ASAP HELP - can some1 explain how we can use colorimetry to find keq (1 Viewer)

[amdspotter]

im confused between how beer lambert law would come into play when we are calculating the concentration of a substance, and by extension the keq.

isn't using calorimetry simply like
creating standard solutions, using a calorimeter to find their absorbance and plotting all of this into a calirometry curve. then using the calorimeter to find the absorbance of the unknown solution and then using the curve to find concentration of the unknown.

like how would we integrate beer lambert's law into all of this.

if we wanted to find concentration using beer lamberts law wouldn't we just sub the value of path length, epsilon etc and solve for concentration of the unknown solution.

should we think of this as two distinct methods to use calorimeter to find keq. like is using standard solutions and then the curve one method, and using the beer lambert law another method.

any help asap would be greatly appreciated.

 

[CM_Tutor]

im confused between how beer lambert law would come into play when we are calculating the concentration of a substance, and by extension the keq.

isn't using calorimetry simply like
creating standard solutions, using a calorimeter to find their absorbance and plotting all of this into a calirometry curve. then using the calorimeter to find the absorbance of the unknown solution and then using the curve to find concentration of the unknown.

like how would we integrate beer lambert's law into all of this.

if we wanted to find concentration using beer lamberts law wouldn't we just sub the value of path length, epsilon etc and solve for concentration of the unknown solution.

should we think of this as two distinct methods to use calorimeter to find keq. like is using standard solutions and then the curve one method, and using the beer lambert law another method.

any help asap would be greatly appreciated.

Firstly, be careful not to mix up the terms here. Calorimetry is a technique for determining enthalpy changes by measuring temperature changes. Colourimetry is for determining concentrations by measuring the absorbance of light in the visible range of the electromagnetic spectrum. And the curve you refer to is a calibration curve.

If I collect a set of data for absorbance at a specified wavelength at different concentrations, I expect to get a linear relationship that passes through the origin - that is, a relationship that looks like , for some positive constant . The Beer-Lambert law says that absorbance is related to concentration by . So, the line of best fit on the calibration curve matches the Beer-Lambert law with the proportionality constant having the value . The two methods are both based in the Beer-Lambert law, but one is used when the molar absorptivity, is known, and the other when we are plotting calibration data to find a concentration without knowing or explicitly finding the value of (and potentially also where the path length is unknown).

If HSC chemistry included finding the least squares regression line, as Advanced Maths does, then questions could be asked to process calibration data to find the value of .

In the real world, the complication for this topic with using colourimetry to find a is that you have multiple species present and they each have non-zero absorbance (and different values of ) at most wavelengths in the visible spectrum. So, say we have a simple equilibrium between A and B: it is practically very difficult to construct calibration curves for absorbance by A at one wavelength and for B at another wavelength such that I can get concentrations of A and B at equilibrium to put into the expression for . In practice, we need to consider models like and then use ICE / RICE table approaches to relating and .

Incidentally, this is an area where most people have encountered this application in real life. A pulse oximeter, that is used to measure the oxygenation level in blood (one of those devices that are put on your finger in hospital, plugged in to a device on a mobile stand, that gives your pulse and an oxygen % that is hopefully something like 98) is using colourimetry. Light from the device on your finger passes through your finger and the absorbance is measured. From the known absorbance of haemoglobin and oxyhaemoglobin, which are in equilibrium, the level of oxygenation can be calculated.

The concept is also applicable to acid-base indicators where the colour changes between the acidic and basic forms due to different absorbance spectra for the two species.

 

[amdspotter]

Firstly, be careful not to mix up the terms here. Calorimetry is a technique for determining enthalpy changes by measuring temperature changes. Colourimetry is for determining concentrations by measuring the absorbance of light in the visible range of the electromagnetic spectrum. And the curve you refer to is a calibration curve.

If I collect a set of data for absorbance at a specified wavelength at different concentrations, I expect to get a linear relationship that passes through the origin - that is, a relationship that looks like , for some positive constant . The Beer-Lambert law says that absorbance is related to concentration by . So, the line of best fit on the calibration curve matches the Beer-Lambert law with the proportionality constant having the value . The two methods are both based in the Beer-Lambert law, but one is used when the molar absorptivity, is known, and the other when we are plotting calibration data to find a concentration without knowing or explicitly finding the value of (and potentially also where the path length is unknown).

If HSC chemistry included finding the least squares regression line, as Advanced Maths does, then questions could be asked to process calibration data to find the value of .

In the real world, the complication for this topic with using colourimetry to find a is that you have multiple species present and they each have non-zero absorbance (and different values of ) at most wavelengths in the visible spectrum. So, say we have a simple equilibrium between A and B: it is practically very difficult to construct calibration curves for absorbance by A at one wavelength and for B at another wavelength such that I can get concentrations of A and B at equilibrium to put into the expression for . In practice, we need to consider models like and then use ICE / RICE table approaches to relating and .

Incidentally, this is an area where most people have encountered this application in real life. A pulse oximeter, that is used to measure the oxygenation level in blood (one of those devices that are put on your finger in hospital, plugged in to a device on a mobile stand, that gives your pulse and an oxygen % that is hopefully something like 98) is using colourimetry. Light from the device on your finger passes through your finger and the absorbance is measured. From the known absorbance of haemoglobin and oxyhaemoglobin, which are in equilibrium, the level of oxygenation can be calculated.

The concept is also applicable to acid-base indicators where the colour changes between the acidic and basic forms due to different absorbance spectra for the two species.

makes a lot of sense now, thanks a lot m8 really appreciate it