Writing Half Reactions

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kamalkolluri
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Writing Half Reactions

Postby kamalkolluri » Thu Feb 21, 2019 9:56 am

When writing half reactions for Redox rxns, are we allowed to add in molecules, or are we restricted to those given in the original equation?

mcredi
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Joined: Fri Sep 28, 2018 12:16 am

Re: Writing Half Reactions

Postby mcredi » Thu Feb 21, 2019 10:06 am

You can add water. Each equation is balanced by adjusting coefficients and adding H2O, H+, and e- in this order: Balance elements in the equation other than O and H then balance the oxygen atoms by adding the appropriate number of water (H2O) molecules to the opposite side of the equation.

A De Castro 14B 2H
Posts: 75
Joined: Fri Sep 28, 2018 12:29 am

Re: Writing Half Reactions

Postby A De Castro 14B 2H » Thu Feb 21, 2019 12:36 pm

When writing half reactions for redox reactions in acidic solution, the only acceptable additions are H2O, H+, and electrons.

For example, balancing this reduction reaction: Cr2O72- → Cr3+
1. Balance elements in the equation other than oxygen and hydrogen.
so, Cr2O72-2Cr3+
2. Balance the oxygen atoms by adding the water (H2O) molecules.
so, Cr2O72- → 2Cr3+ + 7H2O
3. Balance the hydrogen atoms by adding hydrogen protons (H+)
so, Cr2O72- + 14H+ → 2Cr3+ + 7H2O
4. Balance the charge by adding electrons
so, since the products side has a charge of +6 (from 2*3) and the reactants side has a net charge of +12 (-2 + 14), add electrons to the reactants side: Cr2O72- + 14H+ + 6e-→ 2Cr3+ + 7H2O

Final reaction: Cr2O72- + 14H+ + 6e- → 2Cr3+ + 7H2O

Hope this enumeration helped!


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