(1) Half-reactions are written as ______ reactions with charge and mass balanced.
(2) Among the ions, Cd^2+, Pb^2+ & Ca^2+, ________ has the greatest ability to reduce to its neutral metal.
(3) Among the neutral metals; Pb, Ca & Cd, ________ has the least ability to oxidize to the +2 ion.
I really need help for 2 & 3, but for the first one, half-reactions are written as oxidation reactions, right?
I write half reactions as oxidation reactions if that is the way they go in the system or as reduction reactions if that is the way they go. But if the problem is not addressing a specific reaction but the table of potentials, those are written as reduction reactions.[This has been a heated discussion years among chemists with some preferring to write the tables as oxidation and others preferring reduction. As it now stands, the IUPAC has agreed that writing all tables in recution form is will be the standard.
For 2 and 3, look up the activity series in your text or notes. The activity series is a list of metals, not ions, being oxidized with the most reactive on top and the least reactive on bottom.
My text lists the metals in this order:
Ca
Cd
Pb.
That means that Ca is the strongest reducing agent (going to Ca ==> Ca^+2 + 2e). Of course, that means Ca is the easiest to oxidize. That answers #3, but of course you want the one at the bottom of the list if you want the least ability to oxidize. For #2, just reverse the reasoning. If Pb has the least ability to oxidize to the +2 ion, then ____ has the greatest ability for the +2 ion to be reduced.
For the first question, half-reactions are actually written as both oxidation and reduction reactions, depending on whether you are focusing on the oxidation or reduction process. Each half-reaction is balanced individually, both in terms of charge and mass, before being combined to form a complete balanced equation.
Now, let's move on to the second and third questions.
To determine which ion, Cd^2+, Pb^2+, or Ca^2+, has the greatest ability to reduce to its neutral metal (2nd question), we need to look at the reduction potentials of these ions. Reduction potential is a measure of the tendency of a species to gain electrons and undergo reduction. The higher the reduction potential, the greater the ability to be reduced.
You can find reduction potentials in a table called the Standard Electrode Potential table, which provides the standard reduction potentials for various species. The more positive the reduction potential, the greater the ability to be reduced.
Looking at the table, you will find that the reduction potential for Cd^2+ is more positive than both Pb^2+ and Ca^2+. Therefore, Cd^2+ has the greatest ability to be reduced to its neutral metal.
Moving on to the third question, we want to determine which neutral metal, Pb, Ca, or Cd, has the least ability to oxidize to the +2 ion. To do this, we can again refer to the Standard Electrode Potential table and look for the oxidation potential of each metal.
The more positive the oxidation potential, the greater the ability to be oxidized. In this case, we are looking for the metal with the least ability to be oxidized, so we want the metal with the least positive oxidation potential.
By referring to the table, you will find that the oxidation potential for Cd is the least positive among Pb, Ca, and Cd. Therefore, Cd has the least ability to oxidize to the +2 ion.
Remember that using reference materials like the Standard Electrode Potential table is crucial in determining the relative abilities of species to undergo oxidation or reduction reactions.