Identify the equations that represent reactions that could occur. select all that apply. a) bacl2(aq) + na2so4(aq) ==> baso4(s) + 2nacl(aq) b) nabr(aq) + kcl(aq) ==> nacl(aq) + kcl(aq) c) nh4no3(aq) + agclo3(aq) ==> agno3(aq) + nh4clo3(aq) d) hch3coo(aq) + nahco3(aq) ==> nach3coo(aq) + co2(g) + h2o(l) e) 2naoh(aq) + cacl2(aq) ==> 2nacl(aq) + ca(oh)2(s)

A reaction is apparently said to occur if it proceeds via the formation of a precipitate or a gaseous product, or a visible color change is observed after the reaction.

A): This reaction occurs as there is a formation of white precipitate of barium sulfate.

B) NaBr(aq)+KCl(aq)-->NaCl(aq)+KCl(aq): This reaction does not occur because all the ions just remain as spectator ions in the solution as the products are aqueous too.

C) : This reaction does not occur because all the ions just remain as spectator ions in the solution as the products are aqueous too.

D):This reaction can be observed as this proceeds via the formation of gas bubbles of carbon dioxide.

E) :This reaction occurs as there is a formation of white precipitate of barium hydroxide.

So the correct answers are:

A)

D)

E)

your answer is a. the enthalpy of formation for a pure element in its standard state is always positive.