- #1
Faiq
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The bond enthaply of halogens decreases down the group so why is the reactivity of halogens with hydrogens decreases down the group?
Reactivity of Halogens: Why Decreases Down the Group?
- Thread starter Faiq
- Start date
In summary, the bond enthalpy of halogens decreases down the group, which means that the hydrogen-halogen bond energy also decreases. This decrease in bond strength results in a decrease in reactivity of halogens with hydrogen. The concept of electronegativity and ionic size also plays a role in bond strength, with highly electronegative elements weakening covalent bonds in compounds such as N-H, O-H, and F-H. In the halogen family, bond strength decreases with increasing atomic number due to the addition of principle energy levels and the shielding effect. The bond enthalpy of H-X also contributes to the reactivity trends of halogens towards hydrogen.
- #2
Let'sthink
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Bond always involves two atoms. When you say enthalpy which is measure of the bond energy decreases along the group means the hydrogen halogen bond only I think. More the binding energy more reactive it will be because by bonding more energy will be released. So everything is in order in HF binding energy is greatest and F is most reactive with H to form HF. Take the analogy of an iron piece and attach it to a weak magnet and then to a strong magnet. where the binding energy is more that is stronger magnet attracts the piece more strongly than the weak magnet, hence it is more reactive. Think it over.
- #3
Faiq
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Oh so you are saying that hydrogen will be attracted more towards an electro negative atom. Can you explain me in terms of bond strength of H-X and how will it affect the reactivity?
- #4
DrDu
Science Advisor
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The first link I found when looking for the bond enthalpies of the halogens contains both the bond enthalpies of the halogens and the halogen hydrides. You can see that both decrease, so supposing that the enthalpy difference is a proxy for the driving force of the reaction, a decrease in bond strength of the halogens isn't made up by the bond formed in the hydrides. http://www.chemguide.co.uk/inorganic/group7/properties.html
- #5
James Pelezo
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Bond Energy trends are a function of electronegativity in series and of ionic size in groups. In series (consider Series II => Li, Be, B, C, N, O, F & Ne). Electronegativity increases with increasing atomic number due to increasing number of electrons and protons making up the atomic and ionic structures.
The effect of increasing electronegativity of neutral elements in series functions to decrease atomic radius b/c of increased force of electrostatic attraction. Ionizations follow the same trends in series for the Group I - III elements, but C, N, O & F tend to gain electrons causing a large in crease in ionic radius after Boron because of e-/e-repulsion, but electronegative effects still increase from Carbon through Fluoride. The order of increasing electronegativity is C <N<O<F. Applying this concept to chemical bonds (Covalent bonds) consider the C - H bond 1st. The low electronegativity of Carbon does not distort the chemical bond as much as the same bonds in N, O and F leaving a high electron cloud density in the Carbon to Hydrogen bond and results in a reasonably strong bond as compared to N - H, O - H or F - H which are polar covalent in nature. Distortion of the bonds by the highly electronegative N, O and F ions weakens the covalent bonds in these systems.
In groups or families of elements, size of central element plays a greater factor in strength of bond. This is especially true in the halogen family. Considering the compounds H - F, H - Cl, H - Br and H - I, bond strength decreases with increasing atomic number in the order (H - F) > (H - Cl) > (H - Br) > (H - I) due to the addition of principle energy levels (rings) creating a 'shielding effect', 'Shielding' decreases the electrostatic influence of the nucleus on the bond between the valence of the anion and the attached substrate. The acidity of the Hydrogen - Halide group is also referred to as the 'Hydrogen Halide Paradox'
The effect of increasing electronegativity of neutral elements in series functions to decrease atomic radius b/c of increased force of electrostatic attraction. Ionizations follow the same trends in series for the Group I - III elements, but C, N, O & F tend to gain electrons causing a large in crease in ionic radius after Boron because of e-/e-repulsion, but electronegative effects still increase from Carbon through Fluoride. The order of increasing electronegativity is C <N<O<F. Applying this concept to chemical bonds (Covalent bonds) consider the C - H bond 1st. The low electronegativity of Carbon does not distort the chemical bond as much as the same bonds in N, O and F leaving a high electron cloud density in the Carbon to Hydrogen bond and results in a reasonably strong bond as compared to N - H, O - H or F - H which are polar covalent in nature. Distortion of the bonds by the highly electronegative N, O and F ions weakens the covalent bonds in these systems.
In groups or families of elements, size of central element plays a greater factor in strength of bond. This is especially true in the halogen family. Considering the compounds H - F, H - Cl, H - Br and H - I, bond strength decreases with increasing atomic number in the order (H - F) > (H - Cl) > (H - Br) > (H - I) due to the addition of principle energy levels (rings) creating a 'shielding effect', 'Shielding' decreases the electrostatic influence of the nucleus on the bond between the valence of the anion and the attached substrate. The acidity of the Hydrogen - Halide group is also referred to as the 'Hydrogen Halide Paradox'
- #6
Faiq
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X-X + H-H -->2H-XJames Pelezo said:
Can you please also tell me what role does the bond enthalpy of H-X plays in the reactivity trends of halogens towards hydrogen?
- #7
James Pelezo
Gold Member
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I couldn't structure the energy flow diagram directly, so I typed it out in word and posted in jpeg. Hope this helps.
- #8
James Pelezo
Gold Member
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ΔHaX(g) = Heat of atomization for halogen, not hydrogen. My mistake.
- #9
Faiq
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So is this reasoning correct?James Pelezo said:
Down the group the strength of H-X bonds increases and to form a strong bond more energy is required. As a result due to difference in energy requirements the reactivity of H-X is higher up the group
- #10
James Pelezo
Gold Member
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Faiq said:
Not exactly ... 'Down' the group the strength of H-X bonds decrease and form weaker bonds giving up 'less' energy on formation... The bonding energies of the H-X series are 'size of halide ion' dependent, not 'electronegativity' dependent as in series substances. Such is why the trend is called 'The Hydrogen Halide Paradox'. As atomic number increases additional energy levels are present causing a shielding effect that reduces the influence of the nucleus on the valence level electrons. As a result the bond energy trends, from strongest to weakest follow H-F > H-Cl > H-Br > H-I ... The numerical values (supported by many online references) are ...
- ΔHb(H-F) = 135 Kj/mol
- ΔHb(H-F) = 105 Kj/mol
- ΔHb(H-F) = 87.5 Kj/mol
- ΔHb(H-F) = 71 Kj/mol
H-F is a weak acid (~2.5% ionized) in aqueous media where as HCl, HBr & HI are strong acids ionizing 100% in aqueous media b/c of decreasing H/X bond strengths.
- #11
James Pelezo
Gold Member
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Oops in last post, correction ...
- ΔHb(H-F) = 135 Kj/mol
- ΔHb(H-Cl) = 105 Kj/mol
- ΔHb(H-Br) = 87.5 Kj/mol
- ΔHb(H-I) = 71 Kj/mol
1. What is the trend in reactivity of halogens as you go down the group?
The reactivity of halogens decreases as you go down the group. This is because, as you move down the group, the atomic radius of the halogens increases, making it more difficult for them to attract an electron and form a bond.
2. Why does the atomic radius affect the reactivity of halogens?
The atomic radius of an element is a measure of its size. As the atomic radius increases, the outermost electrons are further away from the nucleus, making it more difficult for the nucleus to attract and bond with other atoms. This decrease in attraction leads to a decrease in reactivity.
3. How does the number of electron shells in a halogen atom affect its reactivity?
As you go down the group, the number of electron shells in a halogen atom increases. This means that the outermost electrons are shielded by the inner shells, making it more difficult for them to participate in chemical reactions. This leads to a decrease in reactivity.
4. What is the difference in reactivity between halogens at the top and bottom of the group?
The halogens at the top of the group (fluorine and chlorine) are the most reactive because they have smaller atomic radii and are able to attract electrons more easily. The halogens at the bottom of the group (bromine and iodine) have larger atomic radii and are less reactive because they are not able to attract electrons as easily.
5. How does the reactivity of halogens affect their chemical properties?
The reactivity of halogens plays a crucial role in their chemical properties. The more reactive halogens are more likely to form compounds with other elements, while the less reactive halogens are less likely to form compounds. Additionally, the reactivity also affects the types of compounds that are formed, with more reactive halogens forming more stable and stronger bonds with other elements.
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