Over the years, the thermite reaction has been used for welding railroad rails,

Question

Over the years, the thermite reaction has been used for welding railroad rails, in incendiary bombs, and to ignite solid-fuel rocket motors. The balanced reaction is: Fe_2O_3(S) + 2Al(S) a 2 Fe(l) + Al_2O_3(S) A 6.26 g sample of aluminum was allowed to react with 21.5 g of Fe_2O_3. Which of the following statements are true? The excess reactant of the reaction is aluminum, and 0.134 mol of aluminum will be present at the end of the reaction. The theoretical yield of iron for the reaction would be 15.0 g. None of these statements are true. After the reaction is complete, 0.135 mol of Al_2O_3 is present. After the reaction is complete, 0.232 mol of iron will be present.

Explanation / Answer

Answer – We are given, mass of Al = 6.26 g , mass of Fe2O3 = 21.5 g

Reaction – Fe2O3(s) + 2 Al(s) ----> 2 Fe(l) + Al2O3(s)

Calculating the limiting reactant –

We need to calculate moles of each reactant first –

Mole of Al = 6.26 g / 26.982 g.mol-1

                   = 0.232 moles

Mole of Fe2O3 = 21.5g / 159.687 g.mol-1

                         = 0.135 moles

Now moles of Fe from the Al

From the balanced reaction –

2 moles of Al = 2 moles of Fe

So, 0.232 moles of Al = ?

= 0.232 moles of Fe

moles of Fe from the Fe2O3

From the balanced reaction –

1 moles of Fe2O3= 2 moles of Fe

So, 0.135 moles of Fe2O3= ?

= 0.269 moles of Fe

So lowest moles of Fe got from the Al, so limiting reactant is Al and Fe2O3 is excess.

Moles of Fe = 0.232 moles

Moles of excess Fe2O3 –

From the balanced reaction –

2 moles of Al = 1 moles of Fe2O3

So, 0.232 moles of Al = ?

= 0.116 moles of Fe2O3

So, Moles of excess Fe2O3 = 0.135 -0.116 = 0.0186 moles

Theoretical yield of the Fe = 0.232 moles * 55.845 g/mol

                                     = 12.95 g of Fe

Moles of Al2O3(s)

From the balanced reaction –

2 moles of Al = 1 moles of Al2O3(s)

So, 0.232 moles of Al = ?

= 0.116 moles of Al2O3(s)

So from the above all calculation the true statements are as follow –

After the reaction is completed 0.232 moles of iron will present.

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