Physics of a chemistry experiment

In summary, the fraction of air that is oxygen is related to the pressure exerted by the gas trapped in the tube, the atmospheric pressure acting on the water in the reservoir, and the height of the water column in the cylinder.
  • #1
Fewmet
406
37
There is a simple chemistry experiment for finding the fraction of air that is oxygen. You push some steel wool to the bottom of a graduated cylinder. Partly fill the cylinder with water, and invert the tube into a water reservoir such that the steel wool is now above a pocket of air that is above a raised column of water. (Here is an online account with a http://www.practicalchemistry.org/experiments/intermediate/oxidation-and-reduction/how-much-air-is-used-during-rusting,208,EX.html" ). As the steel wool rusts, oxygen is depleted and the water level rises.

The chem class I teach is wrestling through problems with this and variations on the experiment. We are having trouble with one student question: what is the relationship involving the pressure exerted down by the gas trapped in the tube (Pi), the atmospheric pressure acting on the water in the reservoir,(PA) and the [tex]\rho[/tex]gh pressure of the water column in the cylinder? Is it simply that the three sum to zero?
 
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  • #2
For the sake of the experiment you can safely assume [tex]\rho[/tex]gh=0 because the height of the water column represents a tiny fraction of atmospheric pressure. But there is no reason why you can't calculate what it actually is if you want to. Preservoir + Pwatercolum = Patmospheric
 
  • #3
russ_watters said:
Preservoir + Pwatercolum = Patmospheric

I'm not sure of your meaning, so let me ask to clarify. I used "reservoir" to mean the water in the outer container, whose surface is exposed to the atmosphere. Is that what you mean? If so, why isn't the pressure of the gas trapped in the tube in the calculation?
 
  • #4
That was a poor choice of word - you are correct about what it should be.
 
  • #5
Thanks, Russ. That is very helpful. What you tell is (satisfyingly) what I expected the relationship to be. I was, though, bugged that the surface area of that outer reservoir of water didn't come into play. (I now see my mistake). When the experimental result the student got was inconsistent with my physics, I assumed my physics were wrong.

We'll go back and look at the experimental results again.
 

Related to Physics of a chemistry experiment

1. What is the relationship between physics and chemistry in an experiment?

The two fields are closely intertwined, as physics provides the fundamental principles and laws that govern the behavior of matter, while chemistry deals with the properties and reactions of different substances. In an experiment, the physical properties of the materials used and the laws of physics that govern their interactions determine the chemical reactions that take place.

2. How does temperature affect the outcome of a chemical reaction?

Temperature is a crucial factor in determining the rate and extent of a chemical reaction. As temperature increases, the particles in a substance gain more energy, causing them to move faster and collide more frequently. This leads to an increase in the rate of reactions and can also affect the products formed.

3. What role does energy play in a chemistry experiment?

In a chemical reaction, energy is either released or absorbed. This energy is necessary to break and form bonds between atoms, which results in the formation of new substances. The amount of energy involved can also determine whether a reaction is exothermic (releasing heat) or endothermic (absorbing heat).

4. How does the concentration of a substance affect its reaction in an experiment?

The concentration of a substance refers to the amount of that substance in a given volume. In an experiment, the concentration of a reactant can affect the rate of reaction, as a higher concentration means there are more particles present to react with each other. It can also affect the equilibrium of a reaction, as a higher concentration of products can lead to a reverse reaction.

5. What safety precautions should be taken when conducting a chemistry experiment?

Chemistry experiments can involve hazardous chemicals and processes, so it is important to take proper safety precautions. These may include wearing protective gear such as gloves and goggles, working in a well-ventilated area, and following proper handling and disposal procedures for chemicals. It is also important to carefully read and follow all instructions and guidelines provided for the specific experiment.

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