PRELAB: �USING REDOX TO DETERMINE THE CONCENTRATION OF STORE-BOUGHT H₂O₂

BALANCING EQUATION

H₂O₂(aq) → O₂(g) + H⁺(aq) Oxidation half reaction

MnO₄^–(aq) + H⁺(aq) → Mn²⁺(aq) + H₂O(l) Reduction half reaction

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These are the two half reactions you need to determine the full, balanced reaction for this lab.

BALANCING EQUATION

H₂O₂(aq) → O₂(g) + H⁺(aq) Oxidation half reaction

MnO₄^–(aq) + H⁺(aq) → Mn²⁺(aq) + H₂O(l) Reduction half reaction

Remember:

1. Balance atoms
2. Balance charge with e^-
3. Balance electrons between the two half reactions
4. Combine the two half reactions and cancel as necessary

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TITRATION

The slow addition of one solution of a known concentration (called a titrant) to a known volume of another solution of unknown concentration (called the analyte) until the reaction reaches molar equivalence, which is often indicated by a color change.

MnO₄^- = titrant (known concentration)�H₂O₂ = analyte (unknown concentration)

H₂O₂(aq) + MnO₄^–(aq) + H⁺(aq) →

O₂(g) + Mn²⁺(aq) + H₂O(l) (unbalanced)

purple

colorless

As the purple MnO₄^- is added to the H₂O₂, it immediately reacts to form colorless Mn²⁺.

At the endpoint (5:2 mole ratio), one extra drop of MnO₄^- will turn the solution light purple (pink).

Titration to Endpoint

Add titrant quickly at first until pink/purple disappearance is very slow. Then add drop by drop until pale pink persists. That is the endpoint.

Not there

There

Too far

TITRATION TO ENDPOINT

Cleaning Buret

1. Rinse and roll with distilled water
2. Rinse and roll with small sample of titrant
3. Rinse and roll with 2^nd small sample of titrant

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We are regrettably skipping this step due to time.

Filling Buret

1. Be sure that stopcock is closed
2. Lower top of buret in buret clamp to lowest level possible
3. Add titrant using small beaker and funnel
4. Run small amount of titrant through stopcock
5. Record initial volume of titrant

Reading a Buret

1. Volume markings are from the top to the bottom with 0 mL on the top.
2. Read from the top downward.
3. Read the bottom of the meniscus.
4. Burets read to 2 decimals. Remember: Estimate between the lines!
5. Do not allow titrant to go below the 50.00 mL marking at bottom!

Safety (this will remain on screen)

1. Goggles at all times at lab stations! No exceptions at all!
2. Wear an apron. KMnO₄ will stain anything! Wipe up drips/spills immediately!
3. Sulfuric acid is VERY corrosive. Notify your teacher of any spills and wash skin immediately if you contact it.
4. All used KMnO₄ goes in waste beaker in fume hood.
5. Use small beakers to get small amounts of each reactant:
1. ~ 4 mL of H₂O₂
2. ~ as needed mL of KMnO₄
3. ~ 10 mL of H₂SO₄