Which of the following gives the correct equation for ∆H°f of C20H23PO2(s)? (a) C(s, gr) + H(g) + P(s) + O(g) --> C20H23PO2(s) (b) C(s, gr) + H2(g) + P(s) + O2(g) --> C20H23PO2(s) (c) 20 C(s, gr) + 23 H(g) + P(s) + 2 O(g) --> C20H23PO2(s)
(d) 40 C(s, gr) + 23 H2(g) + P(s) + 2 O2(g) --> 2 C20H23PO2(s)
(e) 20 C(s, gr) + 23/2 H2(g) + P(s) + O2(g) --> C20H23PO2(s)
Solution: Rules for writing equation for the ∆H°f (heat of formation).
Put 1 mole of the substance on the product side. This eliminates D since it has 2 in front of C20H23PO2.
On the left side, put elements that compound is made from in their natural state. Make sure to put diatomic elements with a 2 next to it. These elements are: Br2, I2, N2, Cl2, H2, O2, F2.
The equation must be balanced.
Choice E is the correct answer.