Hey there! Sign in to join this conversationNew here? Join for free

Why are ionic compounds (i.e. sodium chloride) soluble in water? please Watch

    • Thread Starter
    Offline

    0
    ReputationRep:
    Why are ionic compounds (i.e. sodium chloride) soluble in water?

    Could someone please explain this to me. cheers
    Offline

    0
    ReputationRep:
    Forgive me if I'm wrong I'm doing my gcse's however I believe, ionic substances are hydrohpilic. Because the oxygen in water is more electronegative it attracts the shared electrons more , making it electronegative, conversely making the hydrogens electroposive. Because ionic substance are charged when the ionic substance is put into water the there is an electrostatic attraction between the polar water molecule and ionic substance. I could be very wrong
    Offline

    2
    ReputationRep:
    water molecules will coordinate to certain ions, to a greater or lesser extent, giving what are called solvation spheres:


    The stability of these aqueous ions dictates whether an ionic species will dissolve. Group 1 ions, Halides, Nitrates and Sulphates all dissolve very well in water, with a few exceptions. This is driven by the massive entrophy increase between a ionic latice and a dissolved solution, as long as this entrophy increase is large enough and the enthalpy difference small enough (it is a positive enthalpy change, you can tell as when you dissolve a salt the temperature drops) then the gibbs free energy is negative, and thus the reaction is spontaneous.
    • Community Assistant
    • Study Helper
    Offline

    14
    ReputationRep:
    (Original post by TheWonderKid)
    Why are ionic compounds (i.e. sodium chloride) soluble in water?

    Could someone please explain this to me. cheers
    First of all it is important to note that not all ionic compounds are soluble in water. For example, all oxides are insoluble unless they actually react with water. All carbonates except groups 1 are insoluble etc. There are many insoluble ionic compounds.

    What makes soluble ionic compounds dissolve is the interaction of their ions with the water molecules. Water molecules are polar and can surround the charged ions as GB85 has shown above.

    However, the process of dissolution depends on three factors:

    1. The lattice enthalpy of the ionic solid (always endothermic as you break the lattice)
    2. The hydration enthalpy of the ions (always exothermic as you form bonds with water)
    3. The overall entropy change

    From these you can calculate Gibbs Freen energy change for the process and if the Gibbs free energy change is negative then the solid dissolves. If you haven't covered GFE then dont worry.

    NOTE: Dissolution of ionic compounds is not always endothermic, in fact in several cases it is highly exothermic (eg NaOH)

    The conclusion is that it is not easy to predict solubility and you should learn some simple rules:

    Group 1 compounds are always soluble
    Ammonium compounds are always soluble
    Nitrates are always soluble
    Ethanoates are always soluble
    Sulphates Ba, Ca, Pb, Ag insoluble
    Chlorides Pb, Ag insoluble
    Carbonates all insoluble (except group 1 and ammonium)
    Hydroxides all insoluble (except group 1, Ba, Ca (slightly, Mg, sparingly)
    Oxides all insoluble (except group 1, 2, which react with water)

    This is not all encompassing, but good guidelines.
    Offline

    2
    ReputationRep:
    (Original post by charco)
    However, the process of dissolution depends on three factors:
    1. The lattice enthalpy of the ionic solid (always endothermic as you break the lattice)
    2. The hydration enthalpy of the ions (always exothermic as you form bonds with water)
    3. The overall entropy change
    I pushed point 1 & 2 together for simplicity's sake, the combined magnitude of each determines the overall enthalpy change (which to my knowledge is always at least somewhat endothermic), which is the value you punch into dG=dH-TdS.

    Cliffs notes on Gibbs Free energy, it combines the enthalpy (energy change) with entrophy (change in disorder) to tell you if the reaction will occur spontaneously or not. If dG=dH-TdS gives a negative value, the reaction is spontaneous. Thus a large increase in entrophy can drive a reaction even if it is endothermic.

    This handily also explains why salts are more soluble as you increase the temperature. And gases for that matter.
    • Thread Starter
    Offline

    0
    ReputationRep:
    Thanks guys!!
 
 
 
  • See more of what you like on The Student Room

    You can personalise what you see on TSR. Tell us a little about yourself to get started.

  • Poll
    What newspaper do you read/prefer?
  • See more of what you like on The Student Room

    You can personalise what you see on TSR. Tell us a little about yourself to get started.

  • The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

    Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE

    Quick reply
    Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.