Students will be able to understand that equilibrium is achieved when reactions rates are equal and identify equilibrium on a graph of reactants and products vs. time. Students will be asked many questions in order to sythesize the information as it is being presented, and as a self learner, this approach should be effective.
Background Summary:
The way to write an equation that is in equilibrium looks like this:
O2 + H2 ⇔ 2OH
When a chemical reaction happens, there is a specific amount of the compounds
involved. When the compounds are reacting, the amounts change, some are consumed
and others are formed. After a certain period of time, the concentrations
of the compounds will stop changing. This means that the reaction rate to
the equation going forward is equal to the reaction rate going backward.
Example:
H2 + I2 → 2 HI They are both written in a normal chemical equation but
2 HI → H2 + I2 this equation shows that equilibrium has been reached.
Equilibrium: H2 + I2 ⇔ 2 HI
The key to understanding equilibrium is that it doesn’t matter if you start
with two moles of HI or one mole each of H2 and I2 because after the reaction
takes place the quantities of each will become that same and the reaction
rates will be identical.
On these graphs you see the two equations that we have been using on a reactants
& products vs. time chart:
Do you see that equilibrium is reached at the same time and level regardless
of the direction of the equation? That means that the reaction rate of the
H2 and I2 as well as the reaction rate for 2HI is the same!
Lesson:
Part One: Watch this video and answer the questions:
How was equilibrium explained? (2 sentences)
Define equilibrium in terms of H2 and I2. (3 sentences)
Define equilibrium generally. (2 sentences)
What do you see happening in the demonstration?
What is happening at a microscopic level?
Check your understanding:
What is the key concept to equilibrium?
a) the amount of reactants and products is equal. (WRONG - the actual amounts of the molecules don't have anything to do with equilibrium)
b) the product and reactants stop reacting, it is impossible for any more reactions to take place (WRONG - reactions are always happening)
c) the reaction rates are equal to each other (CORRECT - the key to understanding equilibrium is to know that reactions are always taking place, but it is the reaction rate that becomes equal not the mass or amount)
d) all of the above (WRONG - a & b are wrong. the amounts don't effect equilibrium and reactions are always taking place)
Part Two: Now, you have a basic understanding of chemical kinetic equilibrium. To further understand this concept, complete the questions using "learning applet" below.
Select 80 molecules of A and a temperature of "Low." Click the "Start" button and observe the animation for a couple minutes. Describe what you observed.
Select 80 molecules of A and a temperature of 298 K. Click the "On" button and observe the animation for three minutes. Record the value of Kc at three minutes. How did the number of A molecules and the number of B molecules change with time?
Select 40 molecules of B and a temperature of 298 K. Click the "On" button and observe the animation for three minutes. Record the value of Kc at three minutes. How are the plots of the number of A molecules and number of B molecules versus time in this exercise different from the plots in the previous (#2) exercise? In what way are the plots in the two exercises (#2 and #3 ) similar?
What evidence do you have to support the assertion that the equilibrium states in exercises #2 and #3 are effectively the same?
Select 80 molecules of A and a temperature of 380 K. Click the "On" button and observe the animation for three minutes. Record the value of Kc at three minutes. How does the equilibrium state at 380 K in this exercise differ from the equilibrium state at 298 K in exercise #2?
When you raise the temprature of the system, is equilibrium reached quicker?
a) no (WRONG - the temprature of a system allows for more collisions to happen)
b) yes (CORRECT - there are more collisions happening per second so there is more of an oppertunity for moleclues to collide, temprature just speeds up the process)
c) temprature doesn't effect equilibrium (WRONG - temprature speeds up the equilibrium process therefore equilibrium is reached quicker)_
d) all of the above (WRONG - temprature effects the time it takes for a system to reach equilibrium)
- Does the direction of the arrow have any significance to the order of the reaction?
Hint: what does a system in equilibrium mean?
- It is important for you to reflect on your work. Write down the 7-10 main points of equilibrium.
Hint: for guidance, look at the background information.
- Draw the equation: 2H2 + 02 = 2H2O on a reactant and products vs time graph.
Hint: look at the previous graph
- In a system, explain what is happening at a molecular level when it has achieved equilibrium?
Hint: You can use analogies or previous equilibrium problems
Answers Below:
- When a system is in equilibrium, the reaction rates of the same so the reactions are happening at the same time.
- This answer will be different for every person
- It should be accurate enough to see where the products start and and where the reactants end
