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Predicting Reactions
For reactions that follow some reasonable rules
©copyright by James B. Condon (2013) all rights reserved.
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Predicting Reactions
Most chemical reactions are predictable with some
simple rules. It is usually the exceptions that are
concentrated on since there are so many.
Here is a look at the general rules for inorganic
reactions that involve oxidation and reduction.
Specifically, reactions of the elements to form
binary compounds.
To predict reactions, one must first know the
oxidation number rules. The rules that were given
in the lab manual are referred to as “principal
oxidation numbers”.
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Predicting Reactions
Rule 1: If there is no conflict, when elements react
they assume the principal oxidation numbers.
Example 1: What is the reaction product between
Li and F2?
Li + F2 ! ?
Firstly, do not consider what the subscripts are in the
reactants. Consider only what the elements are.
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Predicting Reactions
Rule 1: If there is no conflict, when elements react
they assume the principal oxidation numbers.
Example 1: What is the reaction product between
Li and F2?
Li + F2 ! ?
Li’s principal oxidation number is +1
F’s principal oxidation number is –1
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Predicting Reactions
Rule 1: If there is no conflict, when elements react
they assume the principal oxidation numbers.
Example 1: What is the reaction product between
Li and F2?
+1 –1
Li + F2 ! ? Li F
The oxidation numbers must balance to leave an
uncharged compound.
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Predicting Reactions
Rule 1: If there is no conflict, when elements react
they assume the principal oxidation numbers.
Example 1: What is the reaction product between
Li and F2?
Li + F2 ! LiF
Thus the Li to F ratio in this case is 1:1.
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Predicting Reactions
Rule 1: If there is no conflict, when elements react
they assume the principal oxidation numbers.
Example 2: What is the reaction product between
Ca and Cl2?
+2 –1
Ca + Cl2 ! Ca Cl
Principal oxidation number of Ca is +2
Principal oxidation number of Cl is –1
Thus the ratio for Ca to Cl is 1:2 for neutrality.
Ca + Cl2 ! Ca Cl2
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Predicting Reactions
Rule 1: If there is no conflict, when elements react
they assume the principal oxidation numbers.
Example 3: What is the reaction product between Y
and S8?
+3 –2
Y + S8 !
Y S
Principal oxidation number of Y is +3
Principal oxidation number of S is –2
Thus Y:S must be 2:3 for neutrality.
Y + S8 ! Y2S3
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Predicting Reactions
Rule 1: If there is no conflict, when elements react
they assume the principal oxidation numbers.
Rule 2: If both principal oxidation numbers are
negative, then the least electronegative element
assumes:
–(principal oxidation number) and if possible
8 + (principal oxidation number)
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Predicting Reactions
Rule 1: If there is no conflict, when elements react
they assume the principal oxidation numbers.
Rule 2: If both principal oxidation numbers are
negative, then the least electronegative element
assumes:
–(principal oxidation number) and if possible
8 + (principal oxidation number)
Example 4: What is the reaction products between
Cl2 and F2?
Cl2 + F2 ! ?
Principal oxidation numbers: Cl = –1 and F = –1.
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Predicting Reactions
Rule 1: If there is no conflict, when elements react
they assume the principal oxidation numbers.
Rule 2: If both principal oxidation numbers are
negative, then the least electronegative element
assumes:
–(principal oxidation number) and if possible
8 + (principal oxidation number)
Example 4: What is the reaction products between
Cl2 and F2?
Cl2 + F2 ! ?
F is the most electronegative thus Cl changes.
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Predicting Reactions
Rule 1: If there is no conflict, when elements react
they assume the principal oxidation numbers.
Rule 2: If both principal oxidation numbers are
negative, then the least electronegative element
assumes:
–(principal oxidation number) and if possible
8 + (principal oxidation number)
Example 4: What is the reaction products between
Cl2 and F2?
+7 –1
+1 –1
Cl2 + F2 ! Cl F and Cl F
Thus Cl = +1 and +7
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Predicting Reactions
Rule 1: If there is no conflict, when elements react
they assume the principal oxidation numbers.
Rule 2: If both principal oxidation numbers are
negative, then the least electronegative element
assumes:
–(principal oxidation number) and if possible
8 + (principal oxidation number)
Example 4: What is the reaction products between
Cl2 and F2?
Cl2 + F2 ! Cl F and Cl F7
Cl : F ratios are 1:1 and 1:7
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Predicting Reactions
Rule 1: If there is no conflict, when elements react
they assume the principal oxidation numbers.
Rule 2: If both principal oxidation numbers are
negative, then the least electronegative element
assumes:
–(principal oxidation number) and if possible
8 + (principal oxidation number)
Example 4: What is the reaction products between
Cl2 and F2?
Cl2 + F2 ! ClF and ClF7
However, ClF7 has an invalid Lewis dot.
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Predicting Reactions
Rule 1: If there is no conflict, when elements react
they assume the principal oxidation numbers.
Rule 2: If both principal oxidation numbers are
negative, then the least electronegative element
assumes:
–(principal oxidation number) and if possible
8 + (principal oxidation number)
Example 4: What is the reaction products between
Cl2 and F2?
Cl2 + F2 ! ClF is the only answer.
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Predicting Reactions
Rule 1: If there is no conflict, when elements react
they assume the principal oxidation numbers.
Rule 2: If both principal oxidation numbers are
negative, then the least electronegative element
assumes:
–(principal oxidation number) and if possible
8 + (principal oxidation number)
Example 5: What is the reaction products between
S8 and Br2?
S8 + Br2 ! ?
Principal oxidation numbers: S = –2 and Br = –1.
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Predicting Reactions
Rule 1: If there is no conflict, when elements react
they assume the principal oxidation numbers.
Rule 2: If both principal oxidation numbers are
negative, then the least electronegative element
assumes:
–(principal oxidation number) and if possible
8 + (principal oxidation number)
Example 5: What is the reaction products between
S8 and Br2?
S8 + Br2 ! ?
Br is more electronegative (rule 6 > rule 7)
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Predicting Reactions
Rule 1: If there is no conflict, when elements react
they assume the principal oxidation numbers.
Rule 2: If both principal oxidation numbers are
negative, then the least electronegative element
assumes:
–(principal oxidation number) and if possible
8 + (principal oxidation number)
Example 5: What is the reaction products between
S8 and Br2?
S8 + Br2 ! ?
Thus, S assumes +2 and +6.
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Predicting Reactions
Rule 1: If there is no conflict, when elements react
they assume the principal oxidation numbers.
Rule 2: If both principal oxidation numbers are
negative, then the least electronegative element
assumes:
–(principal oxidation number) and if possible
8 + (principal oxidation number)
Example 5: What is the reaction products between
S8 and Br2?
+2 –1
+6 –1
S8 + Br2 ! ? S Br and S Br
Thus, S assumes +2 and +6.
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Predicting Reactions
Rule 1: If there is no conflict, when elements react
they assume the principal oxidation numbers.
Rule 2: If both principal oxidation numbers are
negative, then the least electronegative element
assumes:
–(principal oxidation number) and if possible
8 + (principal oxidation number)
Example 5: What is the reaction products between
S8 and Br2?
+6 –1
+2 –1
S8 + Br2 ! ? S Br and S Br
Ratios are 1:2 and 1:6
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Predicting Reactions
Rule 1: If there is no conflict, when elements react
they assume the principal oxidation numbers.
Rule 2: If both principal oxidation numbers are
negative, then the least electronegative element
assumes:
–(principal oxidation number) and if possible
8 + (principal oxidation number)
Example 5: What is the reaction products between
S8 and Br2?
S8 + Br2 ! SBr2 and SBr6
Ratios are 1:2 and 1:6
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Predicting Reactions
Rule 1: If there is no conflict, when elements react
they assume the principal oxidation numbers.
Rule 2: If both principal oxidation numbers are
negative, then the least electronegative element
assumes:
–(principal oxidation number) and if possible
8 + (principal oxidation number)
Example 5: What is the reaction products between
S8 and Br2?
S8 + Br2 ! SBr2 and SBr6
SBr6 is allowed by Lewis dot.
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Predicting Reactions
Rule 1: If there is no conflict, when elements react
they assume the principal oxidation numbers.
Rule 2: If both principal oxidation numbers are
negative, then the least electronegative element
assumes:
–(principal oxidation number) and if possible
8 + (principal oxidation number)
Example 5: What is the reaction products between
S8 and Br2?
S8 + Br2 ! SBr2 and SBr6 are both allowed.
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Predicting Reactions
Rule 1: If there is no conflict, when elements react
they assume the principal oxidation numbers.
Rule 2: If both principal oxidation numbers are
negative, then the least electronegative element
assumes:
–(principal oxidation number) and if possible
8 + (principal oxidation number)
There are some prominent exceptions. Some of
these exceptions are given in another slideshow.
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The End
Predicting Reactions
For reactions that follow some reasonable
rules
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