Homework Help: Chemistry

Posted by Rick on Sunday, January 22, 2012 at 7:22pm.

1)Which thermochemical equation and data does not agree with the other two written?

a) 2NO (g)+ O2 (g) ->2 NO2 (g) deltaH=-169.8

b) NO (g) + 1/2 O2 (g) -> NO2 (g) delta H = -56.6

c) 4 NO2 (g) -> 4 NO (g) + 2 O2 (g) delta H = +226.4

d)all three equations are in thermochemical agreement.

I chose answer D because all seem correct.

2)Calculate the value of delta H for the reaction 2 CH4 (g) -> C2H6 (g) + H2 (g) given the following thermochemical equations:

C2H2 (g) + H2 (g) -> C2H4 delta H=-175.1

C2H6(g) -> C2H4 (g) + H2 (g) delta H=+136.4

C2H2 (g) + 3 H2 (g) -> 2 CH4 (g) delta H= -376.8

The answer choices are +65.3, +338.1, -415.5, +688.3

I get Delta H as +65.3. Below is how I did it. Please let me know if incorrect.

C2H4 + H2 --> C2H6 Delta H = -136.4 (flipped)
2CH4 --> C2H2 + 3H2 Delta H = +376.8 (flipped)
C2H2 + H2 --> C2H4 Delta H = -175.1
--------------------------------------------
2CH4 --> C2H6 + H2 Delta H = +65.3

Chemistry - DrBob222, Sunday, January 22, 2012 at 8:15pm
2 looks ok to me.
1 is not correct. a is the bad one.
Note that c is 4xb but a is not 2xb
Chemistry - DrBob222, Sunday, January 22, 2012 at 8:17pm
For the problem you worked for Hannah, just a note that it is not necessary to convert to kelvin. Tfinal0-Tinitial is the same in degrees C and degrees K since 1 degree C = 1 degree K.
Using kelvin is not wrong; it just takes longer.

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