Core practicals Flashcards
CP1: How can you measure the volume of gas released from a reaction?
- Using a gas syringe connected to a bung (via a tube) which stoppers the conical flask containing the reactants
- As gas is released it pushes the syringe outwards
CP1: What is the weighing by difference method?
- Weigh materials accurately
- Mass of substance = mass of weighing dish and substance - mass of dish after substance has been transferred
CP1: Why is it more accurate to find the mass of the calcium carbonate used by weighing the sample tube with calcium carbonate in, then tipping it out and reweighing the sample tube, rather weighing the empty tube at the start?
Weighing by difference
- Ensures the amount of CaCO3 that ends up in the reaction mixture is known
CP1: How would you carry out an experiment to measure the molar volume of a gas?
- React ethanoic acid and solid calcium carbonate together
- Using a gas syringe, measure the volume of gas released
- Repeat for increasing masses of CaCO3
CP1: Write a chemical equation for the reaction between ethanoic acid, CH3COOH and calcium carbonate
CaCO3 + 2CH3COOH -> Ca(CH3COO)2 + CO2 + H2O
CP1: What should the set up for this experiment look like?
Conical flask, bung, delivery tube, gas syringe
CP1: Identify the major source of error caused by the procedure used
- Some gas could escape between the addition of powdered CaCO3 and sealing the test tube
- CO2 is slightly soluble in water, so the exact volume isn’t measured
CP1: How do you prevent gas escaping?
Place the solid reactant upright inside a sample tube in the conical flask, tipping the tube over by moving the conical flask around to start the reaction
CP1: How would you analyse this data?
- Plot a graph of mass of CaCO3 (x-axis) against volume of CO2 collected (y-axis)
- Draw a line of best fit that passes through the origin
- In this reaction, CaCO3 to CO2 is 1:1 ratio
CP1: Suggest why you were told in the procedure not to exceed 0.40g of calcium carbonate
Using over 0.40g of CaCO3 will result in the volume of gas produced exceeding the capacity of the gas syringe (therefore making the experiment unsafe as glass could break)
CP1: Ethanoic acid is a weak acid. HCl is an example of a strong acid. Suggest why a weak acid should produce better results in this experiment than a strong acid
Using a stronger acid would result in a faster reaction, greater loss of gas when the CaCO3 is added to acid using a weak acid is more suitable as the reaction is slower, so less gas will be loss
CP1: What are other limitations or problems using the alternative apparatus?
Water displacement
- Some CO2 would dissolve in the water so the volume collected would be lower
CP2: What is a standard solution?
A standard solution is a solution of known concentration
CP2: How do you make a standard solution?
- Measure, using a balance, the mass of solid required
- Transfer this to a volumetric flask and rinse the remaining weighing bottle content (with distilled water) into the flask so no solid is lost
- Add a volume of distilled water to dissolve the solid. Swirl to mix
- Then add more distilled water up to the line on the neck of the volumetric flask. Invert 20 times to mix
CP2: What is the standard solution for this practical? How is it made?
- Diluted sulfamic acid
- Dissolve 2.5g solid sulfamic acid in 100cm^3 of distilled water
- Transfer to 250cm^3 volumetric flask and fill up to line with distilled water
CP2: What equipment is used to carry out this titration?
- A pipette and pipette filler are used to accurately measure out the volume of NaOH before transferring it to a conical flask
- A burette is used to add small volumes of sulfamic acid solution to the NaOH until the reaction has reached completion
CP2: How do you carry out a titration?
- Once the pipette has been used to place NaOH into the conical flask, fill the burette with the acid solution. Record initial volume
- Add a few drops of methyl orange to the conical flask
- Open the burette tap and allow the sulfamic acid to flow into the conical flask, swirling it to mix the contents
- Close the burette tap once the expected colour change occurs. Use a white tile so the colour change is easy to identify
- Record final burette volume
- Repeat until you get concordant results, then calculate a mean titre
CP2: Why should the pipette be rinsed with the sodium hydroxide solution?
This is because water that gets left in the pipette can dilute the sodium hydroxide solution, therefore changing the actual number of moles used
CP2: Why is there no need to dry the conical flask after washing it out between trials?
This is because the volume of sodium hydroxide is already measured with a volumetric pipette before it gets put into the flask, therefore water won’t change the number of moles used
CP2: Identify another indicator that could be used in this titration and state the colour change that would be seen at the end point
You could use phenolphthalein. It will change from pink to colourless
CP3: Write a chemical equation for the reaction of hydrochloric acid with sodium hydroxide
HCl + NaOH -> NaCl + H2O
CP3: Write an equation for the reaction of sodium hydroxide with carbon dioxide
2NaOH + CO2 -> Na2CO3 + H2O
CP3: When sodium hydroxide solution is stored, it reacts with carbon dioxide in the air. How will this change the concentration of the sodium hydroxide solution?
It will decrease
CP3: When sodium hydroxide solution is stored, it reacts with carbon dioxide in the air. How will this affect the volume of sodium hydroxide solution required to reach the end point in the titration? Explain your answer
The volume of the NaOH (aq) will not change because the Na2CO3 that is made will also react with the HCl. This is because 2 mols of Na2CO3 react to make 1 mol of NaOH. These will react with 2 mols of HCl, therefore the same number of mols of HCl that the NaOH would’ve reacted with
CP3: Explain why it is better to have a titre of around 25cm^3 than of around 10cm^3?
If the titre was smaller, the % uncertainty would be larger
CP4: What is a hydrolysis reaction?
Hydrolysis is a type reaction where water is used to break chemical bonds and split a reactant into two
CP4: How do you test the rate of hydrolysis of different halogenoalkanes?
- In 3 different test tubes add 4 drops of 1-chlorobutane, 1-bromobutane and 1-iodobutane
- To each test tube add 5cm^3 of ethanol. Place all test tubes in a 50C water bath
- Pour about 5cm^3 of silver nitrate into 3 test tubes. Place the test tubes in the water bath
- When all the solutions have reached 50C, add the silver nitrate to the halogenoalkane-ethanol solutions
- Start the stop clock. Measure the time taken for each precipitate to appear
CP4: What are the expected results of these reactions?
1-chlorobutane - white ppt forms slowly
1-bromobutane - cream ppt forms faster than of 1-chlorobutane but slower than 1-iodobutane
1-iodobutan - yellow ppt forms very quickly
CP4: What kind of reaction is the hydrolysis of halogenoalkanes?
Nucleophilic substitution
CP4: Why are water baths used?
To keep the temperature constant (as temperature is a control variable) so it doesn’t interfere with the rate of hydrolysis
CP4: Explain why water is able to act as a nucleophile
Because the oxygen atom has lone pairs
CP4: Explain why water is used as nucleophile rather than hydroxide ions?
If OH- were used as the nucleophile, a precipitate of silver hydroxide would form instead and this would mask the formation of the precipitate you were looking out for
CP4: What is % uncertainty and how do you calculate it?
100 x absolute uncertainty / calculate value
CP4: Explain why ethanol is used in these reaction
Both the halogenoalkane and aq AgNO3 will dissolve in ethanol
CP4: How can you decrease the uncertainty in time taken?
Use a lower temperature to reduce the rate of reaction. This will make the time taken longer and so the % uncertainty will be lower
CP5: What is oxidation?
Oxidation is loss of electrons
CP5: What equipment is used for distillation?
CP5: What happens when you oxidise ethanol by distillation?
- Ethanol is distilled with acidified potassium dichromate (VI) and is oxidised into an aldehyde, ethanal
- Colour change from orange to green
CP5: Why is heating under reflux used?
- Organic reactions involve breaking strong covalent bonds and heating under reflux is used to thermally speed up the rate of reaction by conducting the reaction at an elevated temp
- Allows heating for a long period of time
- Prevents the flask from boiling dry
- Prevents volatile reactants/products escaping
- Ensures even heating
CP5: What does the apparatus for heating under reflux look like?
CP5: Why are anti-bumping granules used when heating under reflux/in distillation?
To allow smooth boiling. They prevent the appearance of bubbles caused by vapour in the hot liquid which would cause splashing up the sides of the flask
CP5: What happens when you oxidise ethanol under reflux?
- Ethanol is refluxed with acidified potassium dichromate (VI) and is oxidised into a carboxylic acid, ethanoic acid
- This is shown by a colour change from orange to green
CP5: Write an ionic equation for the conversion of 1-bromobutane into butan-1-ol
CH3(CH2)3Br + OH- -> CH3(CH2)3OH + Br-
CP5: Draw a mechanism for the reaction and suggest why you have chosen this mechanism
Sn2 - 1-bromobutane is a primary halogenoalkane
CP5: Describe tests that will confirm (or otherwise) the presence of butan-1-ol in the distillate
You could add PCl5, if alcohol is present then there will be steamy fumes that will turn damp blue litmus paper red
- As there is also water mixed in the solution, you could also test for the alcohol with potassium dichromate (VI) in dilute sulfuric acid. It will go from orange to green
CP5: Describe tests that will confirm (or otherwise) the presence of unreacted bromoalkane
Warm it with aqueous sodium hydroxide, acidify with dilute nitric acid, then add silver nitrate. If bromoalkane is present, then a cream precpitate will form
CP5: Analysis of the product shows that there is still unreacted 1-bromobutane present. What modifications could you make to the method to try and improve the conversion to butan-1-ol?
You could heat it for a longer amount of time
CP6: How do you form the crude product?
- Add conc. HCl and 2-methylpropan-2-ol to a conical flask. Put the rubber bung in and swirl the flask
- Open the bung to release the pressure. Repeat this regularly over 20minutes
- Add some anhydrous CaCl2 and shake
- At this point, there should be two distinct layers
- Upper layer = organic product. Lower layer = aqueous layer
CP6: How do you separate the organic layer from the aqueous later?
- Transfer the contents of the flask to a separating funnel
- Allow the layers to separate and remove the lower layer by opening the tap
- Keep the organic layer in the separating funnel
CP6: How do you remove the unreacted HCl?
- Add a solution of NaHCO3 to remove the unreacted HCl. Swirl. Stop the separating funnel and shake it
- Invert and open the tap to release pressure. Repeat a few times
- Remove the stopper and run off the aqueous layer. Then, the organic later into a clear conical flask
CP6: How do you remove any water from the organic liquid?
Add some anhydrous Na2SO4 because it acts as a drying agent
CP6: How do you purify the product?
- Distillation
- This separates the products based on boiling point
- Collect the liquid boiling between 50C and 52C - this should be the pure product
CP6: Write an equation for the reaction of 2-methylpropan-2-ol with concentrated hydrochloric acid
(CH3)3COH + HCl -> (CH3)3CCl + H2O
CP6: What is removed from the crude product when it is shaken with sodium hydrogencarbonate solution? Write an equation for any reaction that occurs
The unreacted HCl
HCl + NaHCO3 -> NaCl + CO2 + H2O
CP6: 2-methylpropan-2-ol has a boiling temperature of 82C and is soluble in water. 2-chloro-2-methylpropane has a boiling temperature of 51C and is insoluble in water. Explain these differences
2-methylpropan-2-ol can form hydrogen bonds with water molecule therefore making it soluble in water. But 2-chloro-2-methylpropan-2-ol has a higher boiling point because hydrogen bonds are stronger than the permanent dipole-dipole and London forces in 2-chloro-2-methylpropane. Therefore 2-methylpropan-2-ol has a higher boiling point
CP7: How can a flame test be used to identify metal ions?
- Clean wire loop with H2SO4 or HCl
- Dip the loop into the sample
- Place the loop into a blue flame (using a bunsen burner)
- Record the colour of the flame
CP7: How can you use sodium hydroxide to identify cations?
- Dissolve the unknown substance in water
- Add aqueous NaOH dropwise, until in excess and no further change occurs
CP7: How do you test for an aldehyde?
- Add Tollen’s reagent to a sample of the suspected aldehyde. Warm -> The presence of a silver mirror confirms an aldehyde was present
- Add Fehling’s/Benedict’s solution -> aldehyde turns clear blue solution to brick red ppt
- Add dichromate ions (eg/ K2Cr2O7) acidified in H2SO4 and heat -> turns orange solution to green
CP7: How do you test for a carboxylic acid?
- Add an alcohol and acid
- Add a metal carbonate -> effervescence as CO2 is released
- Add a reactive metal -> effervescence as H2 is released
- Add an acyl chloride -> misty fumers (HCl)
CP7: How do you test for ammonium (NH4+) ions?
- Add an equal volume of NaOH to the sample. Shake
- Warm the solution in the test tube
- Test the gas released with damp red litmus paper
- If it goes blue, ammonium ions are present
CP7: How do you test for group 7/halide ions?
- To the compound being tested, add nitric acid and silver nitrate
- To samples of this solution, add dilute and then concentrated ammonia
CP7: How do you test for carbonate ions?
Effervescence
CP7: How do you test for sulfate ions?
- Add HCl and BaCl2 to the suspected sulfate solution
- If sulfate ions are present, a white ppt of BaSO4 will form
CP7: What is the order of testing ions?
Carbonate -> sulfate -> halide
This prevents false positive results occuring i.e. unexpected insoluble precipitates such as Ag2SO4, Ag2CO3 and BaCO3 could form
CP7: Why is nitric acid added in the test for halide ions using silver nitrate?
Remove any CO3 2- ions that would also give a white ppt
CP8: What is Hess’ Law?
The enthalpy change for a chemical reaction is the same no matter which route is chosen to get from reactants to products
CP8: Why may an experimental value for enthalpy change be different to the theoretical value?
- Heat loss to apparatus/surroundings
- Incomplete combustion
- Non-standard conditions
- Evaporation of alcohol/water
CP8: How do you prevent heat loss to surroundings/apparatus?
- Insulate by placing the reactants in a polystyrene cup with a lid, into the beaker
- Avoid large temperature differences between surroundings and calorimeter
- Use a bomb calorimeter
CP8: Other than preventing heat loss, how can the accuracy of this experiment be improved?
- Read the thermometer at eye level
- Stir the solution so the temperature is evenly distributed.
- Use a digital thermometer and data logger for more accurate and faster readings
- Use greater concentrations and masses, leading to a greater temperature change and thus smaller uncertainty
CP8: Why is it not possible to measure the enthalpy change for the decomposition of potassium hydrogencarbonate directly?
Because we have to heat it, therefore we can’t differentiate if the temperature changes is from the heating or thermal decomposition
CP13a: Give the reaction between propanone and iodine
I2 (aq) + CH3COCH3 (aq) + H+ (aq) –> CH3COCH2I (aq) + 2H+ (aq) + I- (aq)
CP13a: Similar experiments show that the reaction is first order with respect to both propanone and to hydrogen ions. What is the effect on the rate if the concentration of the hydrogen ions is doubled?
The rate will double
CP13a: Similar experiments show that the reaction is first order with respect to both propanone and to hydrogen ions. What is the effect on the rate if the concentration of the iodine is doubled?
It will stay the same
CP13a: Similar experiments show that the reaction is first order with respect to both propanone and to hydrogen ions. Write the overall rate expression for this reaction
Rate = k[H+][CH3CHOCH3]
CP13b: How can rate of reaction be measured?
Initial rates method - i.e. the iodine clock reaction
A continuous monitoring method
CP13b: What is a continuous monitoring method?
This involves measuring the change in concentration of a reactant or product over time as the reaction progresses
CP13b: Give an example of a continuous monitoring method
- Mix propanone with sulfuric acid and iodine in a beaker. Start a stopwatch
- Using a pipette, remove a sample of the mixture and add NaHCO3. This stops the reaction.
- Titrate the remaining iodine present in the sample with sodium thiosulfate (VI) solution, using starch as the indicator
- Repeat titrations with samples taken every 3 minutes
CP13b: How would you analyse the results from this reaction?
- Plot a graph of titre against time. Concentration of iodine is proportional to titre
- By comparing the shape of the graph to known order concentration-time graphs, determine the order of reaction with respect to I2
CP13b: What is the ‘iodine clock’ experiment?
2S2O3 2- (aq) + I2(aq) –> 2I- (aq) + S4O6 2- (aq)
- The I2 produced reacts with all of the thiosulfate ions present. Excess I2 remains in solution which then reacts with starch to form a blue-black solution
- Time how long it takes for this blue-black colour to appear.
CP13b: Identify the main sources of uncertainty in the procedure used and the measurements recorded in this experiment
The blue colour change is subjective, therefore it depends on each person’s judgement. The apparatus also has uncertainty, making it less accurate
CP13b: Suggest ways of minimising these uncertainties
- You could repeat each experiment to get an average
- You could use a pipette or burette instead
CP14:What is activation energy?
The minimum energy required to break all chemical bonds in the reactants for the reaction to then occur
CP14: What is an example method to determine the activation energy of a reaction?
- Repeat for various temperatures 15C to 75C (in water baths):
- Add equal volumes of bromide/bromate solution and phenol. Add methyl red indicator
- Add H2SO4 solution and time how long it takes for the solution to go colourless
CP14: How would you analyse this data?
- Plot a graph of ln t (y-axis) against 1/T (x-axis)
- The gradient = Ea/R
- Therefore, Ea = gradient x R
- Where t = time, T = temperature, R = 8.314J K-1 mol-1
CP14: Why is the log scale used sometimes?
To show a large range of values without compressing the scale
CP14: Write an equation for the reaction between bromine and phenol
C6H5OH + 3Br2 -> C6H2Br3OH + 3HBr
CP14: What function does the methyl red have in this experiment?
Once all the phenol reacts, the Br2, which is still being produced in the first reaction, then reacts with the methyl red, bleaching the methyl red indicator
CP9: Why are acid-base indicators used?
To detect when a reaction reaches its equivalence point. The indicator should be chosen so that its end point matches the equivalence point of the reaction
CP9: Why does a pH probe need to be calibrated?
So that for each pH reading, the pH value is accurate
CP9: How do you calibrate a pH probe?
Submerge pH probe in buffer solutions of three different pHs including pH 7 and 4 and 10. Each time pressing the calibrate button
CP10: Why do you need to sand away the outer layer of the metal?
It removes the oxide layer on the outside of the metal
CP12: What is the method of preparing a transition metal complex?
- Weigh out the mass of copper sulfate accurately and dissolve in water
- In a fume cupboard, add conc. NH3. Stir the mixture and pour into ethanol. Then, cool the mixture in ice bath. Crystals of product will form
- Filter the crystals via Buchner filtration
- Record the mass and calculate % yield
CP12: How do we prevent solid being lost in vacuum filtration?
Wash the transferring flask/tube with solvent and pour onto the filter paper in the buchner funnel
CP12: What is a fume cupboard used for?
To capture and then remove hazardous substances generated during experiments in the laboratory
CP16: What are the main steps in producing a pure organic solid?
- Synthesis of the compound (usually reflux, distillation)
- Filtration (vacuum filtration)
- Purification (recrystallisation)
CP16: Why is heating under reflux used?
- Allows heating for a long period of time
- Prevents the flask from boiling dry
- Prevents volatile reactants/products escaping
- Ensures even heating
CP16: What compounds are heated under reflux together in the first step to create aspirin?
- 2-hydroxybenzoic and ethanoic anhydride with a few drops of sulfuric acid
- Filter the product using vacuum filtration
CP16: How do you purify a solid product?
Recrystallisation
1. Dissolve the solid in the minimum volume of hot solvent
2. Filter the solution whilst hot (to remove any insoluble impurities)
3. Cool the filtrate (so that, as the solubility drops on cooling, the solid will crystallise out)
4. Filter at the pump and wash with a little cold solvent
5. Blot the crystals dry between filter papers
CP16: How do you determine the melting point of a substance and why can this information be useful?
- Pure substance: melt at a sharp temperature
- Impure: melt over a range of temperatures
- Can then compare the melting point to known values in the data booklet to identify the substance
