Silicon forms a series of compounds that is analogous to alkanes and has the general formula $\mathrm{Si}_{\mathrm{n}} \mathrm{H}_{2 \mathrm{n}+2}$. The first of these compounds is silane, $\mathrm{SiH}_4$, which is used in the electronics industry to produce thin ultrapure silicon films. $\mathrm{SiH}_4(\mathrm{~g})$ is somewhat difficult to work with because it is pyrophoric at room temperature-meaning that it bursts into flame spontaneously.

(a) Write an equation for the combustion of $\mathrm{SiH}_4(\mathrm{~g})$. (The reaction is analogous to hydrocarbon combustion, and $\mathrm{SiO}_2$ is a solid under standard conditions. Assume the water produced will be a gas.)

(b) Use the data from Appendix $\mathrm{E}$ to calculate $\Delta S^{\circ}$ for this reaction.

(c) Calculate $\Delta G^{\circ}$ and show that the reaction is spontaneous at $25^{\circ} \mathrm{C}$.

(d) Compare $\Delta G^{\circ}$ for this reaction to the combustion of methane. Are the reactions in these two exercises enthalpy or entropy driven? Explain.

Distinguish between a reversible and an irreversible process.

Why is the free energy change of a system equal to the maximum work rather than just the work?

The reaction

$$\mathrm{CO}_2(\mathrm{~g})+\mathrm{H}_2(\mathrm{~g}) \rightarrow \mathrm{CO}(\mathrm{g})+\mathrm{H}_2 \mathrm{O}(\mathrm{g})$$

is not spontaneous at room temperature but becomes spontaneous at a much higher temperature. What can you conclude from this about the signs of $\Delta H^{\circ}$ and $\Delta S^{\circ}$, assuming that the enthalpy and entropy changes are not greatly affected by the temperature change? Explain your reasoning.

Discuss the effect of temperature change on the spontaneity of the following reactions at $1 \mathrm{~atm}$. (a) $\mathrm{Al}_2 \mathrm{O}_3(\mathrm{~s})+2 \mathrm{Fe}(\mathrm{s}) \rightarrow 2 \mathrm{Al}(\mathrm{s})+\mathrm{Fe}_2 \mathrm{O}_3(\mathrm{~s}) \Delta H^{\circ}=+851.5 \mathrm{~kJ} ; \quad \Delta S^{\circ}=+38.5 \mathrm{~J} / \mathrm{K}$

(b)

$$\begin{aligned} \mathrm{N}_2 \mathrm{H}_4(\ell) \rightarrow & \mathrm{N}_2(\mathrm{~g})+2 \mathrm{H}_2(\mathrm{~g}) \\ & \Delta H^{\circ}=-50.6 \mathrm{~kJ} ; \quad \Delta S^{\circ}=0.3315 \mathrm{~kJ} / \mathrm{K}\end{aligned}$$

(c) $\mathrm{SO}_3(\mathrm{~g}) \rightarrow \mathrm{SO}_2(\mathrm{~g})+\frac{1}{2} \mathrm{O}_2(\mathrm{~g}) \Delta H^{\circ}=98.9 \mathrm{~kJ} ; \quad \Delta S^{\circ}=+0.0939 \mathrm{~kJ} / \mathrm{K}$

Use the web to learn how many pounds of plastics are recycled in your area each year. How has this value changed during the past decade?

Calculate $\Delta G^{\circ}$ at 45 degree C for reactions for which

(a) $\Delta H^{\circ}=293 \mathrm{~kJ} ; \Delta S^{\circ}=-695 \mathrm{~J} / \mathrm{K}$

(b) $\Delta H^{\circ}=-1137 \mathrm{~kJ} ; \Delta S^{\circ}=0.496 \mathrm{~kJ} / \mathrm{K}$

(c) $\Delta H^{\circ}=-86.6 \mathrm{~kJ} ; \Delta S^{\circ}=-382 \mathrm{~J} / \mathrm{K}$

There is another free energy state function, the Helmholtz free energy (F), defined as F = E - TS. Comparing this to the definition of G, we see that internal energy has replaced enthalpy in the definition. Under what conditions would this free energy tell us whether or not a process is spontaneous?

Under what conditions does $\Delta G$ allow us to predict whether a process is spontaneous?

Describe why it is easier to use $\Delta G$ to determine the spontaneity of a process rather than $\Delta S_{\mathrm{u}}$.

A beaker of water at 40 degree C and a beaker of ice water at 0 degree C are placed side by side in an insulated container. After some time has passed, the temperature of the water in the beaker on the left is 30 degree C and the temperature of the ice water is still 0 degree C. Describe what is happening in each beaker

(a) on the molecular level and

(b) in terms of the second law of thermodynamics.

Look up the value of the standard entropy for the following molecules: $\mathrm{SiO}_2(\mathrm{~s}), \mathrm{NH}_3(\mathrm{~g}), \mathrm{C}_2 \mathrm{H}_6(\mathrm{~g})$. Rank the compounds in order of increasing entropy and then explain why this ranking makes sense.

Look up the value of the standard entropy for the following molecules: $\mathrm{CH}_4(\mathrm{~g}), \mathrm{C}_2 \mathrm{H}_5 \mathrm{OH}(\ell), \mathrm{H}_2 \mathrm{C}_2 \mathrm{O}_4(\mathrm{~s})$. Rank the compounds in order of increasing entropy and then explain why this ranking makes sense.

Find websites describing two different attempts to reach the coldest temperature on record. What features do these experiments have in common?