Chemistry U3 Exam Notes

Flame Test

• Use a nichrome wire instead of copper wire to avoid a green-blue flame from copper.
• Use concentrated hydrochloric acid, not nitric acid, as metal chlorides are more volatile.
• Set up a non-luminous Bunsen flame to easily see the flame color.
• Fully open the air hole on the Bunsen burner to make the flame color easier to see.

Presence of Hydroxyl Group (-OH)

• Add sodium: effervescence occurs due to hydrogen gas being released which burns with a squeaky pop, and sodium dissolves.
• Add carboxylic acid and concentrated sulfuric acid, then warm: a fruity smell indicates ester formation.
• Add phosphorus(V) chloride ( $PCl_5$ ): steamy fumes are given off.

Presence of Ammonium Ions ($NH_4^+$)

• React with $HCl$: white smoke is given off.
• React with sodium hydroxide ( $NaOH$ ): ammonia gas ( $NH_3$ ) is evolved, which turns damp red litmus paper blue.

Primary, Secondary, and Tertiary Alcohols

• Primary alcohols:
  • Oxidized to aldehydes or carboxylic acids with potassium dichromate ( $K<em>2Cr</em>2O_7$ ).
  • The potassium dichromate changes color from orange to green.
• Secondary alcohols:
  • Oxidized to ketones with potassium dichromate.
  • The potassium dichromate changes color from orange to green.
• Tertiary alcohols:
  • Cannot be oxidized, so there is no color change with potassium dichromate.

Reaction with Bromine Water

• Bromine water with sodium chloride:
  • No visible reaction as bromine is less reactive than chlorine.
  • The bromine water turns lighter due to dilution; the colorless sodium chloride solution becomes orange.
• Bromine water with sodium iodide:
  • A black solid forms, and the solution goes darker as bromine is more reactive than iodine.

Barium Chloride and Hydrochloric Acid Reactions

• Ammonium sulfate ( $(NH<em>4)</em>2SO<em>4$ ) and ammonium carbonate ( $(NH</em>4)<em>2CO</em>3$ ) react with barium chloride ( $BaCl<em>2$ ) and hydrochloric acid ( $HCl$ ) to form a white precipitate of barium carbonate ( $BaCO</em>3$ ).
• No reaction occurs with ammonium nitrate ( $NH<em>4NO</em>3$ ).

Reaction with Potassium Manganate(VII) Solution

• Hexane remains purple with potassium manganate (VII) solution ( $KMnO_4$ ).
• Hexene changes the color from purple to colorless when potassium manganate (VII) solution is added.

Enthalpy Change for Magnesium and Hydrochloric Acid

• To reduce percentage uncertainty in temperature measurement, use a larger mass of magnesium or a smaller volume of $HCl$.
• Magnesium is cleaned with sandpaper to remove magnesium oxide ( $MgO$ ) as the two enthalpy changes would be different.

Polystyrene Cup vs. Beaker

• A polystyrene cup insulates better and has a low heat capacity.
• The enthalpy change would be less exothermic with a beaker than with a polystyrene cup.

Ethanol Reacted with Sodium Bromide and Concentrated Sulfuric Acid to Form Bromoethane

• Sulfuric acid is added slowly to prevent spitting as the reaction is exothermic to prevent the mixture from boiling over.

Simple Distillation

• Anti-bumping granules promote smoother boiling.
• Water out is at the top, and water in is at the bottom to prevent the build-up of pressure.

Physical Properties of Bromoethane

• Immiscible with water and denser than water.
• The aqueous layer can be removed from the separating funnel by opening the tap and running off the bromoethane into a beaker.
• Sodium hydrogencarbonate solution is added to the impure bromoethane in a separating funnel to neutralize the $H^+$ ions.

Drying Agent for Bromoethane

• Calcium sulfate ( $CaSO_4$ ) is a suitable drying agent.
• The appearance of bromoethane changes from cloudy to colorless after adding the drying agent and allowing the mixture to stand.

Accurate Temperature Change Measurements

• Using a series of measurements gives a more accurate temperature change than taking the initial and highest temperatures because the reaction is not instantaneous, so the best-fit line allows for the effect of cooling during the reaction.
• The initial line considers temperature changes of the solution before the reaction and accounts for heat losses to the surroundings.

Assumptions in Energy Transfer Calculation

• Specific heat capacity of the metal is zero, and the heat capacity of the polystyrene cup is ignored.
• The density of the solution is $1 g/cm^3$, the same as water.

Improvements for Enthalpy Change Experiment

• Place a lid on top of the polystyrene cup to reduce heat loss.
• Measure the temperature change more often to give a precise extrapolation.
• Use a pipette or burette to measure the volume of the solution to reduce uncertainty in volume measurement.

Uncertainty in Burette Readings

• To reduce percentage uncertainty without changing the apparatus, use a larger mass of $MHCO_3$ to give a larger titration volume, resulting in a smaller percentage uncertainty.

Standard Solution Preparation

• Dissolve the $MHCO_3$ in deionized water.
• Transfer the solution with washings to a volumetric flask using a funnel.
• Make the mark up to $250 cm^3$ on the volumetric flask and shake.

Test for Chloride Ion

• Dissolve in deionized/distilled water.
• Add dilute nitric acid, then add silver nitrate solution.
• A white precipitate indicates the presence of chloride ions.

Measuring Cyclohexanol

• Use a measuring cylinder to measure approximate volumes.
• Pipettes and burettes are used to measure exact volumes.

Yield of Cyclohexene

• Adding phosphoric(V) acid slowly, with cooling and swirling, results in a higher yield of cyclohexene because the reaction is exothermic, and cyclohexene is volatile with a low boiling temperature, so it will evaporate if the acid isn't added slowly.

Anti-Bumping Granules

• Anti-bumping granules are present to promote smooth boiling in the mixture.
• Bumping should be avoided to prevent the product from being contaminated by the reaction mixtures.

Washing the Mixture

• Washing the mixture with sodium carbonate solution removes phosphoric acid.
• Washing the product with deionized water removes ionic compounds, cyclohexanol, or sodium phosphate(V).
• $MgSO_4$ (magnesium sulfate) is an anhydrous salt and doesn’t react.

Stirring the Solution

• Stirring the solution helps the solid (ammonium chloride) to dissolve and ensures a uniform temperature.

Polystyrene Cup vs. Glass Beaker

• Using a polystyrene cup instead of a glass beaker results in a greater temperature change because the polystyrene cup is a better insulator.
• The slope of the graph would be less steep as less heat from the surroundings would enter the solution.
• The temperature would increase but at a slower rate when time is greater than $90 s$.

Assumptions for Solution Properties

• The solution has a density of $1 g/cm^3$, the same as that of water.
• The specific heat capacity of the solution is the same as that of water.
• The mass of the solution is $50 g$, and the mass of ammonium chloride/solid is negligible.
• The heat capacity of the beaker/apparatus can be ignored/is negligible.

Combustion Analysis with $U$-Tubes

• Measure the mass of each $U$-tube and its contents before and after combustion.

Use of Pure Oxygen vs. Air

• Pure $O_2(g)$, not air, should be used to exclude water and carbon dioxide from the air and to ensure complete combustion.

Reaction with Phosphorus(V) Chloride

• When phosphorus(V) chloride is added to $X$, steamy white fumes are seen, indicating that compound $X$ can either be an alcohol or a carboxylic acid.

Addition of Sodium Sulfate Solution

• Sodium sulfate solution is added to remove barium ions, which would otherwise form a precipitate with the chromate ions.

Formation of Silver Chromate(VI) Precipitate

• The red precipitate of silver chromate(VI) only forms after all the chloride ions have reacted because silver chloride is much less soluble than silver chromate(VI).

Purpose of Anhydrous Calcium Chloride

• Anhydrous calcium chloride absorbs water, as ammonium chloride will react with water.

Form of Calcium Chloride

• Granules of anhydrous calcium chloride are used rather than powder to allow the chlorine gas to pass through.

Chlorine Gas Hazards

• The main hazard related to chlorine gas is toxicity, which is minimized by using a fume cupboard.

Adding Hydrochloric Acid Drop by Drop

• Concentrated hydrochloric acid is added ‘drop by drop’ to the pear-shaped flask to control the rate of reaction so chlorine gas is produced slowly.

Delaying Heating of Aluminum

• The heating of the aluminum is delayed by $20 s$ after the initial production of chlorine gas to allow the chlorine to displace the air so the oxygen does not react with aluminum.

Completion of Reaction

• The reaction is complete when the aluminum stops glowing.

Purpose of Potassium Hydroxide

• The potassium hydroxide in the absorption tube absorbs unreacted chlorine gas, preventing it from escaping, as potassium hydroxide is a base and chlorine gas is acidic.

Test for Sulfate Ions

• In the test for sulfate ions, an acid is added, followed by aqueous barium chloride.
  • The mixture needs to be acidified to remove insoluble barium compounds other than barium sulfate, such as carbonates, sulfites, and hydrogencarbonates, and to prevent unwanted carbonate ion compounds from precipitating.
  • Hydrochloric or nitric acid is a suitable acid, but sulfuric acid should not be used as it precipitates barium ions and produces barium sulfate.
  • White crystals tested for sulfate ions should be dissolved in deionized water, and the reagents should be added, forming a white precipitate for a positive result.

Rough Titration

• A rough titration is carried out to determine the approximate endpoint, so other titrations can be completed more quickly, knowing when the color changes and allowing dropwise addition near the endpoint.

Constant Precipitate Height

• The height of the precipitate becomes approximately constant when all of the iodide has reacted, and the nitrate is now in excess.

Precipitate Height Anomalies

• Some precipitate heights may be above the maximum height expected if the precipitate was not allowed to settle down.

Oxidation Products of Propan-1-ol

• Benedict’s or Fehling’s solution and propanal will form a red precipitate.
• Tollen’s reagent and propanal give a silver mirror.
• Addition of carbonate/hydrogencarbonate solution will give effervescence/fizz of carbon dioxide gas, which turns limewater cloudy.
• Addition of a reactive metal will give effervescence/fizz of hydrogen gas, which pops with a lighted splint.
• Addition of a named alcohol and acid catalyst will form a sweet-smelling ester.

Drying Geraniol

• To produce a sample of pure, dry geraniol, use (anhydrous) calcium chloride, (anhydrous) sodium sulfate, or (anhydrous) magnesium sulfate.
• Mix/shake/swirl/wait until it goes clear, then decant (the liquid)/pour off (the liquid)/filter (off the solid).

Burning Organic Compounds

• Burning organic compounds gives black smoke.

Enthalpy Change Discrepancies

• A possible reason for the enthalpy change value differing from a data book value is heat loss to the surroundings or apparatus.
• Other reasons include a mass of solution more than $25 g$, a density more than $1 g/cm^3$, or a specific heat capacity not being $4.2 / 4.18$, and assuming the heat capacity of the polystyrene cup is $0$.

Hydration Enthalpy

• The enthalpy change of hydration cannot be found directly by experiment because it is hard to add the correct amount of water, and it is hard to measure the temperature change of a solid.

Anomalous Data Points

• If all but one point is on the best fit line, and there is one anomaly (at $40°C$), it is not necessary to repeat the experiment as the anomaly has been identified (and excluded from the line of best fit).

Low Percentage Yield of 1-Bromobutane

• A possible reason for the yield of 1-bromobutane being lower if the cold-water bath was not used is that the reaction is exothermic/vigorous/gives off heat, and the reaction mixture may boil, causing reactant/product to escape/evaporate, or the reaction may bubble/fizz/effervesce and overflow the round bottom flask (causing loss of reactant/product).

Sodium Hydrogencarbonate Addition

• Sodium hydrogencarbonate is added to neutralize excess acid to ensure completion of the reaction.
• Vigorous effervescence from adding sodium hydrogencarbonate solution should be dealt with by removing the stopper (with the funnel upright) or opening the tap (with funnel inverted) to release build-up of pressure in the separating funnel.

Drying Agent Purpose

• The purpose of the anhydrous calcium chloride used is as a drying agent.
• The appearance of the organic liquid turns from cloudy to clear.

Less Than 100% Yield

• Reasons for the yield being less than $100\%$ include incomplete reaction or transfer losses.

Titration Preparation

• Before repeating a titration, wash the burette with water to remove traces from the previous solution and then with the solution to remove water, which would change the concentration of the solution.

Standard Solution Preparation

• Weigh the required mass of solid using a balance.
• Put the mass of solid in a beaker and dissolve the solid by adding deionized water.
• Transfer the solution to a volumetric flask using a funnel.
• Make the mark up to $250 cm^3$.
• Close the stopper and shake and invert several times to mix the solution.

Ensuring Accurate Acid Measurement

• Ensure none of the acid weighed out is left behind in the beaker to accurately know the amount of acid in the solution.

Burette Rinsing

• The burette is rinsed with ethandioic acid to ensure only the acid is present in the burette, preventing dilution of the acid and removing remaining water.

Pipette Errors

• If the meniscus is above the pipette mark, the reading should be taken level with the mark to reduce parallax error.
• A drop of the solution should be left at the tip of the pipette.
• If more sodium hydroxide than expected is in the conical flask, the titre volume will be larger and not the same volume as $25 cm^3$.

Preventing Suck Back

• Prevent suck back of water so that the water or liquid does not flow back into the tube.
• Otherwise, the flow of water back into the tube would cause the tube to break or crack.

Reducing Measurement Uncertainty

• Increasing the mass would reduce the percentage uncertainty in the measurement of mass/volume.
• If a larger mass is used, there is a larger production of carbon dioxide, which would exceed the volume of the measuring cylinder.

Hazard Symbols

• The first symbol (corrosive) indicates to wear gloves; the second symbol (harmful/irritant) indicates to conduct the experiment in a fume cupboard.

Organic Compound Properties

• The organic compound only has London forces between molecules, which are not strong enough to overcome the hydrogen bonding in the aqueous layer.

Carbon Dioxide Release

• Carbon dioxide gas must be released to prevent the build-up of pressure.

Hex-1-ene Observations

• Two layers form; the upper organic layer changes color to purple as iodine dissolves in it, and the lower aqueous layer decolorizes.

Shielding Apparatus from Draughts

• It is important to shield the apparatus from draughts to reduce heat losses.
• Some of the ethanol would be burned without heating the water; the mass of ethanol burned would be greater than expected, so the final value for the enthalpy change of combustion would be less exothermic/less negative.

Percentage Uncertainty Reduction

• Increasing the temperature change and mass of ethanol burned improves the percentage uncertainty by reducing it.

Uncertainty Values

• Uncertainty values indicate the range over which the measured value is valid.

Accuracy

• Accuracy is an indication of the difference between the experimental value and the data book value.

Pipette Washing

• The pipette must have been washed with water and again with the solution the pipette is measuring. Simply rinsing with water lowers the concentration of the solution the pipette is measuring as the acid is diluted.

Volumetric Flask Changes

• The volumetric flask has no change as we add water to the volumetric flask to make the mark up to $250 cm^3$.

Ensuring Thorough Mixing

• Mix thoroughly to ensure that the solution has the same concentration throughout. (sodium hydroxide) solution is an irritant/harmful/damages eyes/skin; avoid filling above head height as it might splash into eyes/onto face/onto arm.

Solution Concentration

• If the solution was too concentrated, the values of the titre would have been higher than the $50 cm^3$ of burette if Solution A had not been diluted. For a larger concentration of solution, a larger volume from the burette would be needed for the reaction to occur smoothly.

Calorimetry Measurements

• Measure the volume of water/mass of water as density is $1 g/cm^3$.
• Measure the initial and final temperatures of water.
• Measure the mass of spirit burner before and after.

Heat Loss Prevention

• Put a lid on the beaker.
• Add draught shield around the apparatus.
• Put a lid on the burner before and after combustion.

Ensuring Pure Hydrogen Gas

• Ensure all of the oxygen has been removed from the test tube, and the only gas present is hydrogen gas, as a hydrogen and oxygen mixture is explosive.

Copper Oxide Formation

• Oxide which is black, color change of red copper to black copper oxide can be seen.
• The amount of copper obtained will not be correct as there will be less mass of oxygen calculated since the copper was reoxidized. The hydrogen supply is turned off, for which oxygen again reacts with copper to form copper.

Solution Preparation in Volumetric Flask

• Dissolve the solid in deionized water in a beaker.
• Transfer the solution from the beaker to a volumetric flask with washings using a funnel.
• Make the mark up to $250 cm^3$.
• Invert the volumetric flask and shake to mix the solution in the flask.

Titration Reading Adjustment

• Use a more diluted solution of sodium hydroxide or use a greater volume of acid to make the titration reading higher.

Separating Funnel Usage

• Shake and invert the funnel multiple times.
• Open the tap to release build-up of pressure.
• Leave it to settle, then open the tap to release the lower aqueous layer into a container.
• Then release the pentane layer into a separate container.

Ammonia Test

• Ammonia turns damp red litmus paper blue.
• React ammonia with hydrogen chloride (gaseous) to form white smoke of ammonium chloride OR dip a glass rod in hydrochloric acid to form a white smoke of ammonium chloride.

Water Test

• Add anhydrous copper (II) sulfate, which changes color from white to blue.

Distillation vs. Heat Under Reflux

• Distillation is used when aldehyde is the product we want, as distillation prevents further oxidation as aldehydes are easily oxidized to a carboxylic acid and more easily oxidized than alcohols.
• Heat under reflux is used to ensure complete oxidation, complete oxidation of primary alcohols to carboxylic acid and secondary alcohols to ketones.

Mineral Wool Use

• Mineral wool soaked in alcohol is to hold the alcohol as alcohol is a liquid, and mineral wool is also unreactive, hence does not react with the alcohol.

Delivery Tube Removal

• It is necessary to remove the delivery tube from the heated tube as leaving the test tube causes the suck back of water; cold water into a hot tube causes the tube to break or crack.

Polystyrene Cup Placement

• A polystyrene cup is placed in a glass beaker to ensure the polystyrene cup does not tip over.

Powdered Metal

• Powdered metal reacts faster to minimize heat loss.

Unitary Method in Titration

• USE UNITARY METHOD WHEN FOR EXAMPLE SODIUM HYDROXIDE IS DISSOLVED IN DISTILLED WATER AND TRANSFERRED WITH WASHINGS TO A 250 CM3 VOLUMETRIC FLASK, MARK THE SOLUTION WITH DISTILLED WATER AND MIX THOROUGHLY, HOWEVER PIPETTE MEASURES 25CM 3 OF THE SODIUM HYDROXIDE SOLUTION INTO A CONICAL FLASK IN 250 CM3 THE MOLE OF NaOH IS 10 X THE MOLE OF NaOH IN 25CM3.