells you are trying to support? Thankfully, your body contains a variety of enzymes that speed up the rate of these reactions without the need to raise the temperature. Almost every important biological reaction occurs in living organisms with the assistance of an enzyme.
Enzymes are biological catalysts. A catalyst is a substance that speeds up a reaction without being consumed by the reaction. Catalysts are important in many biological and industrial reactions where the speed of a reaction is important but temperature cannot exceed a certain range. So, how do catalysts manage to speed up a reaction without adding heat to increase the temperature? If temperature cannot be increased to meet the activation energy requirement of a given reaction, another way to speed up the reaction is to provide an alternative reaction pathway with a lower activation energy requirement.
As you can see in the potential energy diagram below, a catalyst provides a different pathway between reactants and products that requires a lower amount of activation energy. Imagine you are hiking through the woods and there are two paths that will take you to your destination. The main path requires you to climb a steeper hill than the alternate path. Both paths take you to the same exact destination; the alternative path just requires less energy to get there. That is how a catalyst works: It provides an alternative path to the same final products but requires less activation energy to get there.
The lower activation energy requirement of this alternative reaction pathway results in a larger percentage of the given collisions between reactants that will be successful and produce products. Notice in the Maxwell-Boltzmann distribution curve below that the lower activation energy requirement of the catalyzed pathway allows many more particles to meet the energy requirement for a successful collision without any increase in temperature. Did You Know? Some substances, called inhibitors, actually decrease the rate of catalyzed chemical reactions.
They do this by interfering with the catalysts’ ability to participate in the reaction. Many biological poisons are inhibitors that interfere with the enzymes in your body. Speeding up a Reaction If all the students in a math class are about to take a very tough exam, what are the possible ways to increase the percentage of the class that successfully passes the exam with a grade of C or higher? If all the students work hard studying and practicing for the test, they would probably see an increase in the number of students who pass the exam successfully.
Another option is for the teacher to lower the standard for a C on the grading scale. If your teacher changed the grading scale to make 55 percent the requirement for a C grade, it would be easier for more students to earn a C or higher with less effort on their part. This story gives an analogy for the two main ways we can increase the rate of a chemical reaction. Just as increasing the effort made by the students will result in more students successfully meeting the grade requirement of a C, increasing the temperature of a system allows more particle collisions to meet the energy requirement for a successful collision.
On the other hand, lowering the standard for a passing grade can also increase the number of students who successfully make the grade. This is how a catalyst works. Lowering the activation energy requirement of a reaction allows more collisions to be successful without changing any properties of the collisions themselves. Although using a catalyst to lower the energy standard in a chemical reaction is a common practice in chemistry, do not expect your teachers to lower their standards for your success anytime soon.
Some practice: e reaction shown below has a positive enthalpy change and a negative entropy change.
2C (s) + 2H2 (g) C2H4 (g)
The reaction will not be spontaneous at any temperature. Which of the following conditions will result in a reaction that is spontaneous only at low temperatures? negative enthalpy change and negative entropy change . Compare the following words. Which word best describes a scientific model? Analogy The collision theory states that the rate of a chemical reaction is determined by collisions occurring at the molecular level.
Which of the following will best help us visualize collision theory? A scientific model Scientific models help predict experimental results. The model of an ideal gas is useful because it approximates the behavior of most gas molecules. Catalytic converters on automobiles use the metals rhodium and platinum as catalysts to convert harmful gases to carbon dioxide, nitrogen, and water. Which answer best explains why rhodium and platinum do not have to be continuously replaced in the system? The metals speed up the reactions but are not used up in the reaction.
Which of the following changes will always be true for a spontaneous reaction? Which of the following changes has a decrease in entropy?
3Fe (s) + 2O2 (g) Fe3O4 (g)
Which of the following combinations will result in a reaction that is never spontaneous? positive enthalpy change and negative entropy change Describe in detail what you expect for the changes in enthalpy, entropy, and free energy when a sample of liquid evaporates.
What factors did you investigate in your procedure, and why did you choose to compare these two factors? I checked if the amount of water and temperature would affect the rate of reaction. I learned that increased temperature means higher kinetic energy, which would affect the movement of the particles in the mixture. I was just really curious if the amount of water would have anything with dissolving the tablet.
What other factors did you need to control during your investigation? Explain how you controlled each one in your procedure. The water need to be controlled during the investigation. So I used filtered water for each procedure (except the ones that I had to heat up). The tablet had to be the same size for each experiment.
What is your prediction about the results of each trial in your lab? Explain your predictions based on your knowledge of the dissolving process, collision theory, and reaction rates. I hypothesize that the hotter water would have a faster rate of reaction than the one with the room temperature water. This will occur because the kinetic energy will cause the particles to move faster, causing them to collide more easily. I also hypothesize that the large amount of water would be cause the tablet to dissolve faster because the collision theory states that some factors such as the change in amount, progress, motion etc, can affect the rate of reaction.
Explain the collision theory, in your own words, and what is necessary for a collision to be successful. According to the theory, if the molecules in the reaction are allowed to collide more, the faster the reaction rate would be. In order for a collision to be successful, sufficient energy and orientation must be true for the collision to be successful.
A specific catalyst was not provided for this reaction, but catalysts are useful for increasing the rate of many slow reactions. In your own words, give a detailed explanation of how catalysts can increase the rate of a reaction or process. A catalyst can speed up the reaction rate, by allowing the reactants and products to collide to only require a lower amount of activation energy.
