Balance the following redox equation using the oxidation-number-change method. Describe each step you use to balance the reaction.
HCl(aq) + Zn(s) --> ZnCl2(s) + H2(g)
I am really confused on how to do this as my book is very vague on this subject. Any help is greatly appreciated!
1. Go through and assign oxidation numbers to EACH element/ion/etc. Do you know how to do that? I can give you links if you don't.
On the left, H is +1, Cl is -1, Zn is 0.
On the right, Zn is +2, Cl is -1 (each) and H2 is 0.
2. Look to see which elements/ions/etc have changed.
H and Zn have changed.
3. Assign loss or gain or electrons to those elements/ions/etc that have changed. Ignore the others.
This is VERY important and usually is the most troublesome part. You MUST compare apples with apples and oranges with oranges; i.e., you see you have 1 H on the left and 2 on the right so you MUST compare the total electrons change for TWO Hs. So you have 2H (place a 2 there as a coefficient now so you don't forget it), total charge on H is +2. 2H going from +2 to 2H on the right at 0 means gain of 2 electrons. Zn on the left to Zn on the right is loss of 2e(e for electrons). I like to draw a line above the equation between 2H and H2 and write gain of 2e above it. It looks like this. |---------|. I draw another line below the equation between Zn on the left and ZnCl2 on the right like this and write on it, gain of 2e. |____________|.
4. Make the electrons lost equal to electrons gained by multiplying each one by a whole number.
You have 2e lost and 2e gained so you need to multiply by 1. So 1x2 for H on the left = 2 and that requires no change. On the right 1 x 1 = 1 and that requires no change. For the Zn 1 x 1 on the left and 1 x 1 for ZnCl2 on the right makes those numbers right. That finishes balancing the redox part.
5. Whatever number you've used as a multiplier goes in as a coefficient to that part of the equation.
So 1x2 for H on the left = 2 and that requires no change. On the right 1 x 1 = 1 and that requires no change for H2. For the Zn ,1 x 1 on the left and 1 x 1 for ZnCl2 on the right makes those numbers right. That finishes balancing the redox part.
6. Finish by balancing the non-redox part by trial and error.
In this case you check to make sure Cl is balanced. You have 2 Cl on the left and 2 on the right.
I hope this helps. It took a long time to type all of this. Most text books leave out the part about the apples and oranges and if you don't start by balancing elements on the left and right and comparing TOTAL charges, you never get it right.
To balance a redox equation using the oxidation-number-change method, here are the step-by-step instructions:
Step 1: Assign oxidation numbers to each element in the equation.
In this equation, H has an oxidation number of +1, Cl has an oxidation number of -1, and Zn has an oxidation number of 0.
HCl(aq) + Zn(s) → ZnCl2(s) + H2(g)
Step 2: Identify the element undergoing oxidation and the element undergoing reduction.
In this equation, Zn is oxidized (its oxidation number increases from 0 to +2), and H is reduced (its oxidation number decreases from +1 to 0).
Step 3: Write separate half-reactions for oxidation and reduction.
In the oxidation half-reaction, Zn is oxidized and loses two electrons:
Zn(s) → Zn^2+(aq) + 2e^-
In the reduction half-reaction, H is reduced and gains two electrons:
2H^+(aq) + 2e^- → H2(g)
Step 4: Balance the atoms and charges in each half-reaction separately.
Since the oxidation half-reaction already has a balanced charge, we only need to balance the atoms:
Zn(s) → Zn^2+(aq) + 2e^-
In the reduction half-reaction, we need to balance both atoms and charges:
2H^+(aq) + 2e^- → H2(g)
Step 5: Multiply the half-reactions to equalize the number of electrons transferred.
To do this, you need to multiply the oxidation half-reaction by 2:
2Zn(s) → 2Zn^2+(aq) + 4e^-
Now the number of electrons transferred in each half-reaction is equal.
Step 6: Combine the half-reactions to create the overall, balanced equation.
Add the two half-reactions together, canceling out any species that appear on both sides of the equation:
2Zn(s) + 4H^+(aq) → 2Zn^2+(aq) + 2e^- + H2(g)
Step 7: Finally, simplify the equation if necessary.
Divide all coefficients by the greatest common factor (in this case, 2):
Zn(s) + 2H^+(aq) → Zn^2+(aq) + 2H2(g)
And there you have it! The balanced redox equation using the oxidation-number-change method is:
Zn(s) + 2H^+(aq) → Zn^2+(aq) + 2H2(g).