Achieve #6 (Week 5)

[asheagg3I]

Hi,

Can someone help with this? I don't know how to start this question.

[Hayley Vu]

You can start by looking at the answer choices and trying to match them to their respective equations. First, we can focus on the ones that say "4xbond energy of C-H" and "-4xbond energy." These choices refer to the energy that is either released or absorbed as four C-H bonds are formed or broken. This means that we should look for an equation that shows each of the four H-C bonds being formed or broken individually and match them based on whether energy is released or absorbed in the process. For the choice that says "enthalpy of combustion of CH4," we should look for a combustion equation. And for the choice that says "enthalpy of formation of CH4(g)," we should look for the equation that shows the formation of CH4 from its elements when they are in their standard states (such as O2 being the standard state for the element oxygen rather than just O by itself). Hope this helps!

[Alice Guey 1B]

You can start by looking at the answer choices and trying to match them to their respective equations. First, we can focus on the ones that say "4xbond energy of C-H" and "-4xbond energy." These choices refer to the energy that is either released or absorbed as four C-H bonds are formed or broken. This means that we should look for an equation that shows each of the four H-C bonds being formed or broken individually and match them based on whether energy is released or absorbed in the process. For the choice that says "enthalpy of combustion of CH4," we should look for a combustion equation. And for the choice that says "enthalpy of formation of CH4(g)," we should look for the equation that shows the formation of CH4 from its elements when they are in their standard states (such as O2 being the standard state for the element oxygen rather than just O by itself).

This is a great way to start on the problem. To add on, for the answer choices that say "4xbond energy of C-H" and "-4xbond energy of C-H", recall that the bond enthalpy method of determining ΔH is the bond enthalpies of all of the products - the bond enthalpies of all of the reactants. For a reaction such as CH4(g)⟶C(g)+4H(g), the bond enthalpies of the products are 0 so ΔH will be negative, explaining the negative sign before one of the answer choices. For the answer choice that says "enthalpy of combustion of CH4", recall that combustion reactions involve oxygen and often produce carbon dioxide and water.