Consider a mixture of gases in a cylinder with a piston. The original volume of

Question

Consider a mixture of gases in a cylinder with a piston. The original volume of the cylinder is 500. cm3. If the combustion of this mixture releases 650. J of energy, to what volume will the gases expand against a constant pressure of 750. torr if all the energy of combustion is converted into work to push back the piston? Sand is converted to pure silicon in a three step process. The third step is SiCl_4(g) + 2Mg(s) rightarrow 2MgCl_4(s) + Si(s) DeltaH = -625.6 kJ What is the enthalpy change when 25.0 kg of silicon tetrachloride is converted to elemental silicon? Galena (PbS) is the ore from which elemental lead is extracted. In the first sten of the extraction process, galena is heated in air to form lead (II) oxide 2PbS(s) + 3O_2(g). 2PbO(s) + 2SO_2(g) DeltaH = -827.4 kJ What mass of galena is converted to lead (II) oxide if 1975 kJ of heat are liberated?

Explanation / Answer

1)

we know that

work done is given by

W = PdV

given

pressure (P) = 750 torr

P = 750 / 760 atm

P = 750 x 101325 / 760 Pa

P = 99991.776 Pa

and

so

W= P x dV

650 = 99991.776 x dV

dV = 6.5 x 10-3 m3

we know that

1 m3 = 10^6 cm3

so

dV = 6.5 x 10-3 x 10^6

dV = 6500 cm3

so

Vf = 500 + 6500

Vf = 7000

so

the gases will expant to a volume of 7000 cm3

2)

we know that

moles = mass / molar mass

also

molar mass of SiCl4 = 170 g/mol

so

moles of SiCl4 = 25000 / 170 = 147.059

now

consider the given reaction

SiCl4 + 2 Mg ---> 2 MgCl2 + Si dH = -625.6 kJ

so

1 mole of SiCl4 ---> -625.6

147.059 mole of SiCl4 ---> y

y = -625.6 x 147.059

y = -92000 kJ

so

-92000 kJ of enthalpy change

3)

we can see that

2 mole of PbS ---> -827.4

y moles of PbS --> -1975

y = -1975 x 2 / -827.4

y = 4.774

so

4.774 moles of PbS is converted

now

mass = moles x molar mass

also

molar mass of PbS = 239.30 g/mol

so

mass = 4.774 x 239.30

mass = 1142.4 g

so

1142.4 grams of galena is converted

Related Questions

Navigate

• Browse (All)
• Browse C
• Subjects

---

---

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.