Why did copper come before iron?

Varanasi

1. The problem statement, all variables and given/known data
10) Both iron (III) oxide and copper (IV) oxide are mined for use as primary ores in the production of pure iron and copper respectively. Historically, the earliest use of copper tools appears to be around 4500 BC. Iron tools appear much later; European artifacts have been dated to around 1200BC. Assuming that both metal ores were easily found, why do you think that iron tools were developed more recently than copper tools?

a) CuO2 --> Cu + O2 ΔGº = -129 kJ/mole
b) Fe2O3 --> Fe + 3/2 O2 ΔGº =-742 kJ/mole

2. Its a thinking question.

3. I'm unsure exactly where to go with this. I tried doing dG = -rtln(k) for each and trying to find some justification that way. I'm completely lost with these question. It has something to do with the fact that dG is higher for iron but that almost sounds like more of a reason for it to be earlier than copper rather than the other way around.
Something about the larger dG value has to explain it but I'm completely lost. I even tried dG = h - tdS because dS will be larger for iron because it makes 1.5 o2 but that means it needs less heat which gets me even farther from an answer...

CompuChip

Are you sure you didn't get it backwards with the minus signs?
If I look at this page, for most substances the [itex]\Delta G^0[/itex] is negative for the formation of compound substances from their components. So I would expect there to be a positive difference in energy for the decomposition of such an element. In other words, if you put copper and oxygen together, they will prefer combining to CuO₂ and you'll actually have to do work to take them apart, rather than the other way around.

HallsofIvy

Simply put, the fact that iron ore requires more energy to smelt means that you need a hotter fire. The smelting of iron ore first required the development of charcoal and the bellows.

Varanasi

How do you deduct from the dG values that iron ore requires more energy to smelt?
I tried to come to that conclusion but am unsure how...

Mapes

I agree with CompuChip that the equations or the [itex]\Delta G[/itex] values must be flipped. The oxide is a lower energy state at standard temperature and pressure. Once this is worked out, the [itex]\Delta G=\Delta H-T\Delta S[/itex] approach (which is the key to identifying smelting temperatures) will work out.

chemisttree

This is an interesting theory but I'm not convinced that it is true. Bloom iron was first produced in copper smelters so the temperature of the fire isn't key. The Gibbs energy only gives you the amount of energy required which isn't the same thing as a temperature requirement.

Given the temperature of copper smelters is high enough to produce the iron, the higher energy requirement might better translate to a time requirement. Copper is produced faster in the smelt than iron perhaps.

gneill

Persée : Some Experiments in Prehistoric Copper Smelting