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Hi I just took EK Lecture 7 Chemistry Exam and I had a question about a question.
So the reaction that they are referring to is Reaction 1:
6Fe(2+) + Cr2O7(2-) + 14H+ <-------> 2Cr(3+) + 7H2O + 6Fe(3+) Eº=1.33V
Question 139 is:
Which of the following is true concerning Reaction 1?
A. Cr is oxidized and Cr2O7(2-) is the oxidizing agent.
B. Cr is reduced and Cr2O7(2-) is the reducing agent.
C. Cr is reduced and Cr2O7(2-) is the oxidizing agent.
D. Cr is oxidized and Cr2O7(2-) is the reducing agent.
Obviously, Cr2O7(2-) is the oxidizing agent as it is reduced in reaction 1. But I was confused as to which Cr they are referring to. Since they did not say the Cr in Cr2O7(2-), I assumed that it was the Cr from 2Cr(3+), which is oxidized in the reverse direction. However, according to EK "C is correct. Cr2O7(2-) is reduced to 2Cr(3+). Although this doesn't look like reduction from the charges, Cr in the dichromate has an original oxidation state of +6." So EK was obviously referring to the Cr in Cr2O7(2-), however, how was I supposed to know this? If I get a question like this on the MCAT, am I supposed to assume that they are talking about the atom in the molecule? Or is this just a bad MCAT question and I shouldn't worry about it because real MCAT questions will be clearer?
Thank you for all your help.
You are all awesome!
So the reaction that they are referring to is Reaction 1:
6Fe(2+) + Cr2O7(2-) + 14H+ <-------> 2Cr(3+) + 7H2O + 6Fe(3+) Eº=1.33V
Question 139 is:
Which of the following is true concerning Reaction 1?
A. Cr is oxidized and Cr2O7(2-) is the oxidizing agent.
B. Cr is reduced and Cr2O7(2-) is the reducing agent.
C. Cr is reduced and Cr2O7(2-) is the oxidizing agent.
D. Cr is oxidized and Cr2O7(2-) is the reducing agent.
Obviously, Cr2O7(2-) is the oxidizing agent as it is reduced in reaction 1. But I was confused as to which Cr they are referring to. Since they did not say the Cr in Cr2O7(2-), I assumed that it was the Cr from 2Cr(3+), which is oxidized in the reverse direction. However, according to EK "C is correct. Cr2O7(2-) is reduced to 2Cr(3+). Although this doesn't look like reduction from the charges, Cr in the dichromate has an original oxidation state of +6." So EK was obviously referring to the Cr in Cr2O7(2-), however, how was I supposed to know this? If I get a question like this on the MCAT, am I supposed to assume that they are talking about the atom in the molecule? Or is this just a bad MCAT question and I shouldn't worry about it because real MCAT questions will be clearer?
Thank you for all your help.
You are all awesome!