How to balance redox reactions in acidic and basic solutions

academics AP exams chemistry College High School MCAT

Statistical Mediation & Moderation in Psychological Research (1)-2Balancing redox reactions is an essential skill for the Chemical and Physical Foundations section of the MCAT, the GRE Chemistry Subject Test, and the AP Chemistry Exam. Today, we will learn how to use the half-cell method for balancing redox reactions in acidic and basic solutions. We will first balance a redox reaction in acidic solution, then we will balance the same redox reaction in basic solution.

Balancing Redox Reactions in Acidic Solution

Consider the following unbalanced redox reaction:

Let’s try to balance this reaction in acidic solution.

Step 1: Divide the reaction into half reactions

 

Step 2: Balance the elements other than H and O

Fortunately, all atoms other than H and O are already balanced, so we can move on to the next step.

Step 3: Balance the O atoms by adding H2O

Step 4: Balance the H atoms by adding H+

 

Step 5: Balance the charges by adding e-

 

Step 6: Add the half reactions and simplify

Because the number of electrons in the first half reaction (3e-) does not equal the number of electrons in the second half reaction (2e-), we must multiply the reactions by coefficients so that the electrons will cancel out when we add them. In this case, we multiply the first reaction by 2 to yield 6e-, and the second reaction by 3 to yield 6e- as well.

Carrying this out, we get:

Now we can add the two reactions, and we notice that the electrons on both sides cancel out.

Thus, our balanced reaction is:

To summarize, the steps for balancing redox reactions in acidic solution are as follows:

  1. Divide the reaction into half reactions
  2. Balance the elements other than H and O
  3. Balance the O atoms by adding H2O
  4. Balance the H atoms by adding H+
  5. Balance the charges by adding e-
  6. Add the half reactions and simplify

Now that we have learned how to balance redox reactions in acidic solution, we will learn how to balance the same reaction in basic solution.

Balancing Redox Reactions in Basic Solution

Balancing redox reactions in basic solution is slightly more complicated than balancing in acidic solution because we must add both H+ and OH- to each half reaction. I will highlight the additional steps while we move through the process.

Let’s return to the same unbalanced reaction as above:

We will try to balance this reaction in basic solution.

Step 1: Divide the reaction into half reactions

 

Step 2: Balance the elements other than H and O

Fortunately, all atoms other than H and O are already balanced, so we can move on to the next step.

Step 3: Balance the O atoms by adding H2O

 

Step 4: Balance the H atoms by adding H+

 

Step 5: Add OH- to BOTH SIDES neutralize any H+ [ADDITIONAL STEP]

 

Step 6: Combine H+ and OH- to make H2O [ADDITIONAL STEP]

 

We simplify the equations to get:


Step 7: Simplify by cancelling out excess H2O [ADDITIONAL STEP]

 

Step 8: Balance the charges by adding e-

 

Step 9: Add the half reactions and simplify

Recall that we must multiply the reactions by coefficients so that the electrons will cancel out when we add them. In this case, we multiply the first reaction by 2 to yield 6e-, and the second reaction by 3 to yield 6e- as well.

Carrying this out, we get:

Now we can add the two reactions, and we notice that the electrons on both sides cancel out.

Thus, our final balanced reaction is:

To summarize, the steps to balancing a redox reaction in basic solution are as follows:

  1. Divide the reaction into half reactions
  2. Balance the elements other than H and O
  3. Balance the O atoms by adding H2O
  4. Balance the H atoms by adding H+
  5. Add OH- ions to BOTH SIDES neutralize any H+
  6. Combine H+ and OH- to make H2O
  7. Simplify by cancelling out excess H2O
  8. Balance the charges by adding e-
  9. Add the half reactions and simplify

And there we have it! I hope you enjoyed this guide and found it helpful. For more practice on balancing redox reactions or other general chemistry concepts, I recommend checking out Khan Academy, the UWorld MCAT Question Bank, or the ExamKrackers 1001 Questions in MCAT Chemistry workbook. If you are interested in working with me to prepare for the MCAT, please reach out via the contact button below!

Comments

topicTopics
academics study skills MCAT medical school admissions SAT expository writing college admissions English MD/PhD admissions strategy writing LSAT GMAT GRE physics chemistry math biology graduate admissions academic advice ACT interview prep law school admissions test anxiety language learning premed MBA admissions career advice personal statements homework help AP exams creative writing MD study schedules computer science test prep Common Application summer activities history mathematics philosophy organic chemistry secondary applications economics supplements research 1L PSAT admissions coaching grammar law psychology statistics & probability legal studies ESL CARS SSAT covid-19 dental admissions logic games reading comprehension engineering USMLE calculus PhD admissions Spanish mentorship parents Latin biochemistry case coaching verbal reasoning DAT English literature STEM excel medical school political science skills AMCAS French Linguistics MBA coursework Tutoring Approaches academic integrity chinese letters of recommendation Anki DO Social Advocacy admissions advice algebra artificial intelligence astrophysics business cell biology classics diversity statement gap year genetics geometry kinematics linear algebra mechanical engineering mental health presentations quantitative reasoning study abroad technical interviews time management work and activities 2L DMD IB exams ISEE MD/PhD programs Sentence Correction adjusting to college algorithms amino acids analysis essay art history athletics business skills careers cold emails data science dental school finance first generation student functions information sessions international students internships logic networking poetry resume revising science social sciences software engineering tech industry trigonometry writer's block 3L AAMC Academic Interest EMT FlexMed Fourier Series Greek Health Professional Shortage Area Italian Lagrange multipliers London MD vs PhD MMI Montessori National Health Service Corps Pythagorean Theorem Python Shakespeare Step 2 TMDSAS Taylor Series Truss Analysis Zoom acids and bases active learning architecture argumentative writing art art and design schools art portfolios bacteriology bibliographies biomedicine brain teaser campus visits cantonese capacitors capital markets central limit theorem centrifugal force chemical engineering chess chromatography class participation climate change clinical experience community service constitutional law consulting cover letters curriculum dementia demonstrated interest dimensional analysis distance learning econometrics electric engineering electricity and magnetism escape velocity evolution executive function freewriting genomics graphing harmonics health policy history of medicine history of science hybrid vehicles hydrophobic effect ideal gas law immunology induction infinite institutional actions integrated reasoning intermolecular forces intern investing investment banking lab reports linear maps mandarin chinese matrices mba medical physics meiosis microeconomics mitosis mnemonics music music theory nervous system neurology neuroscience object-oriented programming office hours operating systems