Question

H2PO4- ⇄ OH- + HPO4-

OH- ions are added to this system. What happens?

I know that the OH- will join with the H+ to form water and that the equilibrium will shift right. BUT why will it shift right?

Answer 1

a system at equilibrium is like a delicately balanced see-saw. a stress to one side will make the system shift to re-establish a balance and restore equilibrium. by addin OH- ions to the product side, you've made the products "too heavy" so the system has to shift away from that stress

Answer 2

so then shouldn't the backward reaction be favoured?

Answer 3

sorry, i wrote the question wrong

Answer 4

it will. we will get a decrease in the HPO4- concentration, and an increase in the H3PO4 concentration

Answer 5

the products are H+ and HPO4-

Answer 6

ok, then the situatino changes silghtly, but not much. You've got H+ and HPO4- ions as products. When you add OH- ions, which product are they likely to react with?

Answer 7

with H+ to produce water

Answer 8

so what does that do to the H+ concentration?

Answer 9

it increases the H+ conc

Answer 10

it's goign to DECREASE the H+ concentration, because the H+ ions will be used up by the addition of OH- ions

Answer 11

ohhhhh

Answer 12

so that means the stress isn't an addition on one side, it's a REMOVAL of product. so that stresses the system to move towards what side?

Answer 13

the equilibrium will shift right as there will be a greater conc of the product. by shifting right, a greater amount of products will be formed, restoring equilibrium

Answer 14

am i correct?

Answer 15

you're mostly right

Answer 16

reactants**

Answer 17

that's better

Answer 18

reactant***

Answer 19

haha, thanks

Answer 20

yw