A Isomers are molecules with the same elemental composition...

A Isomers are molecules with the same elemental composition but a different atomic arrangement. Three isomers with the formula $\mathrm{C}_{4} \mathrm{H}_{8}$ are shown in the models below. The enthalpy of combustion $\left(\Delta_{c} H^{\circ}\right)$ of each isomer, determined using a calorimeter, is as follows: Compound $\quad \Delta_{c} H^{\circ}(k J / \text { mol butene })$ $\begin{array}{ll}\text { cise-2.butene } & -2709.8 \\ \text { trans 2.butene } & -2706.6 \\ \text { 1-butene } & -2716.8\end{array}$ (a) Draw an energy level diagram relating the energy content of the three isomers to the energy content of the combustion products, $\mathrm{CO}_{2}(\mathrm{g})$ and $\mathrm{H}_{2} \mathrm{O}(\ell)$ (b) Use the $\Delta_{c} H^{\circ}$ data in part (a), along with the enthalpies of formation of $\mathrm{CO}_{2}(\mathrm{g})$ and $\mathrm{H}_{2} \mathrm{O}(\ell)$ from Appendix $\mathrm{L},$ to calculate the enthalpy of formation for each of the isomers. (c) Draw an energy level diagram that relates the enthalpies of formation of the three isomers to the energy of the elements in their standard states. (d) What is the enthalpy change for the conversion of cis-2-butene to trans-2-butene? (FIGURE CANNOT COPY)

A Isomers are molecules with the same elemental composition but a different atomic arrangement. Three isomers with the formula $\mathrm{C}_{4} \mathrm{H}_{8}$ are shown in the models below. The enthalpy of combustion $\left(\Delta_{c} H^{\circ}\right)$ of each isomer, determined using a calorimeter, is as follows:
Compound $\quad \Delta_{c} H^{\circ}(k J / \text { mol butene })$ $\begin{array}{ll}\text { cise-2.butene } & -2709.8 \\ \text { trans 2.butene } & -2706.6 \\ \text { 1-butene } & -2716.8\end{array}$
(a) Draw an energy level diagram relating the energy content of the three isomers to the energy content of the combustion products, $\mathrm{CO}_{2}(\mathrm{g})$ and $\mathrm{H}_{2} \mathrm{O}(\ell)$
(b) Use the $\Delta_{c} H^{\circ}$ data in part (a), along with the enthalpies of formation of $\mathrm{CO}_{2}(\mathrm{g})$ and $\mathrm{H}_{2} \mathrm{O}(\ell)$ from Appendix $\mathrm{L},$ to calculate the enthalpy of formation for each of the isomers.
(c) Draw an energy level diagram that relates the enthalpies of formation of the three isomers to the energy of the elements in their standard states.
(d) What is the enthalpy change for the conversion of cis-2-butene to trans-2-butene?
(FIGURE CANNOT COPY)

Key Concepts

Isomerism

Isomerism refers to the phenomenon where compounds have the same molecular formula but different arrangements of atoms in space. This difference in structure can lead to variations in physical and chemical properties, as seen in the different stabilities and reactivities of the isomers.

Enthalpy of Combustion

The enthalpy of combustion is the amount of heat released when a substance undergoes complete combustion in oxygen, producing carbon dioxide and water. It serves as an indicator of the relative energy content of different compounds; lower energy content leads to less heat released during combustion.

Standard Enthalpy of Formation

Standard enthalpy of formation is the change in enthalpy when one mole of a compound is formed from its elements in their most stable, standard states. It provides a measure for the intrinsic stability of compounds relative to their constituent elements.

Energy Level Diagrams

Energy level diagrams are graphical representations that show the relative energies of reactants, products, and intermediates. These diagrams help visualize the energy changes in chemical processes, including combustion and formation, and are useful for comparing the stability of different isomers.

Thermochemical Calculations

Thermochemical calculations involve using known enthalpy changes, such as enthalpies of combustion and formation, to indirectly determine unknown energy values through algebraic relationships like Hess's Law. This approach helps in comparing the relative stabilities and energetics of different chemical species.

Transcript

00:02 Okay, let's consider isomers which are molecules with the same elemental composition but a different atomic arrangement.

00:09 We've got three isomers are shown here.

00:17 Um, okay, my isomers are cis 2 -butene and i'm just going to draw a simplified version.

00:38 This doesn't look right.

00:59 Our trans 2 -butene, which will be very similar but, and our 1 -butene, which will be c.

01:52 Okay, so now what are we doing? we are going to first, make this a little smaller, draw an energy level diagram relating energy content of the three isomers to energy content of combustion products, co2 and h2o.

03:03 And they want me to use l.

03:07 Okay, let's see if we can figure out how to do that.

03:34 So we have our 1 -butene was the highest, then our cis -butene, and then our trans 2 -butene was next.

04:04 So this will be 1 -butene plus 6 -o2, and our delta h for this one is negative 2716 .8 kilojoules per mole.

04:30 Here we have cis -butene plus 6 -o2g, and our delta h is 2709 .8 kilojoules per mole.

04:56 And finally, my trans -butene plus 6 -o2, and our delta h for this one is negative 2706 .6 kilojoules per mole.

05:16 And these will all go down to 4 -o2, whoops, 4 -co2 plus 4 -h2o -l.

05:36 So i think that's all they want there.

05:38 I'm not doing anything to scale.

05:41 Now let's see what they want next.

05:43 So this will next be used to calculate the enthalpy of formation for each isomer.

06:44 Okay, this will be pretty easy because a lot of them are the same here.

06:52 Okay, so delta h of formation for co2g is negative 393 .509, and i'll take that times 6, and that will equal negative 393 .509 times 6.

07:21 This is negative 2361 .054.

07:26 Oh, by 4.

07:28 I'm sorry, by 4.

07:38 Negative 393 .509 times 4.

07:47 This is negative 1574 .36 kilojoules...

Please give Ace some feedback