Carbon disulfide (CS2) is a toxic, highly flammable...

Carbon disulfide $\left(\mathrm{CS}_{2}\right)$ is a toxic, highly flammable substance. The following thermodynamic data are available for $\mathrm{CS}_{2}(l)$ and $\mathrm{CS}_{2}(g)$ at $298 \mathrm{~K}$ : (a) Draw the Lewis structure of the molecule. What do you predict for the bond order of the $\mathrm{C}-\mathrm{S}$ bonds? (b) Use the VSEPR method to predict the structure of the $\mathrm{CS}_{2}$ molecule. (c) Liquid $\mathrm{CS}_{2}$ burns in $\mathrm{O}_{2}$ with a blue flame, forming $\mathrm{CO}_{2}(g)$ and $\mathrm{SO}_{2}(g)$. Write a balanced equation for this reaction. (d) Using the data in the preceding table and in Appendix $\mathrm{C},$ calculate $\Delta H^{\circ}$ and $\Delta G^{\circ}$ for the reaction in part $(c)$. Is the reaction exothermic? Is it spontaneous at $298 \mathrm{~K}$ ? (e) Use the data in the table to calculate $\Delta S^{\circ}$ at $298 \mathrm{~K}$ for the vaporization of $\mathrm{CS}_{2}(l)$ Is the sign of $\Delta S^{\circ}$ as you would expect for a vaporization? (f) Using data in the table and your answer to part (e), estimate the boiling point of $\mathrm{CS}_{2}(l)$. Do you predict that the substance will be a liquid or a gas at $298 \mathrm{~K}$ and $1 \mathrm{~atm} ?$

Carbon disulfide $\left(\mathrm{CS}_{2}\right)$ is a toxic, highly flammable substance. The following thermodynamic data are available for $\mathrm{CS}_{2}(l)$ and $\mathrm{CS}_{2}(g)$ at $298 \mathrm{~K}$ :
(a) Draw the Lewis structure of the molecule. What do you predict for the bond order of the $\mathrm{C}-\mathrm{S}$ bonds? (b) Use the VSEPR method to predict the structure of the $\mathrm{CS}_{2}$ molecule.
(c) Liquid $\mathrm{CS}_{2}$ burns in $\mathrm{O}_{2}$ with a blue flame, forming $\mathrm{CO}_{2}(g)$ and $\mathrm{SO}_{2}(g)$. Write a balanced equation for this reaction.
(d) Using the data in the preceding table and in Appendix $\mathrm{C},$ calculate $\Delta H^{\circ}$ and $\Delta G^{\circ}$ for the reaction in part $(c)$. Is the reaction exothermic? Is it spontaneous at $298 \mathrm{~K}$ ?
(e) Use the data in the table to calculate $\Delta S^{\circ}$ at $298 \mathrm{~K}$ for the vaporization of $\mathrm{CS}_{2}(l)$ Is the sign of $\Delta S^{\circ}$ as you would expect for a vaporization? (f) Using data in the table and your answer to part (e), estimate the boiling point of $\mathrm{CS}_{2}(l)$. Do you predict that the substance will be a liquid or a gas at $298 \mathrm{~K}$ and $1 \mathrm{~atm} ?$

Key Concepts

Lewis Structures

A Lewis structure is a diagrammatic representation of the valence electrons of atoms within a molecule. It shows how electrons are arranged among atoms and how they form bonds, providing insight into the molecule’s stability and reactivity.

Bond Order

Bond order refers to the number of chemical bonds between a pair of atoms, indicating the strength and stability of the bond. It can be predicted by examining the distribution of electrons in bonding and antibonding orbitals or by considering formal charges in a Lewis structure.

Molecular Geometry (VSEPR Theory)

VSEPR (Valence Shell Electron Pair Repulsion) theory is used to predict the three-dimensional shape of a molecule based on the repulsions between electron pairs around a central atom. This method helps determine the arrangement of atoms and the bond angles in a molecule.

Reaction Balancing

Balancing a chemical equation involves ensuring that the number of atoms for each element is equal on both the reactant and product sides of the reaction. This is fundamental to obeying the law of conservation of mass in chemical reactions.

Thermochemical Calculations (?H° and ?G°)

Thermochemical calculations involve determining the enthalpy change (?H°) and Gibbs free energy change (?G°) for a reaction using standard thermodynamic data. These quantities help assess whether a reaction is exothermic or endothermic and if it is spontaneous under given conditions.

Entropy and Phase Changes

Entropy (?S°) is a measure of the disorder or randomness in a system. In phase changes such as vaporization, the entropy generally increases as a substance transitions from a more ordered liquid phase to a less ordered gaseous phase.

Boiling Point Estimation

Boiling point estimation involves using thermodynamic relationships, particularly the condition where the Gibbs free energy change (?G) equals zero, to determine the temperature at which a liquid’s vapor pressure equals the external pressure. This method integrates the concepts of enthalpy and entropy changes associated with phase transitions.

Transcript

00:01 Alright, so here is the problem 108 in chapter 19, and this question are going to ask various questions about carbon disulfide.

00:11 Part a is asking us to draw the lewd structure of this molecule, cs2.

00:19 So the lewd structure of cs2 will look like this.

00:24 Carbon as the center atom and sulfur around carbon.

00:29 So carbon has two of four valence electrons and sulfur will have six.

00:35 So as a result, one, two, three, four, five, six, one, two, three, four, five, six.

00:43 So these two electrons bond together and this bonds together.

00:51 So there are two pairs of double bond between c and s.

00:56 So what do you predict the bond order of cs bond? in this case, the bond order of c and s will be part b.

01:12 Use the vs epr method to predict the structure of cs2 molecule.

01:17 So from the fluid structure above, we can see that the central atom c is bonding with two sulfurs, and there are no long pairs of electrons around the carbon.

01:33 So according to the vs epr mode, this is a linear structure.

01:48 Now we look at part c.

01:51 Liquid c .s2 burns in oxygen with a blue in fame, forming co2 gas and so2 gas.

01:59 Write the balance equation for this reaction.

02:03 So we have c .s2 liquid, burns in water, burns in oxygen, so it reacts.

02:12 With oxygen and result in co2 and so2.

02:23 So oxygen is a gas, co2 is a gas, so2 is a gas, co2 is a gas, so2 is a liquid.

02:31 So looking at the carbon, carbon, 2s, 2, so2, so there are two oxygens, 2 to 4, 6 oxygens in total, so there's a 3 before the oxygen.

02:45 So this will be the balanced equation for part c.

02:54 Now let's look at the question d.

02:58 Using the data in the table and in the appendix c, calculate delta h and delta g.

03:07 So after the calculation, we also need to determine the reaction is it endothermic or exothermic, and whether it's spontaneous or not.

03:18 So to calculate delta h values of the reaction, that will be the delta h formation of the products minus the delta h formation of the reactants.

03:33 So it will be equal to the delta h0 of carbon dioxide gas plus two times the delta h0 of so2 gas minus the delta h0 of cs2, liquid minus 3 times of delta h0 of oxygen gas.

03:59 So this number is equal to negative 393 .5 plus 2 times negative 296 .9 minus 896 .9.

04:19 So by doing the math, we will get a number delta h0 equal to negative 1077 kilojoules.

04:35 So then we calculate for delta g0 of the reaction.

04:40 That will be the delta g0 of the products minus the delta g zero of the reactants.

04:47 So the products are co2 gas, two times the delta g0 of so2 gas minus delta g0...

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