Ok this is for a lab we are doing for equilibrium and absorbance and concentration, so i don't know if anyone can answer this.
Question: Describe how you plan to determine the temperature dependence of K sub c. Use a step by step process.
We can't help if we don't know what you did. There are a lot of experiments out there on equilibrium, absorbance, concn, etc. Without knowing the details we can't help.
As you say Kc can be determined by using absorbance when the starting material(s) and the product(s) absorb at different wavelengths. To do this at a different temperature the easiest way is to have themostatically controlled cells and stage.
To answer the question it will depend on how far back you need to start. You will need Beer-Lambert plots for the various species and, for more precise work, at the temperatures you are going to use. For many Kc expeiments which are equilibria this can be done by forcing the equilibrium to one side or the other, e.g. by adjusting the pH in some cases. So say we have two coloured species X- and HX
HX->X- + H+
so determine Kc we need the [HX],[X-][H+]. the latter is from pH measurement. But before we start we need a calibration plot of absorbance for [HX] and [X-] on their own, which we get from determining the absorbance at various concentrations at low and high pH, which will give us HX and X- on their own respecively.
To determine the temperature dependence of the equilibrium constant, Kc, you can follow these step-by-step procedures:
Step 1: Gather the required materials and chemicals
For this experiment, you will need a few materials such as a spectrophotometer or colorimeter to measure absorbance, a set of cuvettes or test tubes, a temperature-controlled water bath or heater, and the necessary solutions or compounds involved in the equilibrium reaction.
Step 2: Prepare the sample solutions
Prepare several solutions with different concentrations of the reactants involved in the equilibrium reaction. It is important to ensure that these solutions have measurable absorbance and cover a suitable range of concentrations. For example, if you are investigating a reaction involving a colored compound, make sure you have solutions with different known concentrations of the colored compound.
Step 3: Set up the spectrophotometer
Set up the spectrophotometer or colorimeter according to the instrument's instructions. Ensure that the wavelength selection matches the absorbance characteristics of your reaction system. Also, ensure that the instrument is properly calibrated.
Step 4: Establish the initial temperature
Start by setting the initial temperature, which will be the temperature at which you will initially measure the equilibrium constant. It is important to choose an appropriate temperature range that allows you to observe changes in absorbance and measure the equilibrium constant accurately.
Step 5: Measure absorbance
Using the spectrophotometer or colorimeter, measure the absorbance of each of the prepared sample solutions at the initial temperature. Record these absorbance values. The absorbance is directly related to the concentration of the colored compound, which is an indication of the progress of the equilibrium reaction.
Step 6: Determine the equilibrium constant (Kc)
Based on the absorbance values obtained, calculate the concentration of the colored compound using the Beer-Lambert Law or any other appropriate method. Then, using the known initial concentrations of the reactants and the concentrations of the colored compound at equilibrium, calculate the equilibrium constant, Kc, for each temperature.
Step 7: Repeat steps 4-6 for different temperatures
Now, repeat steps 4 to 6 for different temperatures within your desired range. Ensure that each measurement is performed at a stable temperature. The temperature can be controlled using a water bath or a heater with precise temperature settings.
Step 8: Plot a graph
Plot a graph of the natural logarithm of the equilibrium constant (ln Kc) against the reciprocal of the absolute temperature (1/T). This is known as the Van't Hoff plot. The slope of this graph will give you the value of the enthalpy change (ΔH) for the reaction.
Step 9: Analyze the data
Analyze the data obtained from the Van't Hoff plot to determine the temperature dependence of the equilibrium constant (Kc). The sign and magnitude of the slope provide insights into the enthalpy change and can help determine if the reaction is exothermic or endothermic.
By following these step-by-step procedures, you will be able to experimentally determine the temperature dependence of the equilibrium constant, Kc, for your specific reaction.