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Marble Chips and Acids
Name: alana
Status: student
Age: 16
Location: N/A
Country: N/A
Date: Around 1999
Question:
I would be ecstatic if you have, or know of any site
that does, an investigation on the reaction between hydrochloric acid and
marble chips. (I am investigating how the concentration of acid affects
the
marble chips.)
It is for my GCSE coursework, and as i have missed the last 2/3 months of
school; due to serious health problems, i am desperately stuggling.
I would really appreciate it if you could help in all areas (plan,
conclusion and analysis etc.). If anyone has handed in or re-written a
GCSE investigation to do with this experiment, i would be SO grateful if
you would email me with either a web page or details.
Replies:
Newton isn't in the business of doing people's schoolwork for them. We're
here to help them learn, which isn't the same thing.
You have chosen a somewhat difficult system to investigate, as it involves
two different phases (solid marble chips and liquid HCl in solution). The
progress of the reaction will depend on many variables other than the
concentration of the HCl. Two that are especially prominent because of the
two-phase nature are: 1. The ratio of surface area of the chips to the mass
of the chips. Smaller chips or chips of irregular shape will have
proportionately more surface area than large or blocky chips. 2. The
mixing between the two phases. As the reaction proceeds, HCl near the chips
will be depleted, and the concentration there will be less than the average
concentration throughout the reaction mixture. This could really throw off
your investigation. To minimize problem 1, you will need to use similar
marble chips for each experiment. They should be the same size and general
shape from run to run to give you interpretable results. For your analysis,
it may be a good idea to estimate the surface area (in, say, square
centimeters per gram) of the chips. This number may change as the reaction
proceeds, so you may also want to determine the area as a function of time
or extent of reaction as well. To minimize problem 2, you will need to keep
the reaction mixture well-mixed throughout the course of the reaction.
Before I go further, I should establish exactly what reaction is occurring.
CaCO3(s) + H+(aq) + Cl-(aq) --> Ca++(aq) + HCO3-(aq) + Cl-(aq)
HCO3-(aq) + H+(aq) + Cl-(aq) --> H2CO3(aq) + Cl-(aq)
H2CO3(aq) --> H2O + CO2(g)
Add these all together to get
CaCO3(s) + 2H+(aq) --> Ca++(aq) + H2O + CO2(g)
Depending on what the rate-determining step of this reaction is, the
kinetics should be either first or second order in acid.
There are several ways that you can follow the reaction, and it may be a
good idea to try more than one of them. As the reaction proceeds, solid
marble (CaCO3) disappears, acid (H+) is consumed, and carbon dioxide gas
(CO2) is produced. You could measure these by:
CaCO3: Stop the reaction after some period of time by filtering out and
washing the marble, and weigh it.
acid: Measure the pH of the reaction as it proceeds (requires a pH meter,
which is expensive), or fish out the marble and titrate the remaining acid.
CO2: Trap the gas evolved into an inverted graduated cylinder or a balloon,
and measure its volume. You could also trap it onto a chemical adsorbent,
such as Ascarite of soda lime, and weigh the charged sorbent. (Both
measurements would need to be corrected for water vapor in the collected
CO2.)
As for analysis and conclusions, you will need some data first. This should
be enough to get you started.
Richard E. Barrans Jr., Ph.D.
Assistant Director
PG Research Foundation, Darien, Illinois
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