[Boxxxed]

In anticipation to the building of a permanent colony on the moon, scientists are planning to provide a self-sustaining source of oxygen by extracting it out of lunar rocks (which are composed primarily of silica). Thus, both silicon (Si(s)) and oxygen gas (O2(g)) can be produced by the electrolysis of molten silica (SiO2(l)). (Note:  in the liquid state, SiO2 can be considered as a mixture of Si4+ and 2 O2- ions)  Hint:  O2 + 4e- → 2O2-


a.  Write both half-reactions involved in this process, identifying the oxidation and reduction half reactions.  Also, write the balanced equation describing the global reaction.

Is the overall reaction Si⁴⁺+2O^2- ----> Si + O₂?

Si⁴⁺ is gaining electrons so it's the cathode and O₂ is losing it so it's the anode.



b.  How many hours are required to convert 1.00 x 103 kg of SiO2(l) by passing a constant current of 5.00 x 104 A through the melt?


Why are they putting '1.00x103' and '5.00x104', why don't they use a single number? Is there something I am missing here?

[rabolisk]

You are correct for a. For b, I think they mean 1.00 x 10^3, and 10^4.

[Boxxxed]

I realized for b) it is scientific notation, the powerpoint didn't use superscripts

[Boxxxed]

For b) time=charge/current

Moles SiO2 = 1714 x 4 = 16856 moles e^-

Charge = Moles e^- x 96500

Charge = 6.61x10⁸

time = 6.61x10⁸/ 5x10⁴ = 13232 seconds

Is this right?

[Borek]

3666.7

3666.7 of what?

[Boxxxed]

c) How many litres of O2(g) will be obtained from this process (at 25 °C and 1 atm)?


Volume = 1714 moles O2 x 0.0821 x 298 / 1 atm = 41934 litres.

d) The O2(g) produced is compressed into a 100 L gas tank.  If the temperature on the moon reaches 107 °C during the day, what will be the pressure inside the gas tank (assume that ideal gas conditions apply here)?

P = nRT/v = 1714(R)(380)/100 = 534 atm

Please let me know if I did these correctly.

[Boxxxed]

3666.7

3666.7 of what?

Calculated molar mass wrong. Got it

[rabolisk]

Check your gas constant again.

[Boxxxed]

Added in the extra decimal place

[Borek]

Calculated molar mass wrong. Got it

Still wrong.

[Boxxxed]

Si = 28.09
O2 = 32

Adds up to 60.09 g/mol

103000g/60.09 = 1714 moles? What is wrong?

[rabolisk]

103000 grams is 1.00 x 10³ kilograms?

[Boxxxed]

thanks, getting things mixed up again.