1st post = 10 points
2nd post (reply to someone else's) = 5 points
total = 15 points
When we explored stoichiometry earlier in the course, we worked with reactions in which the limiting reagent was entirely consumed. This is characteristic of reactions that go to completion. The products are formed until one of the reactants has been completely consumed.
As we've just seen, many reactions don't go to completion. They reach a state in which there is a mixture of reactants and products. We say that the reaction is reversible because when equilibrium is reached, the forward and reverse directions of the reaction are occurring at the same rate.
In theory, all reactions are reversible, as long as the reactants and products remain in contact with one another. But what does this mean in practice? Did we make some incorrect assumptions about the reactions we studied earlier in the course?
Consider the combustion of methane:
CH4 (g) + 2 O2 (g) 
CO2 + 2 H2O (g)
Notice that we've shown this as a reversible reaction. However, when we studied the ideal gas law, we used the reaction stoichiometry to calculate the volume of carbon dioxide that would be produced from a certain mass of methane. In this case, we assumed that the reaction went to completion.
Now that you know about dynamic equilibrium and the equilibrium constant, consider the following question: When is it safe to assume that a reaction goes to completion, and when must you treat it as a reversible reaction? What information would you need in order to make a decision, and how would you use that information to decide whether or not a reaction is reversible?
Andrew Park
ReplyDeleteI believe that it is only safe to assume that a reaction is reversible only when the products can be contained and turned back into reactants. If the products cannot be retrieved after the reaction has been completed, then it is safe to say that the reaction has gone to completion. Otherwise, all reactions are reversible. That's what I think anyway.
Megan Dickson
ReplyDeleteI think that you can assume that a reaction is in a state of chemical equilibrium when the products are able to be reversed back to their states as reactants. When there is a change in which the products are consumed and cannot make products, then you cannot say that the equation is reversible or that the equation is in a state of equilibrium.
Megan Dickson
ReplyDeleteAndy, I agree with your definition. I think that for the most part, equations that involve products that are able to reverse back to their respective reactants are in a state of equilibrium. I think that you can only know this by studying the chemical reaction.
Ellen Cho
ReplyDeleteI think that reversible reaction occurs when the products of a reaction can be converted back to the reactants. Also, to be a reversible reaction, I think the reaction has to be in equilibrium state since reversible reaction needs to take place in both back and forth directions.
I agree with all ideas. I think they are all basically same ideas that the reversible reaction occurs when the reaction can be reversed in a equilibrium state.
ReplyDelete↑Ellen Cho
ReplyDeleteJosh Dos Santos
ReplyDeleteI think that one can safely assume that every reaction is reversible as long as the reactants and products remain in contact.
However, I do not believe that a mistake was made when studying limiting reagants because in practice it makes sense.
For example, in a precipitation reaction, the limiting reagant will produce as much precipitate as possible, and it is very unlikely that the precipitate will dissolve once more in practice.
Moreover, as long as the circumstance, such as the combustion of methane, is not specifically described as being in a closed container over a period of time, it should be assumed that the reaction went to completion and the produced gas left as it was formed, in this way all our studies on limiting reactants and the ideal gas law is completely valid.
Josh Dos Santos
ReplyDeleteAndy, I totally agree. As long as the products can be made into reactants again, it is in equilibrium.
But, what if the rate of the reverse reaction is extremely slow at first.. would this be why theoretical yield and actual yield always differ.
So.. reactions not involving gases that escape can go to completion theoretically, but could they have started the reverse reaction before the actual yield in measured in the lab?
What do you guys think?
As stated before, both products and reactants must be in contact with one another, which means that they need to be kept in a closed system that does not allow either of each to escape nor to be interfered with, only then is it safe to assume that a reaction might go to equillibrium.
ReplyDeleteAllie VanO-
I am rather curious about the idea that the reactions might move at such a slow pace in the reverse that scientists might mistake that the reaction has gone to completion.
ReplyDeleteMistakes can be made not necessarily by tampering with material but by misjudging exactly how the reactants might work together as Josh was suggesting with the theoretical and actual yield.
-Allie VanO
Moon Kyu Kim
ReplyDeleteI will not assume that the reaction in reversible unless it shows the evidence that the product had gone to the reactant.
In reactions that forms precipitate, I have rarely seen the precipitate going back to the liquid form. However, it is distinguishable if the reactant and product have colors. These are reactions that can be easily distinguished.
Moon Kyu Kim
ReplyDeleteI agree with Josh, Megan, and Andy. Megan and Andy made a clear definition of reversible reaction. I have the same thought with you guys. I also agree with Josh about the stoichiometry we did in the previous sections. I also think that it was not wrong because it perfectly made sence when I was doing it in both practically and theoratically.
Okay, I guess I'm the last runner up.
ReplyDeleteI think it is safe to assume that reaction is reversible when activation energy required for products to change into reactants is smaller than the energy produced from reactants to products. For this reason, the information that is required for us to determine whether the reaction goes to completion or not will be the activation energy of each half reaction, from reactant to product and product to reactant.
Example of these information would enthalpy. The enthalpy of the equation would help to figure out if the equation will go to completion or not.
I agree with what Andy says. Since the Product of the equation becomes the reactant of the equation in the reverse reaction, the products must be contained, and it should be one of the factors.
ReplyDeleteThis brings to the other point with the comment that I wrote earlier. Energy cannot be contained like other products can be. So when we are comparing the required activation energy for each half reactions, we need to take account the fact that the energy may be lost, since the energy produced will be in form of heat energy, and there fore may be lost.