Question

Phosphorous and chlorine react to form phosphorous pentachloride according to the following reaction:

P4 (g) + 10 Cl2 (g) ---> 4 PCl5 (s) and delta H of the reaction = -1835 kJ

If 20.0 g of P4 are allowed to react with 20.0 g of Cl2, how much heat would be produced?

Answer 1

Molar mass of P4 = 123.88 g/mol

mass of P4 = 20.0 g

we have below equation to be used:

number of mol of P4,

n = mass of P4/molar mass of P4

=(20.0 g)/(123.88 g/mol)

= 0.1614 mol

Molar mass of Cl2 = 70.9 g/mol

mass of Cl2 = 20.0 g

we have below equation to be used:

number of mol of Cl2,

n = mass of Cl2/molar mass of Cl2

=(20.0 g)/(70.9 g/mol)

= 0.2821 mol

1 mol of P4 reacts with 10 mol of Cl2

for 0.1614 mol of P4, 1.614 mol of Cl2 is required

But we have 0.2821 mol of Cl2

so, Cl2 is limiting reagent

we will use Cl2 in further calculation

Since delta H is negative, heat is released

when 10 mol of Cl2 reacts, heat released = 1835.0 KJ

So,

for 0.2821 mol of Cl2, heat released = 0.2821*1835.0/10 KJ

= 51.76 KJ

Answer: 51.8 KJ