Visual Representation of Activation Energy

What you can expect to learn from this page:

Users will learn how activation energy affects the number of particles which can react. They will interpret Activation Energy graphs and create their own. Users will be able to draw information on the reaction type from graphs such as exothermic vs endothermic, and information on the equilibrium constant.

Background Summary:

The student should know about: Reaction rates, collision models, and equilibriums/equilibrium constants.

Lesson:

READ THE BACKGROUN INFORMATION BEFORE ANSWERING ANY QUESTIONS OR COMPLETING ACTIVITIES!

 

If you have a situation involving two species (types of atom), they can only react together if they come into contact with each other. They first have to collide. Even then, they only may react.

Why "may react"? It isn't enough for the two species to collide - they have to collide with the right orientation, and they have to collide with enough energy for bonds to break.

In this site, we will focus on the energy requirements of reactions. The amount of energy it takes for a reaction to take place between two colliding molecules is called the "Activation Energy" of the reaction. this energy is different from each reaction to the next. The process of a reaction and the energy of the reaction can be expressed in a graph. This graph is called an "Activation Energy Graph".

Follow the explanation of the animation below to increase your understanding of what the graph means.

Specific and important info can be drawn just from the simple shape of the graph. This can save time in difficult equations. One such important observation is the relation between the equilibrium constant (Keq), and the heat exchange of the reaction. The equilibrium constant is the ratio of the concentration of products to the concentration of the reactants.

(the exponents are the coefficients of the molecules in the chemical equation)

The heat exchange is the difference in temperature or energy between the reactants and the products. The heat exchange, it just so happens, directly affects the ratio of the concentration of reactants to products. If the heat exchange of the reaction is positive, the products have a higher potential energy than the reactants, and are less stable (endothermic). If the heat exchange is negative, then the products have a lower potential energy than the reactants and are more stable (exothermic). If a reaction is exothermic, it can be expected that there will be a higher concentration of products than reactants, and vice versa for endothermic reactions. By determining whether the heat exchange ('enthalpy' or 'change in H') of the reaction is positive or negative (endothermic or exothermic), one can determine whether Keq will be greater than, less than, or equal to one, since Keq depends on the concentration of products to reactants.

Note: change in H or enthalpy is equal to the ending temprature minus the starting temprature.

* This makes sense because if the Keq is the ratio of products to reactants, a greater concentration of products will make Keq greater than one. A greater concentration of reactants will make Keq less than one.

Different variables affect the shape of the graph and the reaction itself. Among these are: The starting temperature of the reaction, the amount of energy released or taken up in the reaction, and the ending energy of the reaction (or the energy of the products).

Alternatively, some variables have no affect on the activation energy graph. Note how the number of particles changes the concentration of the reaction but has no effect on the graph or the equilibrium of the reaction. A greater concentration would increase the rate of reaction, but would not change the ratio of products to reactants of the reaction at equilibrium (Keq).

CHANGE THE VARIABLES ON THE APPLET BELOW TO GAIN UNDERSTANDING OF FOW DIFFERENT VARIABLES AFFECT THE ACTIVATION ENERGY GRAPH:

Chose a random activation energy, a random starting temperature, and a random ending temperature for the reaction. All variables you select should be on a scale of one to one hundred for the purposes of this activity only. Graph these results in the form of an activation energy graph.

The concentration of reactants in the a reaction affects:

a) The activation energy

b) The starting and ending temperatures (energy)

c) The equilibrium constant

d) None of the above

Determine the energy released or absorbed from the reaction, determine whether the reaction you represented in your graph is exothermic, endothermic, or at equilibrium.

If the change in H (enthalpy) is positive, then the reaction is:

a) exothermic

b) endothermic

c) at equilibrium

Use your knowledge of equilibrium constants to determine whether Keq will be greater than , less than, or equal to one for your reaction. (Hint: This should be easy if you remember the relationship between exothermic, endothermic, etc, and the equilibrium constant)

If the reaction is endothermic, you would expect the equilibrium constant to be:

a) Greater than one

b) Less than one

c) Equal to one

d) Cannot determine with given information

Once you get your results, what conclusions can you make about the connection between the equilibrium constant and the shape of the graph?

Mark a, b, c, or d for all true statements:

1) the starting energy can be greater than the activation energy.

2) The ending energy can be greater than the activation energy.

3) Reactions generally occur spontaneously if the ending energy is lower than the starting energy.

 

a) 1 & 2 only

b) 3 only

c) 1, 2, & 3

d) None of the above

Homework (if any):

Record these results in your notebook, and then check the flash applet for the correct results. Repeat this process until your results match the flash applet. Keep in mind that your starting or ending energy cannot be greater than the activation energy! (I know, I just gave you an answer)

If your answers don't match completely the first time, feel free to try it again! Chose different variables and try to fix what went wrong! Good luck, and I hope you learned a lot through this site!

Answers to the multiple choice questions are below. Please look over your own answers making sure you have answered to the best of your ability before checking.

 

 

 

 

 

 

 

1) d

Because: Think. Adding particles to the reaction will not increase the temperature. Nor is there any reason for a greater concentration to affect the activation energy, since the activation energy simply varies from reaction to reaction and the only way to lower the activation energy is to add a catalyst. The equilibrium constant is a ratio of reactants to products at equilibrium. Adding more reactants will not change the equilibrium constant, but will simply force the reaction to compensate to reach equilibrium once more. Therefore, the answer is 'd', none.

 

2) b

Because: if enthalpy is equal to the ending energy minus the starting energy, then a positive enthalpy would imply a greater ending energy. A greater ending energy implies that energy was absorbed by the system. Therefore, the reaction must be 'b', endothermic.

 

3) a

Because: If the reaction is endothermic, you would expect more reactants than products, since the activation energy is so high. If the equilibrium constant is a ratio of reactants to products, and there are more reactants than products, the equilibrium constant would have to be 'a', greater than one.

 

4) b

Because: You would never draw an activation energy graph with the activation energy below the starting or energy energy. Everything represented in the graph is a mean or average. Sure, some particles will react, but the vast majority will simply not have enough energy. The one true statement, though, is '3' which states that if the reaction is exothermic, it will occur spontaneously. All this means is that the reaction wants to happen since lower energy levels are more stable than higher energy levels. Generally, if a reaction can release energy, it will. The answer, therefore, is 'b'.