Question

(5). The following reaction is the thermite reaction used in underwater (and space) welding: 8A1, +Fe, A1,0 + 2Fe + 202.8 kcal/mol How many kilocalories of heat are generated when 30.0 g of Al are reacted completely in the above thermite reaction? ii) If you need 1.00x10 kcal of heat, how many grams of Al and Fe:Os are needed?

Answer 1

i)

Molar mass of Al = 26.98 g/mol

mass(Al)= 30.0 g

use:

number of mol of Al,

n = mass of Al/molar mass of Al

=(30 g)/(26.98 g/mol)

= 1.112 mol

Since Δ H is negative, heat is released

when 8 mol of Al reacts, heat released = 202.8 kcal

So,

for 1.112 mol of Al, heat released = 1.112*202.8/8 kcal

= 28.19 kcal

Answer: 28.19 kcal

ii)

when -202.8 kcal of heat is involved, 8 mol of Al is reacting

So,

for -1*10^3 kcal, mol of Al = -1*10^3*8/-202.8 kcal

= 39.45 mol

Molar mass of Al = 26.98 g/mol

use:

mass of Al,

m = number of mol * molar mass

= 39.45 mol * 26.98 g/mol

= 1.064*10^3 g

Answer: 1.064*10^3 g