Enthalpy of Reaction: Using standard enthalpies of formation (AHY) from Table A4.3 in your textbook or below, calculate AH° for the following reactions.
2 NH3(g) + 3 O2(g) + 2 CH4(g) → 2 HCN(g) + 6 H2O(g) (1pt)
Ca3(PO4)2(s) + 3 H2SO4(aq) + 3 CaSO4(s) + 2 H3PO4(aq) (1 pt)
2 NO2(g) → N204(g) (1 pt)
2 Al(s) + Fe2O3(s) → Al2O3(s) + 2 Fe(s) (1 pt)
Substance
Ca3(PO4)2(s)
H3PO4(aq)
AH (kJ/mol)
-4132
-1277

i believe you are right for all except the 3rd ( look at sqdancefan his answer is much better)

in 1783 cavendish was working with his hydrogen gas and found that when he burned it and condensed the vapor it was water. this was very significant since it disproved the greek theory of elements completely since water was not a simple substance but the product of two gases. it was lavoisier who hearing of the experiment gave cavendish's gas the name hydrogen ("water-producer") and added that hydrogen burned by combining with oxygen and that water was a hydrogen-oxygen combination.

explanation: