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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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