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Activity Series
The activity series is a list of metals and their half-reactions arranged in order of
decreasing ease of oxidation or increasing ability to take an electron.
How do you use this series? Very simply, any substance on the right-hand side of the arrows
will be strong enough to take an electron from any substance above it on the left-hand side
of the arrows.
For example, if a Ag+ cation gets close to an Fe atom then the Ag+ cation will take an
electron away from the Fe atom. In terms of a chemical reaction the activity series predicts
2Ag+ + Fe → 2 Ag + Fe2+
In contrast, when a Fe2+ cation meets a Ag atom, the Fe2+ cation will not be strong
enough to take electrons away from the Ag atom. That is,
2 Ag + Fe2+ —X→ 2Ag+ + Fe
What happens when we mix Ca metal and HCl acid?
Ca(s) + HCl(aq) → ?
First, we write the complete ionic equation for the reactants.
Ca(s) + H+(aq) + Cl-(aq) →
Then we ask if H+(aq) is strong enough to take an electron away from
Ca(s)?
We look at the activity series and see that H+ is below Ca so H+(aq) can
indeed take an electron from Ca(s). Now we can write the two half-reactions
for this reaction. Since H+(aq) gains electrons, it undergoes reduction, so
we write its half-reaction as
2H+(aq) + 2 e- → H2(g)
And, since Ca loses electrons, it undergoes oxidation, so we we write its halfreaction as
Ca(s) → Ca2+(aq) + 2eFinally to get a net ionic equation we add these two half-reactions together
to obtain:
Ca(s) + 2H+(aq) → Ca2+(aq) + H2(g)
Again making certain that all the electrons on both sides of the equation
cancel. After putting the spectator ions back in we can write the balanced
molecular equation:
Ca(s) + 2HCl(aq) → CaCl2(aq) + H2(g)
Using the activity series can you explain why the metals gold, silver, platinum, and
palladium are so highly valued and called the precious metals?
Generally, many metals will react with acids to form a salt and hydrogen gas.
Metal + Acid → Salt + H2(g)
Let's look at some other examples. What do you expect will happen when Zn metal is put
into a beaker of HCl(aq)
Using the activities series we can predict:
Zn(s) + 2 HCl(aq) → Zn2+ + 2 Cl- + H2(g)
how about when Cu metal is put into a beaker of HCl(aq)? In this case the activity series
tell's us there's no reaction. So in summary, by simply looking at the Activity Series you can
predict which redox reactions will occur and which won't.
What will happen when copper wire is placed into a AgNO3
solution?
The complete ionic equation for the reactants is
Cu(s) + Ag+(aq) + NO3-(aq) → ?
From the activity series we see that Ag+ is stronger than Cu so a reaction
will occur. The half-reactions are:
Cu(s) → Cu2+(aq) + 2 e2(Ag+(aq) + e- → Ag(s))
__________________________________
Cu(s) + 2 Ag+(aq) → Cu2+(aq) + Ag(s)
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