NO_2 gas is formed in automotive engines as a byproduct and contributes to atmos

Question

NO_2 gas is formed in automotive engines as a byproduct and contributes to atmospheric pollution. Given the following equations: N_2 + O_2 rightarrow 2NO delta H = +180.8 kJ NO + 1/2 O_2 rightarrow NO_2 delta H = -56.0 kJ Calculate the heat of formation of NO_2 gas. 34 kJ 68 kJ 124.8 kJ 236.8 kJ Which of the following equations represents a reaction for which the delta S is positive? 3O_2(g) rightarrow 2O_3(g) 2H_2(g) + O_2(g) rightarrow 2H_2O(l) CaCO_3(s) rightarrow CaO(s) + CO_2(g) l_2(g) + 2K(s) rightarrow 2Kl(s)

Explanation / Answer

Question 8.

Apply Hess Law

If NO2 is required

then

from equation (1), divide by 2 so we get NO alone

1/2N2 + 1/2O2 = NO H = 180.8 = 90.4 kJ

NO + 1/2 O2 = NO2 H = -56 kJ

add all

NO + 1/2 O2 + 1/2N2 + 1/2O2 = NO + NO2     H = 90.4- 56 = 34. 4kJ

cancel common terms

1/2N2 + + O2 = NO2     H = 34.4 kJ

note that this is the heat formation equation of NO2, therefore

choose A

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