Question

Sand is converted to pure silicon in a three step process. the third step is sicl4(g) + 2mg(s) → 2mgcl2(s) + si(s) δh = –625.6 kj what is the enthalpy change when 25.0 mol of silicon tetrachloride is converted to elemental silicon? multiple choice –7820 kj –25.0 kj –3.13 × 104 kj none of these choices are correct. –1.56 × 104 kj

Answer 1

The reaction is that,
1 mole is SiCL4 will produce 1 mole of Si(s) Since there is 1 to 1 relationship then we just multiply the enthalpy of reaction by the number of moles
Enthalpy = -625.5 kJ/mole × 12.5 Moles 
= -7,818.75 KJ
= -0.78 ×10⁴KJ

Answer 2

Answer:

The correct answer is -1.56 × 10⁴ KJ

Explanation:

SiCl₄ (g) + 2Mg (s) → 2MgCl₂ (s) + Si (s), the enthalpy change of the step is -625.6 KJ

From the mentioned reaction, one can see that 1 mole of SiCl₄ will generate 1 mole of Si, as there is a one to one relationship, thus, one can just multiply the enthalpy of the reaction with the number of moles. Therefore,  

Change in enthalpy = -625.6 KJ × 25 moles = -15640 KJ

Thus, the correct answer is -1.564 × 10⁴ KJ