Question

1. The integrated circuits in your cell phone and computer are made from the semiconductor silicon. The silicon is obtained from a really inexpensive starting material, sand, which is primarily silicon dioxide (SiO2). One step in the purification of silicon is to separate it from its solid impurities by forming the gas silicon tetrachloride (SiCl4). Given the following reactions, what is the overall enthalpy change in converting 1 mol of silicon dioxide into pure silicon?

SiO2(s) + 2C(s) → Si(impure, s) + 2CO(g); ΔH = +690 kJ

Si(impure, s) + 2Cl2(g) → SiCl4(g); ΔH= -657 kJ

SiCl4(g) + 2Mg(s) → 2MgCl2(s) + Si(s); ΔH = -625 kJ

2. What is ∆H°rxn for the following reaction under standard conditions?

2CH3OH(l) +3O2(g) → 2CO2(g) + 4H2O(l)

Given that ∆H°f CH3OH(l) = -239 kJ/mol, ∆H°f O2(g) = 0 kJ/mol, ∆H°f CO2(g) = -394 kJ/mol, and ∆H°f H2O(l) = -285.83 kJ/mol.

3. Ethanol (CH3CH2OH) has been suggested as an alternative fuel source. Ethanol’s enthalpy of combustion is ΔHcomb = -1368 kJ/mol and its density is 0.789 g/mL. What is the fuel density of ethanol (in kJ/mL)?

Answer 1

1) Answer:

Explanation:

From the equation 1 and 2 the impure Si can be cancelled. In the same way from equation 2 and 3 the SiCl4 can be cancelled.

The overall enthalpy change can be caluculated by the addition of the individual reaction enthalpies.

2) Answer:

2CH₃OH + 3O₂ → 2CO₂ + 4H₂O

∆H°_rxn = Products Enthalpies - Reactants Enthalpies

∆H°f CH₃OH = -239 kJ/mol, ∆H°f O₂ = 0 kJ/mol, ∆H°f CO₂ = -394 kJ/mol, and ∆H°f H₂O = -285.83 kJ/mol.

∆H°_rxn = [(2 x ∆H°f CO₂) + (4 x ∆H°f H₂O)] - [(2 x ∆H°f CH₃OH) + (3 x ∆H°f O₂)

∆H°_rxn = [(2 x -394) + (4 x -285.83)]  - [(2 x -239) + (3 x 0)]

∆H°_rxn = [-788 + (-1143.32)]  - [(-478) + 0]

∆H°_rxn = -1931.32 + 478

∆H°_rxn = -1453.32 KJ