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    Can someone please explain how to write ionic equations of halogen displacement reactions? Why does the Cl2, Br2 and I2 not form 2Cl-, 2Br- and 2I- ions? Thanks
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    In terms of reactivity, the order is chlorine, bromine, then iodine. Fluorine would be more reactive than chlorine as it has a higher electronegativity, actually the most reactive element. Astatine is very rarely found occurring, but would be the least reactive according to the trend. A more reactive halogen will displace the less reactive halogen.

    e.g. Cl2 + 2KBr --> 2KCl + Br2

    this would be expressed ionically as:

    Cl2 (aq) + 2Br^- --> 2Cl^- + Br2

    Potassium (K^+) is omitted as it is the spectator ion, i.e. it doesn't take place in the reaction.

    Similarly, chlorine would also react with KI to displace the I^- ion and bromine would displace the I^- ion also, as chlorine and bromine are both more reactive than iodine, so the chloride and bromide ions are both capable of displacing the iodide ions.
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    (Original post by jsg9)
    In terms of reactivity, the order is chlorine, bromine, then iodine. Fluorine would be more reactive than chlorine as it has a higher electronegativity, actually the most reactive element. Astatine is very rarely found occurring, but would be the least reactive according to the trend. A more reactive halogen will displace the less reactive halogen.

    e.g. Cl2 + 2KBr --> 2KCl + Br2

    this would be expressed ionically as:

    Cl2 (aq) + 2Br^- --> 2Cl^- + Br2

    Potassium (K^+) is omitted as it is the spectator ion, i.e. it doesn't take place in the reaction.

    Similarly, chlorine would also react with KI to displace the I^- ion and bromine would displace the I^- ion also, as chlorine and bromine are both more reactive than iodine, so the chloride and bromide ions are both capable of displacing the iodide ions.
    Hi!
    Thanks for this.
    But how come the Cl2 doesnt cancel out because you start with 2Cl- and eng with 2Cl-, as with bromine too
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    (Original post by Laraib Chaudhry)
    Hi!
    Thanks for this.
    But how come the Cl2 doesnt cancel out because you start with 2Cl- and eng with 2Cl-, as with bromine too
    Okay, so take the equation:

    Cl2 + 2KBr --> 2KCl + Br2

    ionic: Cl2 + 2Br^- --> 2Cl^- + Br2

    On the left hand side, you have molecular chlorine (Cl2), existing diatomically. You also have 2KBr, composed of 2K^ + ions and 2Br^- ions.

    On the right hand side, you have 2KCl, composed of 2K^ + ions and 2Cl^- ions and molecular bromine (Br2) existing diatomically.

    You can see that from left to right, the Cl2 is reduced to 2Cl^-, each Cl^- having gained an electron, and the 2Br^- in KBr have been oxidised to Br2 (each having lost an electron). From this we can see both Cl2 and Br^- are involved in the reaction, they are not spectator ions because they change from molecular to ions, or ions to molecular from left to right.

    K^ + however, is a spectator ion, because both KBr and KCl are formed of a K^ + ion, and either a Cl^- or Br^- ion. In both of these ionic compounds, the K^ + remains the same, i.e. it is neither oxidised or reduced (and so no electron transfer) and so does not take place in the reaction. Therefore, it can be omitted (cancels out).

    Hope that helps.

    EDIT: every K^ + ion should have a 1 plus charge, and the plus should show in the equations, for some reason this is being omitted.
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    (Original post by jsg9)
    Okay, so take the equation:

    Cl2 + 2KBr --> 2KCl + Br2

    ionic: Cl2 + 2Br^- --> 2Cl^- + Br2

    On the left hand side, you have molecular chlorine (Cl2), existing diatomically. You also have 2KBr, composed of 2K^ + ions and 2Br^- ions.

    On the right hand side, you have 2KCl, composed of 2K^ + ions and 2Cl^- ions and molecular bromine (Br2) existing diatomically.

    You can see that from left to right, the Cl2 is reduced to 2Cl^-, each Cl^- having gained an electron, and the 2Br^- in KBr have been oxidised to Br2 (each having lost an electron). From this we can see both Cl2 and Br^- are involved in the reaction, they are not spectator ions because they change from molecular to ions, or ions to molecular from left to right.

    K^ + however, is a spectator ion, because both KBr and KCl are formed of a K^ + ion, and either a Cl^- or Br^- ion. In both of these ionic compounds, the K^ + remains the same, i.e. it is neither oxidised or reduced (and so no electron transfer) and so does not take place in the reaction. Therefore, it can be omitted (cancels out).

    Hope that helps.

    EDIT: every K^ + ion should have a 1 plus charge, and the plus should show in the equations, for some reason this is being omitted.
    Thank you so much! But why is it that on the left side the Cl2 is not written as 2Cl-? Does it have to be ionic to be split up? Why is that H2O(aq) is split up as H+ and 2O- even though it is a molecule not an ionic compound? Thankss
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    (Original post by Laraib Chaudhry)
    Thank you so much! But why is it that on the left side the Cl2 is not written as 2Cl-? Does it have to be ionic to be split up? Why is that H2O(aq) is split up as H+ and 2O- even though it is a molecule not an ionic compound? Thankss
    Because on the left side it simply is not an ion, it's molecular chlorine whether it's gas or aqueous, written Cl2, when it's in a compound as on the right, it's Cl^-.

    H2O would never split into H+ and 2O-, if anything it splits into H+ and OH-.
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    (Original post by jsg9)
    Because on the left side it simply is not an ion, it's molecular chlorine whether it's gas or aqueous, written Cl2, when it's in a compound as on the right, it's Cl^-.

    H2O would never split into H+ and 2O-, if anything it splits into H+ and OH-.
    Omg! That actually makes sense! So it has to basically be an ionic compound? Why is that water despite being a molecule splits up into H+ and OH-? Also why is it that NH3Cl forms an ionic bond despite all the atoms being non metals? Is this the only exception? Last question, would an aqueous metal e.g. Mg(aq) be written as Mg2+ or would this not be written as it is not ionic? Thanks
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    Yes KCl is an ionic compound. Water can behave in that way as it's a polar covalent structure. The 2H atoms in water are slightly positive and the O atom is slightly negative because oxygen has 2 lone pairs of electrons as it has electron attracting properties. However, they are not ions, just slightly charged atoms. Water is not ionic because the bonding occurs with shared pairs of electrons between two non-metals.

    I assume by NH3Cl you actually mean ammonium chloride, NH4Cl - this has a dative covalent bond, a covalent bond, and an ionic bond. The reason this occurs between two "non-metals" is because the covalent bonds exist between the N-H atoms, 3 covalent and 1 dative bond. This forms the ammonium ion, NH4+. This compound is an ion in it's own right, and so can ionically bond to a Cl- ion. It is the NH4+ as a whole and the Cl- which have an ionic bond, whereas the bonding within NH4+ is covalent and dative.

    Magnesium forms ionic compounds, salts such as MgCl2 for example, meaning when it is in aqueous solution with the solvent being water for example, the polar nature of water (slightly positive H atoms and slightly negative O atom) means the Mg2+ ion in MgCl2 will be attracted to the slightly negative O atom in H2O, and the 2Cl- ions in MgCl2 will be attracted to the slightly positive H atoms in H2O. This "pulls apart" the ionic compound, MgCl2. Therefore, magnesium in aqueous solution would exist as an ion, not a molecule or elementally - thus you write it as Mg2+(aq). In its standard state Magnesium is written as Mg(s).
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    (Original post by jsg9)
    Because on the left side it simply is not an ion, it's molecular chlorine whether it's gas or aqueous, written Cl2, when it's in a compound as on the right, it's Cl^-.

    H2O would never split into H+ and 2O-, if anything it splits into H+ and OH-.
    Omg! That actually makes sense! So it has to basically be an ionic compound? Why is that water despite being a molecule splits up into H+ and OH-? Also why is it that NH3Cl forms an ionic bond despite all the atoms being non metals? Is this the only exception? Last question, would an aqueous metal e.g. Mg(aq) be written as Mg2+ or would this not be written as it is not ionic? Thanks
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    (Original post by jsg9)
    Yes KCl is an ionic compound. Water can behave in that way as it's a polar covalent structure. The 2H atoms in water are slightly positive and the O atom is slightly negative because oxygen has 2 lone pairs of electrons as it has electron attracting properties. However, they are not ions, just slightly charged atoms. Water is not ionic because the bonding occurs with shared pairs of electrons between two non-metals.

    I assume by NH3Cl you actually mean ammonium chloride, NH4Cl - this has a dative covalent bond, a covalent bond, and an ionic bond. The reason this occurs between two "non-metals" is because the covalent bonds exist between the N-H atoms, 3 covalent and 1 dative bond. This forms the ammonium ion, NH4+. This compound is an ion in it's own right, and so can ionically bond to a Cl- ion. It is the NH4+ as a whole and the Cl- which have an ionic bond, whereas the bonding within NH4+ is covalent and dative.

    Magnesium forms ionic compounds, salts such as MgCl2 for example, meaning when it is in aqueous solution with the solvent being water for example, the polar nature of water (slightly positive H atoms and slightly negative O atom) means the Mg2+ ion in MgCl2 will be attracted to the slightly negative O atom in H2O, and the 2Cl- ions in MgCl2 will be attracted to the slightly positive H atoms in H2O. This "pulls apart" the ionic compound, MgCl2. Therefore, magnesium in aqueous solution would exist as an ion, not a molecule or elementally - thus you write it as Mg2+(aq). In its standard state Magnesium is written as Mg(s).
    Thank youuu! I get the splitting into ions now!! But about the NH4Cl, why doesnt the chlorine form a covalent bond with the ammonium ion as ammonium is made up of non-metals only. I get the amonium has a positive charge and chloine a negative, but why? If so,would a hydronium ion (H30+) form an ionic bond with chlorine too? Would a H+ ion form an ionic bond with Cl-? Thanks
 
 
 

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