DOES IRON REACT WITH OXYGEN

IN THE PRESENCE OF ACID?

Knight Foundation Summer Institute

Terry Newirth, Haverford College

Introduction:

In the "Effect of Acid Rain on Marble Statues" experiment, we observed that marble chips in vinegar generated bubbles and neutralized the acid in vinegar. Therefore we saw evidence of a reaction (bubbles) and evidence that it reacted with the acid, since the acid was neutralized. In addition, in "Must it Rust?" we observed that iron wool reacts in the presence of water and reacts even more in the presence of acid as evidenced by the appearance of rust.

In this experiment we place iron wool, soaked in vinegar in a graduate cylinder that we invert in a beaker of water. If the iron reacts with oxygen, it removes oxygen from the air. i.e. It is removing a portion of the air, much like we do when we suck on a straw. We know that when we suck on a straw which is placed in a liquid, the liquid is drawn into the straw to replace the air we suck out. In the same way, water will rise in the cylinder to replace the volume of oxygen which reacts with the iron. We assume that we use enough iron to react with all of the oxygen in the inverted cylinder.

The purpose of this experiment it to determine if iron reacts with oxygen in air in the presence of acid. Also as a bonus, this experiment will allow us to determine the percentage of oxygen in air.

Objectives:

  1. To help the students understand the idea of rusting and what causes it
  2. To let the students complete an experiment where the results are easily seen

Materials:

Procedure:

  1. Put 10-15 ml vinegar in the 20 ml beaker
  2. Break off a piece of steel wool about O.95g (It should be between 0.9 and 1.0 g for this experiment.) You can have an appropriate sized piece for student to look at and they can pull off about that size, or they can weigh their piece of steel wool.)
  3. Measure and record the internal height in mm of the 25 ml graduated cylinder. Measure and record the thickness of the bottom of the 150 ml beaker
  4. Put about 50 - 60 ml of water in the 150 ml beaker
  5. Soak the small ball of steel wool completely in the vinegar for at least one minute, pushing it around with the tweezers to get the bubbles out
  6. Using the tweezers, lift the steel wool out of the vinegar, shake if off slightly into the 20 ml beaker or onto a paper towel, and then stretch it out and loosely push it into the bottom of the graduated cylinder
  7. Turn the cylinder upside down and carefully rest it in the 150 ml beaker with water. At this point no water should be in the graduated cylinder. You have essentially trapped a graduate cylinder full of air in the water-f~lled beaker
  8. As the acid soaked steel wool rusts, it consumes the oxygen from the trapped air. As the oxygen is removed, water moves in to replace the volume occupied by the oxygen
  9. The reaction is essentially complete in 20 minutes, and about 80-85% complete in 10 minutes. After 20 minutes ( or less if time is short) one measures the height of water in the graduated cylinder

Observations / Conclusions:

The internal height of the cylinder is proportional to the volume of air in the cylinder at the start of the reaction. The height of the water in the cylinder at the end of the reaction is proportional to the volume of oxygen that was in the cylinder originally. Therefore the % oxygen in air is

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The actual percentage of oxygen in air is close to 20%. This experiment, if done carefully, gives numbers very close to that figure. Most of the air we breath (close to 80%)is inert nitrogen gas, N,

[Actual numbers for dry air are O2 = 20.9489% : N2 78.084%. the remaining 1% is mostly the inert noble gas Argon. Of course in Philadelphia there is always some water in the air as well as many pollutants like carbon dioxide, and nitrogen and sulfur oxides.]

This experiment confirms that iron reacts with oxygen (or at least some component of air) to form rust. We had previously observed that water is necessary for the reaction and that acid is a catalyst for the reaction; i.e., we saw that the reaction went faster in acid.

If students ask if the marble chips might also be reacting with air, one could perform this same experiment with the marble chips. Soak the marble chips in acid; wrap them in cotton and place them in the bottom of the graduate cylinder and invert in a beaker of water. In this case water will not rise up the cylinder, as the reaction is a true acid/base reaction and not a oxidation/reduction ( redox for short) reaction.

 

Assessments:

The students could create a lab report to be handed in that has their interpretation of the results included.

Extensions:

Have the students write a story about some area where they find rust around or in their house. They could explain why rust would be found in this area based on what they learned in this experiment.

Philadelphia Science Content Standards:

SCIENCE CONTENT STANDARD # 1: NATURE OF SCIENCE

This content allows the students to "design, modify and conduct an investigation through testing, revising, and occasionally discarding idea, all of which lead to a better understanding of how things work." Also, as benchmark number 2 states, the students will learn to "collect and summarize data from an experiment and interpret results in terms of the data."

Cross References:

The students will have to learn to interpret information based on data collected, which uses analytical skills. They also will have to explore their environment and write about it in the extension.