Sapling Week 3/4 #6

[luludaly2B]

Could someone help me with this problem?
The problem says: For each reaction, identify another quantity that is equal to ΔH∘rxn.

[LReedy_3C]

To start this one, I wrote out which bonds were breaking or forming for each reaction, and in the case of the one you added, you have to recognize that its a combustion reaction because its reacting with oxygen

[Nathan Lao 2I]

It's really subtle but I overlooked the actual type of reaction this is when I first did this question. It's a combustion reaction, so I went with "enthalpy of combustion of CH4." :)

[Jayasree Peri 2J]

For this one specifically, one of the main things I looked at was what was specifically going on in the reaction, as compared to the answer choices. Since CO₂ and H₂O are the products of the reaction, I can assume that the most accurate representation of ∆Hº_rxn would be the enthalpy of combustion of CH₄.

[Jamie2002]

For this one, it's good to look over all the answer choices first. Think about what is going on in the reaction. Is it combustion? Is something specific being formed?

[EmilyGillen_1A]

This question took me a lot of tries! But I realized that I had to look at what my options are, and then compare those enthalpies/energies to the given equation. For this reaction, you see that a carbon compound and Oxygen are reacting together to form CO2 and water. So the enthalpy of this system is best summed up by the enthalpy of combustion.

For the questions with C , H, and CH4 you just have to look at if the CH bonds are being formed or broken. If the bonds are being formed then energy will be released ( -bond energy). If the bonds are being broken between C-H, then energy is required (+bond energy).

And for the reaction involving graphite, you know that graphite is the most stable form of carbon so the reaction will involve the enthalpy of formation (seeing as the other reactants are also in their most stable state).

Hope this helps!

[Sahaj Patel Lec3DisK]

It would be the enthalpy of combustion of CH4 since that is the exact reaction that is happening, When an organic compound (usually made of Carbon, Hydrogen, and/or Oxygen) is burned with Oxygen (O2), and the products are Carbon Dioxide and Water, that is a combustion reaction. Hope this helps!

[Bella Wachter 1A]

If we are looking to calculate the delta H rxn using bond enthalpies, we would look at the bonds formed and bonds broken. In the reactants, we can see that there are 4 C-H bonds broken and one O-O double bond broken. In the products, we have 2 C-O bonds and 4 O-H bonds formed. Therefore, we know the last two options are false - the value obtained will not be equal to -4 x the bond energy of C-H or 4 x the bond energy of C-H.

If we are looking to solve using enthalpies of formation, we would add up the enthalpy of formation for CO2 and H2O (times 2 since we have 2 water molecules) and subtract the sum of the enthalpies of formation for CH4 and O2 (times 2 since we have 2 O2 molecules). This is not equal to the enthalpy of formation of CO2, so we know the second option is false.

The enthalpy of combustion of a substance is defined as the change in enthalpy of one mol of a substance as it combusts - aka as it reacts with O2. Since we have one mole of CH4 reacting with O2, delta H rxn will be equal to the enthalpy of combustion of CH4. Therefore, option 1 is correct.

[Joshua Chung 2D]

Since this reaction is a textbook example of a combustion reaction, we can immediately single that out as the most likely answer. If we look at the other possibilities, the bond enthalpies don't line up (as described above). Thus, the enthalpy is equal to the combustion of CH4.

[Sophia Kalanski 1A]

well here you can tell combustion is involved because the products are CO2 and H2O and the reactant has O2

[Rachel Kho Disc 2G]

You could solve this problem thinking about the [bonds broken]-[bonds formed], but there is a much easier and faster way to figure this out. Just remember that a combustion reaction usually consists of x compound + O2(g) --> CO2(g) + H2O(l)!

[Sonel Raj 3I]

If you look at the equation, you can see it is a combustion reaction, which has the basic format [species] + O2 (yields) CO2 + H2O. Therefore, you would choose the answer which deals with the enthalpy of combustion of CH4.

[Can Yilgor 2D]

Generally, when an organic compound like (CH4) is involved in a combustion reaction the products are CO2 and H2O. Therefore, the enthalpy of combustion of methane can be another expression for the change in enthalpy of the reaction. I agree that looking over the answer choices can help you eliminate unrelated options.

[Manseej Khatri 2B]

Using process of elimination you can remove the bond enthalpy ones because c-h bonds are not the only bonds being broken and formed. The enthalpy of formation of CO2 does not work since that is the energy required to form CO2 from C and O2, you would need the enthalpies of formation for H2O and CH4 to find the enthalpy of reaction. Enthalpy of combustion of CH4 works because the reaction is a combustion reaction since there is a reaction with oxygen to form CO2 and H2O and that is a combustion reaction, meaning the enthalpy of combusting CH4 is the same as the enthalpy of the reaction.

[Giselle Granda 3F]

This one would be enthalpy of combustion, which makes sense right away because the reaction is producing CO2 and H2O after CH4 reacts with O2. Remember that anytime you deal with a combustion reaction CO2 and water are always the products, and O2 is always the reactant!