4.2 Direction of a chemical reaction 4.2.1 The standard equilibrium constant expression *Equilibria involving gases H2 (g+12(g=2HI(g Ke- [p(H)/p9]2 p=100kpa p(H2)/p9]Lp(L2)/pe] Equilibria in aqueous solution: Sn2+(aq)+2Fe3+(aq)-Sn4+(aq)+2Fe2(aq) ko_[e(Sn)]Ic(Fe2+/ce) [c(Sn2ce )]c(Fe+ce)12 c=1 mol.I-1☆Equilibria in aqueous solution： H (g) I (g) 2HI(g) 2  2 ☆Equilibria involving gases ： Sn2+(aq)+2Fe3+(aq) Sn4+ (aq)+2Fe2+(aq) [ (H )/ ][ (I )/ ] [ (HI)/ ] 2 2 2 p p p p p p K  p  = 100kpa 2 3 2 4 2 2 [ (Sn )][ (Fe )] [ (Sn )][ (Fe )] c /c c /c c /c c /c     K  c  = 1 mol.l-1 4.2.1 The standard equilibrium constant expression §4.2 Direction of a chemical reaction