ap e unit 3 - intermolecular forces & properties · 2021. 6. 3. · ap e e 2020 page 1 eea e...

AP Chemistry

Intermolecular ForcesISPS Chemistry Oct 2020 page 1

Unit 3 - Intermolecular Forces& Properties

3.1 Intermolecular Forces 3.2 Properties of Solids 3.3 Solids, Liquids & Gases 3.4 Ideal Gas Law 3.5 Kinetic Molecular Theory 3.6 Deviation from Ideal Gas Law 3.7 Solutions & Mixtures 3.8 Representations of Solutions 3.9 Separation of Solutions & Mixtures Chromatography 3.10 Solubility 3.11 Spectroscopy & Electromagnetic Spectrum 3.12 Photoelectric Effect 3.13 Beer-Lambert Law

AP Chemistry


This logo shows it is a Topic Question - it should only require knowledge included in this Topic and it should be giving practice in the Science Practice associated with this Topic.

AP Chemistry


AP Chemistry


3.13 Beer-Lambert Law

AP Chemistry


Colorimetry - Origin of ColourColorimetry is a widely used analytical tool. Electron transitions within atoms, ions & molecules are often within the visible region of the Electromagnetic Spectrum. Energy will be absorbed to temporarily boost electrons to higher energy levels - they will then release heat energy as they return to ground state.

For example, Indigo carmine dye:

absorbs light from the visible andultra-violet parts of the electromagnetic spectrum.

It absorbs mainly in the red-orange-yellow part of the visible spectrum but transmits the blue-purple light which accounts for the indigo colour observed.

Best practice is to take all measurements at the wavelengthcorresponding to the strongest absorbance - about 285 nmin this case. However, this is likely to be outwith the range of most UV-Visible spectrometers, someasurements are more likely to be taken at about 615 nm.

Spectrophotometers use prisms and will be able to select any wavelength required. Colorimeters rely on coloured filters that are placed in front of the light source - in this case a red filter would be selected which may not match exactly with the maximum absorbance.

Issue: Use of a filter (eg 650 nm) will require solution to be of a higher concentration to get 'decent' absorption values.

Result: Linear relationships 'tail off' at higher concentrations (see later).

Before taking measurements, a cuvette filled with colourless solvent (often water) is used and the Intensity of light transmitted (It) can be measured and this value can be assumed to be the same as the Intensity of the Incident light ( Ii or Io ).

The use of a 'blank' cuvette will calibrate the appartaus so that any absorption due to the solvent or the glass/plastic cuvette is subtracted from any measurements of subsequent solutions.

AP Chemistry


Issue: Failure to calibrate using same cuvette filled with same solvent mixture.

Result: Subsequent measurements may include some absorbtion due to cuvette and/or solvent which will lead to higher concentrations being calculated.

Preparation of SolutionsBefore analysing a sample to determine the amount of a coloured substance that is present, it will be necessary to prepare solutions of known concentration to help determine the linear relationship between concentration and absorption.

A Standard Stock solution will be prepared accurately (Lesson 3.7) and then diluted (Lesson 3.7) to produce a series of solutions of accurately known concentrations.

Issue: Failure to weigh accurately / transfer completely to standard flask / make up to exactly the mark on the standard flask

Result: Subsequent calculations will be based on inaccurate concentrations.

Determination of Linear RelationshipOnce the calibration solutions have been prepared, they can each have their absorbance measured and plotted against their calculated concentrations.

It is quite common for a 'first attempt' to fail to produce a satisfactory straight line.

One of the main problems is that, at higher concentrations, intermolecular interactions can have an effect and will reduce absorption.

AP Chemistry


Issue: Using too high concentrations

Result: Absorbances will 'tail off' resulting in a curve rather than a straight line.

Poor technique can also be an issue. Ideally, the same cuvette should be used for every measurement (though expensive 'matched' cuvettes can be available) but that will require the cuvette to be flushed between solutions.

Issue: Flushing cuvettes with water

Result: Some water remains in the cuvette (sticking to sides) which slightly dilutes next solution.

This can be avoided by using the 'new solution' to flush out the 'previous solution'. The first flush may have its concentration altered but by the third flush changes should be minimal.

Using lower concentrations and working carefully should result in a straight-line relationship between concentration and absorbance.

A ∝ c The size of the cuvette (usually 1 cm) determines the path length for the light travelling through the solution.

A ∝ b (or l)

Issue: Changing cuvette

Result: A smaller cuvette will result in a reduced absorbance while a larger cuvette will result in an increased absorbance.

Mixtures / ImpuritiesWavelengths must be chosen to avoid detecting other chemicals present.

AP Chemistry


Beer-Lambert LawFirstly, Spectrophotometers and colorimeters measure the intensity of light hitting theirdetector - often a light dependent resistor.

The Absorbance is then calculated using the following relationship:

Absorbance is proportional to concentration (c): A ∝ c

Absorbance is proportional to path length (b): A ∝ b (or l)

The constant of proportionality is referred to as the Molar Absortivity ( ε - epsilon) and is a measure of how strongly a chemical species or substance absorbs light at a particular wavelength. It is an intrinsic property of chemical species that is dependent upon their chemical composition and structure.

It is normally determined by experiment and can be calculated from the gradient of a plot of absorbance against concentration.

It will only be valid for the particular wavelength, cuvette, species, solvent, temperature etc. that were used when taking measurements.

Gathering all our terms together produces the Beer-Lambert Law:

A = ε b c where A = absorbance and has no units due to Io / It

b = path length (in cm) c = concentration (in mol l-1 or M) ε = molar absortivity ( in M-1 cm-1 )

Unit for molar absortivity is largely meaningless - chosen to ensure all units cancel out to leave Absorbance with no unit.

Most of the time, the species being measured (eg Cu2+ ), the cuvette, the wavelength chosen, thetemperature etc. are not changing so Absorbance ∝ c

Issue: Not controlling variables

Result: Absorbances will not be trustworthy.

AP Chemistry


3.13 Practice Problems1.

The diagrams above show the ultraviolet absorption spectra for two compounds.

Diagram 1 is the absorption spectrum of pure acetone, a solvent used when preparing solutions for an experiment. Diagram 2 is the absorption spectrum of the solute for which the absorbance needs to be measured to determine its concentration.

When the student reads the absorbance of the solution at 280nm, the result is too high.

Which of the following is most likely responsible for the error in the measured absorbance?

A The student added too little solute to the acetone before measuring its absorbance. B The student rinsed the cuvette with the solution before filling the cuvette with the solution.

C The student forgot to calibrate the spectrophotometer first by using a cuvette containing only acetone.

D The wavelength setting was accidentally changed from 280nm to 300nm before the student made the measurement.

O

AP Chemistry


2. A student uses visible spectrophotometry to determine the concentration of CoCl2(aq) in a sample solution. First the student prepares a set of CoCl2(aq) solutions of known concentration.

Then the student uses a spectrophotometer to determine the absorbance of each of the standard solutions at a wavelength of 510nm and constructs a standard curve. Finally, the student determines the absorbance of the sample of unknown concentration.

The student made the standard curve above. Which of the following most likely caused the error in the point the student plotted at 0.050 M Co2+

(aq) ?

A There was distilled water in the cuvette when the student put the standard solution in it. B There were a few drops of the 0.100 M Co2+

(aq) standard solution in the cuvette when the student put the 0.050 M standard solution in it.

C The student used a cuvette with a longer path length than the cuvette used for the other standard solutions.

D The student did not run a blank between the 0.050 M Co2+(aq) solution and the one

before it.

O

AP Chemistry


3. Fe3+(aq) + KSCN(s) → FeSCN2+

(aq) + K+(aq)

To determine the moles of Fe3+(aq) in a 100. mL sample of an unknown solution, excess

KSCN(s) is added to convert all the Fe3+(aq) into the dark red species FeSCN2+(aq), as

represented by the equation above.

The absorbance of FeSCN2+(aq) at different concentrations is shown in the graph below.

If the absorbance of the mixture is 0.20 at 453 nm, how many moles of Fe3+(aq) were present

in the 100. mL sample? (Assume that any volume change due to adding the KSCN(s) is negligible.)

A 4 x 10-4 mol B 3 x 10-4 mol C 4 x 10-6 mol D 3 x 10-6 mol

4. Using a spectrophotometer, a student measures the absorbance of four solutions of CuSO4 at a given wavelength.

The collected data is given in the table opposite.

Which of the following is the most likely explanation for the discrepant data in trial 4 ?

A The solution was at a lower temperature than the solutions in the other trials. B The measurement was made using a different spectrophotometer that uses a cell with a longer path length.

C The solution was saturated and the flow of light through the solution was restricted.

D The concentration of the solution was actually lower than 0.150M .

O

O

AP Chemistry


5. A student prepared five solutions of CuSO4 with different concentrations, and then filled five cuvettes, each containing one of the solutions.

The cuvettes were placed in a spectrophotometer set to the appropriate wavelength for maximum absorbance. The absorbance of each solution was measured and recorded.

The student plotted absorbance versus concentration, as shown in the figure above.

Which of the following is the most likely explanation for the variance of the data point for the 0.600 M CuSO4 solution?

A The cuvette into which the 0.600 M solution was placed had some water droplets inside. B The cuvette into which the 0.600 M solution was placed was filled slightly more than the other cuvettes.

C The wavelength setting was accidentally moved away from that of maximum absorbance.

D The cuvette used for the 0.600 M solution had not been wiped clean before being put in the spectrophotometer.

O

AP Chemistry


6. Cu(s) + 4 HNO3(aq) → Cu(NO3)2(aq) + 2 NO2(g) + 2 H2O(l)

Each student in a class placed a 2.00 g sample of a mixture of Cu and Al in a beaker and placed the beaker in a fume hood.

The students slowly poured 15.0 mL of 15.8 M HNO3(aq) into their beakers. The reaction between the copper in the mixture and the HNO3(aq) is represented by the equation above.

The students observed that a brown gas was released from the beakers and that the solutions turned blue, indicating the formation of Cu2+

(aq). The solutions were then diluted with distilled water to known volumes

To determine the number of moles of Cu in the sample of the mixture, the students measured the absorbance of known concentrations of Cu(NO3)2(aq) using a spectrophotometer.

A cuvette filled with some of the solution produced from the sample of the mixture was also tested. The data recorded by one student are shown in the table opposite.

On the basis of the data provided, which of the following is a possible error that the student made?

A The Cu(NO3)2(aq) from the sample of the mixture was not diluted properly. B The spectrophotometer was calibrated with tap water instead of distilled water.

C The student labeled the cuvettes incorrectly, reversing the labels on two of the solutions of known concentration.

D The spectrophotometer was originally set to an inappropriate wavelength, causing the absorbance to vary unpredictably

O

AP Chemistry


3.9 & 3.11 - 3.13 Quick Check - AP FRQ Questions1. A student investigates the reaction between Ag(s) and HNO3(aq) represented by the equation below. 3 Ag(s) + 4 HNO3(aq) → 3 AgNO3(aq) + NO(g) + 2 H2O(l)

The student runs the reaction using a 3 to 4 mole ratio of Ag(s) to HNO3(aq) . Suggest a method the student can use to isolate solid AgNO3 from the other products of the reaction.

The student can evaporate the water, leaving behind solid silver nitrate.

2. To spectrophotometrically determine the mass percent of cobalt in an ore containing cobalt and some inert materials, solutions with known [Co2+] are prepared and the absorbance of each of the solutions is measured at the wavelength of optimum absorbance.

The data are used to create a calibration plot, shown opposite.

A 0.630 g sample of the ore is completely dissolved in concentrated HNO3(aq).

The mixture is diluted with water to a final volume of 50.00 mL. Assume that all the cobalt in the ore sample is converted to Co2+

(aq) .

a) What is the [Co2+] in the solution if the absorbance of a sample of the solution is 0.74?

An absorbance of 0.74 corresponds to a concentration of 0.0130 M (0.0125 M to 0.0135 M).

b) Calculate the number of moles of Co2+(aq) in the 50.00 mL solution.

1 point is earned for the correct calculation of the number of moles.

0.05000 L x (0.0130 mol Co2+/ 1.00 L) = 6.50 x 10-4 mol Co2+

c) Calculate the mass percent of Co in the 0.630 g sample of the ore.

1 point is earned for a correct calculation of the number of grams of Co that is consistent with the number of moles of Co2+ in part b). (may be implicit) 1 point is earned for a correct calculation of the mass percent that is consistent with the number of grams of Co.

6.50 x 10-4 mol x 58.93 g Co / 1.00 mol = 0.0383g Co 0.0383g Co / 0.630g ore x 100 % = 6.08 %

AP Chemistry


3. A student has 100. mL of 0.400 M CuSO4(aq) and is asked to make 100. mL of 0.150 M CuSO4(aq) for a spectrophotometry experiment.

The following laboratory equipment is available for preparing the solution: centigram balance, weighing paper, funnel, 10 mL beaker, 150 mL beaker, 50 mL graduated cylinder, 100 mL volumetric flask, 50 mL buret, and distilled water.

a) Calculate the volume of 0.400 M CuSO4(aq) required for the preparation.

1 point is earned for the correct volume. M1V1 = M2V2

V2 = (0.150M)(0.100L) / 0.400M V2 = 0.0375 L x (1000 mL/1L) = 37.5 mL

b) Briefly describe the essential steps to most accurately prepare the 0.150 M CuSO4(aq) from the 0.400 M CuSO4(aq) using the equipment listed above.

1 point is earned for using the buret to measure 37.5 mL of 0.400 M CuSO4 solution.

1 point is earned for adding the CuSO4 solution to the volumetric flask and filling to the mark with distilled water.

The student plans to conduct a spectrophotometric analysis to determine the concentration of Cu2+

(aq) in a solution. The solution has a small amount of Co(NO3)2(aq) present as a contaminant.

The student is given the diagram below, which shows the absorbance curves for aqueous solutions of Co2+

(aq) and Cu2+(aq).

c) The spectrophotometer available to the student has a wavelength range of 400 nm to 700 nm. What wavelength should the student use to minimize the interference from the presence of the Co2+

(aq) ions?

1 point is earned for a correct wavelength. 700 nm (Any wavelength from 650 to 700 nm is acceptable.)

AP Chemistry


4. A student is given a sample of CuSO4(s) that contains a solid impurity that is soluble and colorless. The student wants to determine the amount of CuSO4 in the sample and decides to use a spectrophotometer.

First, the student prepares a calibration graph by measuring the absorbances of CuSO4(aq) solutions of known concentrations. The graph is shown below

a) The student dissolves the entire impure sample of CuSO4(s) in enough distilled water to make 100.mL of solution. Then the student measures the absorbance of the solution and observes that it is 0.30. Determine the concentration of CuSO4(aq) in the solution.

The response indicates that the absorbance is 0.15M . (Unit not required. Any value from 0.14M to 0.16M is acceptable.)

b) Calculate the number of moles of CuSO4 that were in the impure sample of CuSO4(s).

The response gives the following calculation (or an equivalent): Unit not required.

0.15 mol /1000 mL x 100. moL = 0.015 mol

c) In addition to the number of moles of CuSO4 calculated in part b) , what other quantity must be measured in order to calculate the mass percentage of CuSO4 in the impure sample of CuSO4(s) ?

The response indicates that the mass of the sample must be measured.

AP Chemistry


5. The complete photoelectron spectrum of an element in its ground state is represented below.

a) Based on the spectrum, i) write the ground-state electron configuration of the element, and

The response gives one of the following two answers.

1s2 2s2 2p6 3s2 3p6 4s2 or [Ar] 4s2

ii) identify the element.

The response indicates Ca

b) Calculate the wavelength, in meters, of electromagnetic radiation needed to remove an electron from the valence shell of an atom of the element.

The response meets both of the following criteria.

The response indicates that the energy required is 0.980 x 10-18 J . The response shows a calculation similar to the following.

E = h𝛎 = h c /λ so λ=hc/E

λ=(6.626 x 10-34 J)(2.998x108ms-1)/(0.980x10-18J)

λ=2.03x10-7m

AP Chemistry


6. A student is given a solution of phenol red of unknown concentration. Solutions of phenol red are bright pink under basic conditions. The student analyzes a sample of the solution using a spectrophotometer set at a wavelength of 559 nm, the wavelength of maximum absorbance for phenol red.

The measured absorbance of the phenol red solution at 559 nm is 0.35.

a) Based on the calibration curve shown below, what is the concentration of the solution in micromoles per liter (μM) ?

The response gives a value between 5.2 μM and 5.6 μM.

b) If the student mixed 10mL of distilled water with 10mL of the sample, would this diluted solution have an absorbance greater than, less than, or equal to the absorbance of the original solution? Justify your answer.


The response indicates that the diluted solution would have an absorbance less than that of the original solution. The response indicates that the diluted solution would have a lower concentration and therefore a lower absorbance, in accordance with Beer’s law.

c) If the student measured the absorbance of the solution at a wavelength of 650nm, would the absorbance be greater than, less than, or equal to the absorbance of the solution at 559nm? Justify your answer.


The response indicates that the absorbance at 650 nm would be less than the absorbance at 559 nm .

The response indicates that 599 nm is the wavelength of maximum absorbance so the absorbance at any other wavelength will be lower.

ap e unit 3 - intermolecular forces & properties · 2021. 6. 3. · ap e e 2020 page 1 eea e...

Documents