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35, 36- Equilibria between salts and precipitates.docx

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CHEM 204
Christopher Barrett

CHEM 204 04/08/2013  04/10/2013 Lecture 35, 36 Martin M. PHGY Tutor Equilibria between salts and precipitates Recall that CaCO3 has one of the lowest Ksp so it doesn't fall out of solution very easily. 2- Question: If you have 0.005 M of Ca(aq), how many CO 3 can you add and still keep all the Ca ions in solution? [ ][ ] [ ][ ] [ ] Therefore if you add more than this, some Ca will fall out of solution. In the body this is NOT desirable because the ions are valuable. Even if CO3 concentrations are lowered later, re- dissolution is usually too slow to be effective. The Ks for calcium oxalate is , and Ca in the blood is about 0.005 M. With the same calculation as above, the critical concentration that drives Ca out of solution is . More than this causes the precipitation of CaC2O4 which builds kidney stones  Don't eat too much spinach, or hydrate a lot Precipitation isn't a bad thing always, our bones and teeth are made of precipitated CaCO3. Eggshell is actually 95% CaCO3 (other 5% is a soft protein). An average shell weighs about 5g, and it's very complex to make. The Ca++ and CO3-- have to be kept separate until just the right time, and have to be stratified into layers with soft spacers. CO (2) + H O 2H CO (aq) 2 3 - + H 2O (a3) + H O (aq2  HCO (aq) + H O (aq3 3 HCO (a3) + H O (aq2  CO 3--(aq) + H O 3aq) ++ -- Ca (aq) + CO 3 (aq)  CaCO (s) 3 Note that this is 3 coupled reactions encorporating acids so pH and PCO2 is important. However, since pH is fairly constant, the HCO3- ions is limited by PCO2 (governed by respiration and temperature).  When temperature is too high, CO2 is given off for cooling (panting) and the equilibrium is driven backwards Bone is similar to eggshell, with many thin wrapped layers and interspersed pores. It maximizes all the interfaces and is made of calcium salts wrapped around a thick collagen layer. The pores save weight and stop crack propagation. Different bones are optimized for different functions (ie femur for compression and hips for twisting) so histologically you can determine what region of the body a bone sample is from. CHEM 204 04/08/2013  04/10/2013 Lecture 35, 36 Martin M. PHGY Tutor Defining some properties:  Strength o Tensile: highest load before the material snaps in 2 from pulling.  Plotting stress vs strain: Hand weight on a material. There will be a linear relationship Tensile between stress and strain until it snaps. There are 3 important measurements: Ultimate stress, the slope, and toughness (area under curve, or energy absorbed) o Compression: The highest load applied before squeezing a material causes a crack and failure. Femur bone leads this category. o Strength can be very different depending on the application. Some materials can be strong i
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