Esimating delta S and delta H from temp and Kb

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In summary, the relationship between free energy and equilibrium constant is given by ΔGo = -RT ln K. By measuring the pH at various temperatures, the temperature-dependent Kb for NH3(aq) was determined to be 1.34 X 10-5 at 283K, 1.42 X 10-5 at 293K, and 1.50 X 10-5 at 303K. The enthalpy and entropy can be estimated by plotting lnKb as a function of temperature and using the slope and y-intercept to find ΔH and ΔS. The x-axis is temperature and the y-axis is lnKb.
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chazgurl4life
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The relationship between free energy and the equilibrium constant is

Go = -RT ln K

By measuring the pH at various temperatures, the Kb for NH3(aq) was found to be temperature dependent, yielding the following values:

temperature (K) Kb
283 1.34 X 10-5
293 1.42 X 10-5
303 1.50 X 10-5

Over moderate temperature ranges (ranges less than 100 K degrees) both deltaSo and deltaHo can be considered approximately temperature independent. By plotting the available information appropriately, obtain estimates for deltaSo and delayHo for the reaction:


ok so i figured out that my y=mx+b equation should loook lik:

lnKb=delta H/R(gas constnt) - delta S/r

bbut what is my x-axis and y axis?



ok so i figured out that my y=mx+b equation should loook lik:

lnKb=delta H/R(gas constnt) - delta S/r

delat g=delta H-Temp(delta S)
 
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  • #2
You lost a factor of T in your enthalpy term. By restoring that, you can find a linear relationship between a function of ##K## and a function of ##T##, which will allow you to read off the enthalpy and entropy directly from a graph of the data.
 

FAQ: Esimating delta S and delta H from temp and Kb

1. What is the formula for calculating delta S and delta H from temperature and Kb?

The formula for calculating delta S is ΔS = nRln(T2/T1), where n is the number of moles of the substance and R is the gas constant. The formula for calculating delta H is ΔH = -nRTln(Kb), where Kb is the equilibrium constant at a given temperature.

2. How do you determine the number of moles in a substance for calculating delta S and delta H?

The number of moles can be determined by dividing the mass of the substance by its molar mass. This can be found using the periodic table or by using a molar mass calculator.

3. Can delta S and delta H be calculated for any chemical reaction?

Yes, delta S and delta H can be calculated for any chemical reaction, as long as the temperature and equilibrium constant (Kb) are known. These values can also be experimentally determined through calorimetry.

4. How does temperature affect the values of delta S and delta H?

As temperature increases, the value of delta S also increases. This is because at higher temperatures, particles have more energy and are more disordered, leading to a larger change in entropy. However, the value of delta H may increase or decrease depending on the direction of the reaction and the value of the equilibrium constant.

5. How can the values of delta S and delta H be used to predict the spontaneity of a reaction?

If the value of delta S is positive and the value of delta H is negative, the reaction is spontaneous at all temperatures. If both values are negative, the reaction is non-spontaneous at all temperatures. If delta S is negative and delta H is positive, the spontaneity of the reaction depends on the temperature and the magnitude of the values. A positive delta S and a positive delta H indicate a spontaneous reaction at high temperatures, but non-spontaneity at low temperatures.

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