Practice English Speaking&Listening with: IB Chemistry 17.1 Experimental determination of Keq

Hi. Welcome to this video on equilibrium.

Please have a careful look at the contents over here and jump to the parts that you think will be helpful.

The experiment used spectrophotometry to measure changes in equilibrium.

Here are the concentrations and volumes we will be using for our calculations.

Please have these handouts open in front of you or open and pause this screen on a second window for reference.

To determine the concentrations in this experiment, you need to realise that the substance you add to the final mixture is diluted.

So first you need to determine how many moles of substance you added.

Only then can you determine concentration by dividing the number of moles added by the total volume.

As we were using high concentrations of iron (III) ions we can assume that all the thiocyanate ions are used up in the reaction

as the equilibrium pushes the reaction toward the right.

Hence any formation and absorbance readings of red iron thiocyanate can also be assumed to be solely due the initial concentration of thiocyanate.

The concentration values were determined by multiplying out the initial concentration determined by solution stoichiometry with the experimental ratios used.

The experimentally determined absorbance readings of the A to F standards, and the unknown are given here in blue.

The graph of the A to F standards of concentration vs absorbance gives us a standard curve.

We then substitute the unknown value into the equation of the line to determine the concentration of iron thiocyante in the unknown.

This is the application of Beer’s law.

Again we use solution stoichiometry to determine the initial concentrations of reactants added in the first unknown.

The reaction stoichiometry are all ones in this reaction.

We can now write in the initial concentrations here.

From Beer’s law we have determined the equilibrium concentration of the iron thiocyanate product.

Hence using the reaction stoichiometry we can write in the changes that have occurred.

The final equilibrium values of the reactants is determined by comparing the initial concentrations to the changes that have occurred.

Now we can work out the equilibrium constant by substituting the equilibrium concentrations into the equation.

I’m using the American Chemical Society referencing style to reference the published value.

Substituting the experimental and published values into the equation for percent error and rounding to one significant figure gives me a final 20% error.

The uncertainty propagation is only necessary for your internal assessment reports. This then is just an example of how to do it.

The volumetric pipette measures in 0.1cm3 increments, so the uncertainty on the volumes is half of this which is 0.05cm3. This works out to 0.00005dm3.

Initally for standard A, 5cm3 was measured, so divide the uncertainty by this and you have a 10% uncertainty.

This final volume of 50cm3 is made up by adding two more volumes so the uncertainty is double.

This divided by 50cm3 gives a 0.2% uncertainty .

As this calculation is divide, we add percent uncertainties rather than actual uncertainties

This gives a total uncertainty of 10.2%.

The uncertainty for the largest standard F has a larger volume measurement so the uncertainty is smaller.

The calculation here comes to 1.2%.

We take the largest uncertainty, so for the standards this will be standard A, at 10.2%.

Now we can work out the uncertainty for the unknown.

The spectrophotometer manual states the uncertainty at 13%.

As the numerator is addition we add the actual uncertainty which then changes the percent uncertainty from 13% to 22%.

The uncertainty for the gradient is y divided by x and so is determined by adding the percentage uncertainties which is 13% for y and 10.2% for x.

This adds up to 23.2

Add the numerator uncertainty of 22% and denominator uncertainty of 23.2% and you have a 45.2% uncertainty for the final FeSCN concentration.

For the unknown 1, the initial concentration uncertainties are worked out in a similar way as to the standards

So the SCN has a 3.25% uncertainty and the Fe has a 1.25% uncertainty.

Now moving onto the RICE table.

The actual uncertainties of the initial concentrations is determined.

The actual uncertainty of the final FeSCN concentration is determined and this is the same for the reactants.

he final equilibrium uncertainties are then the addition of the actual uncertainties as this is an addition and/or subtraction, to obtain the final values.

These final actual uncertainties are then converted back to percent uncertainties for the final Keq equation which is multiply and divide.

The final percent uncertainty is used to calculate the final actual uncertainty.

This is rounded to one significant figure.

The final result is then adjusted to match the unit placings of the uncertainty giving a value of 200 plus or minus 100.