Question

Acetylene torches are used for welding, but the thermite reaction can be useful in situations in which acetylene torches are inappropriate, such as welding underwater. The thermite reaction is described by the equation below: A welder has 1.873 × 102 g Fe2O3 and 94.51 g Al in his welding kit. Which reactant will he run out of first?

Answer 1

limiting reagent is given by

• calculated or given no. of moles
• divided by
• stocheomatric or no. of moles from equation.

Answer 2

The Thermite Reaction is

`2Al(s)+Fe_2 O_3 (s)>2Fe(l)+Al_2 O_3 (s)`

The conversions are

mass
`Fe_2 O_3` to moles
`Fe_2 O_3`to moles Fe to mass Fe

`$1.873 * 10^(2) g \mathrm{Fe}_(2) \mathrm{O}_(3) * \frac{1 \mathrm{mol} \mathrm{Fe}_(2) \mathrm{O}_(3)}{159.7 \mathrm{g} \mathrm{Fe}_(2) \mathrm{O}_(3)} * \frac{2 \mathrm{molFe}}{1 \mathrm{mol} \mathrm{Fe}_(2) \mathrm{O}_(3)} * \frac{55.85 \mathrm{g} \mathrm{Fe}}{1 \mathrm{mol} \mathrm{Fe}}$`

=131 g Fe is obtained

mass Al to moles Al to moles Fe to mass Fe

`$94.51 g A l * (1 m o l A l)/(26.98 g A l) * (2 m o l F e)/(2 m o l A l) * (55.85 g F e)/(1 m o l f e)$`

=196 g Fe is obtained

The reactant which runs out first is called as Limiting reactant.

Less mass of the Product is obtained from the limiting reactant.

So we see here
`Fe_2 O_3` produces less product (131g Fe) than the product produced by Al (196 g Fe)

Hence
`Fe_2 O_3` (Answer) is the Limiting reactant and it runs out first