Effects of a Buffer

Description

Two solutions of acetic acid are prepared, one of which contains a large amount of sodium acetate. Equal amounts of magnesium metal are added to each solution. Both solutions produce the same volume of dihydrogen gas. The buffered solution produces the dihydrogen more slowly.

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Procedure
  1. Weigh two 3-gram samples of magnesium metal powder. Each sample must be in the range 2.9-3.1 g.
  2. Place a powder funnel in a dry balloon. Transfer one of the magnesium samples to one balloon. Transfer the second sample to another balloon.
  3. Weigh 15 g of sodium acetate. Transfer the sodium acetate to one of the clear colorless soda bottles.
  4. Place 250 mL of vinegar in each soda bottle.
  5. Swirl the bottle containing sodium acetate to dissolve the solid.
  6. Connect one balloon to each bottle. Fasten with a twist tie or other fastener.
  7. Begin with the bottle containing the buffer. Add the magnesium metal to the liquid. As soon as possible, add the magnesium to the liquid in the other bottle. Shake all of the magnesium powder down into the liquid.
  8. Carefully note and compare the sizes of the two balloons.

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Handout Makeup

Name ___________________________ Class ________

Teacher__________________________

DoChem 108 Effects of a Buffer

Watch the movie.

  1. Record the relative size of the balloons on each slide showing the reaction.
    Slide vinegar vinegar+buffer
    1    
    2    
    3    
    4    
    5    
  2. Explain your observations in terms of the chemical reactions taking place.
  3. Predict the outcome of using vinegar in one bottle and 0.5 M hydrochloric acid in the other.

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Teachers Guide

Purpose

To illustrate the effect of buffering a solution on the rate and extent of a chemical reaction.

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Materials

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Lab Hints

Use magnesium powder and not magnesium ribbon or turnings.

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Time

Teacher preparation: 15 minutes

Presentation: 10 minutes

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Hazards

Magnesium metal and dihydrogen gas are flammable substances.

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Precautions

Keep magnesium metal and hydrogen gas away from flames and ignition sources.

Release the dihydrogen from the balloons in a well ventilated area away from flames or sparks.

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Disposal

The solutions produced in this experiment may be disposed of safely at the sink. Discard balloons in ordinary trash. Use 0.5 M HCl to dissolve any unreacted magnesium that cannot be stored; neutralize the resulting solution, and discard at the sink.

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Presentation?

Presentation Question:

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Background

There is enough acid in each bottle to react completely with all of the added magnesium. In the buffer system, however, the concentration of hydrogen ion is lower so the reaction is slower. Eventually, the extent of reaction in each bottle is the same -- all of the magnesium is reacted.

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Makeup Ans.
  1. Record information about size of the balloons on each slide showing the reaction.
    Slide vinegar vinegar+buffer
    1 Beginning to inflate limp (1/2)
    2 Balloon taut Beginning to inflate (2/3)
    3 4" diameter 3" diameter
    4 5" diameter 4" diameter
    5 5" diameter 5" diameter
  2. The reaction of Mg with the acid takes place more slowly in the buffered solution where the hydrogen ion concentration is lower. All of the Mg reacts in both vessels eventually. The amount of H2 gas produced is the same because amount of Mg used is the same.
  3. The concentration of hydrogen ions is higher in the hydrochloric acid, so its reaction is expected to be faster. Since all of the magnesium metal dissolves, this is the limiting reagent, and the extent of reaction for the HCl solution should be the same as for the vinegar.

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Key Words

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