10. Sand is converted to pure silicon in a three step Si C14(g) + 2Mg(s) 2MgCl2(

Question

10. Sand is converted to pure silicon in a three step Si C14(g) + 2Mg(s) 2MgCl2(s) + Si(s) What is the enthalpy chang e when 25.0 mol of silicon tetrachloride is converted to elemental silicon? A. B. C. D. E. -25.0 kJ -7820 kJ -1.56 × 104 kJ -3.13 × 104 kJ none of these choices is correct CH5OH, is being promoted as a clean fuel and is used as an additive in many gasoline mixtures. Calculate the Han for the combustion of ethanol. AH r [C2HsOHD]--277.7 kJ/mol; AH r [CO-(2)]-393.5 kJ/mol; AHr [H,0(3)]-241.8 kJ/mol B. -751.1 kJ A. -1234.7 kJ C. -357.6 kJ D. 357.6 kJ E. 1234.7 kJ 12. a. State Hess's Law. for the reaction: b. Use the Ho data given below to calculate C2H4(g) + H2(g) C2H6(g) Data: C,H6(g) + 3.502(g) 2CO2(g) + 3H20(1) C2H4(g) + 302(g) 2CO2(g) + 2H20(1) 2H2(g) + O2(g) 2H2O(/) -1560 -1411 -572

Explanation / Answer

10. 1 mol sicl4 = -625.6 kj

enthalpy change(DH) = -625.6*25

                     = -15640 kj

answer: C.-1.56*10^4 kj

11.   2c2h5oh(l) + 7o2(g) ---> 4co2(g) + 6h2o (g)

2 mol c2h5oh

DH0rxn = (4*-393.5+6*-241.8)-(2*-277.7+7*0)

       = -2469.4 kj

per mol = -2469.4/2 = -1234.7 kj/mol

answer: A

12.Hess law : the energy change takes place in chemical process is same,whether it is takes place in one step or series of steps.

DH = (1560-1411+(-572/2))

   = -137 kj

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