Physical PAGs Flashcards
Plan and carry out a practical to determine the Mr of Mg by reacting it with dilute HCl at RTP
Mg + 2HCl -> MgCl2 (aq) + H2
Plan how they could establish the mr of Mg, answer should have detail of practical procedure carried out, measurements that would need to be taken and steps for the calculation
- use 2d.p balance to record mass of strip of Mg ribbon
-place Mg ribbon in conical flask
-using measuring cylinder measure excess of hcl so that all mg reacts
-carefully add acid to the flask and quickly fit a bung connected to a gas syringe to record the mass of gas produced
-once syringe has stopped moving record final volume of gas produced
How would you calculate the mr of Mg in the magnesium ribbon reaction
Mg + 2HCl -> Mgcl2 + H2
-mol of hydrogen by vol of it over 24000 as measuring cylinders are in cm3
-mg mol= H mol as 1:1
-calculate Mr by Mg mass/mol
Plan how a student could prepare 250cm3 of a standard solution from solid LiOH, with a conc of 0.125moldm3
Provide practical procedure, appropriate quantities and necessary calculations.
-mol of 250cm3 LiOH needed= 0.125/4= 0.03125 (as 250 x 4= 1000) and its mol per dm3. Then calculate mass. - 0.75
-Dissolve 0.75 LiOH in 100cm3 of distilled water , must be less than 250cm3
-transfer into 250cm3 volumetric flask, rinse beaker with distilled water and add washings to flask.
-make solution up to 250cm3 with distilled water, insert stopper and invert several times to ensure thorough mixing
How could a student calculate the conc of sulfuric acid by using 0.125moldm of a standard solution of LiOH?
Provide details of practical procedure including steps for calculations
-Rinse burette with small quantity of distilled water followed by acid (doesnt matter if acid or lioh goes here)
-fill burette with acid ensuring the jet below the tap is filled -overfill burette slightly then run liquid through tap so bottom part gets filled up.
-record initial burette reading
-pipette 25cm3 of LiOH solution into clean dry conical flask
-add 3-5drops of phenphthalein
-carry out trial titration by adding 1cm3 of acid at a time, recording volume of acid required to turn indicator from pink to colourless
- carry out further accurate titrations until 2 concordant results within 0.10cm3 are achieved
-calculate average titre of concordant accurate results, never use trial
Calculation:
H2SO4 + 2LiOH -> Li2SO4 aq + 2H2O
Mol of LiOh- 0.0125x 0.025= 0.003125
Mol of H2SO4 in average titre= 0.0015625
Conc= 0.0015625\ average titre
How could a student establish a value for enthalpy change of neutralisation for:
HCl + NaOH -> NaCl + H2O
Include practical procedure, measurements needed to be taken and steps for calculation
-measure 50cm3 of 1.0 moldm3 of HCl using a measuring cylinder
- measure 50cm3 of 1.0 moldm3 of NaOH in a separate measuring cylinder. Should be same temp
-allow the solution to reach the same temperature by leaving to stand for 5 minutes
-measure temp of solutions using thermometer to check theyr same temp
-place 50cm3 1moldm3 hcl into polystyrene cup
-carefully add 50cm3 of 1moldm3 NaOH to acid and record max temp reached
Calculation
-Temp rise of reaction
-Calculate energy released by reaction in joules, Q=mc t
-convert joules to Kj by /1000
-calculate n by n= cv, where youre calculating moles of water produced by the reaction. (We used equal volumes and conc of acid and alkali as theyr reacting in 1:1 reaction, moles of water is same so then so 1moldm3 x 0.05dm3 = 0.05
Calculate enthalpy change- Q/n, Q/0.05
M
What do you need to know for enthalpy change of combusion?
Mass of H2O heated by fuel, temperature rise, moles of fuel burnt
What do you need to know for enthalpy change of reaction, usually displacement of cuso4 and Sn
Volume of solution (mass), temperature rise, moles of limiting reagent
Plan a synthesis of cyclopentene from cyclopentanol.
The cyclopentene is heated under reflux with conc H3PO4. In the second part, the reaction mixture is heated under distillation to get crude product. Finally, crude product is separated from any impurities
Give apparatus details and experimental methods that should be followed for three steps. Outline potential risks.
Boiling point of cyclopentanol- 139
Cyclopentene= 44
Risks:
-wear eye protection and lab coats at all times
-before practical, use hazard sheets to identify hazards associated with all chemicals involved
-specifiy extra safety precautions that should be taken to reduce risks involved eg: conc H3PO4 acid is corrosive + can cause burns so chemical resistant gloves should be worn
Reflux:
-10cm3 of cyclopentanol to a pear shapes flask
- slowly add H3PO4 4cm3
-fit leibeg condenser to flask for reflux
- connect lower rubber tube to cold water tap and turn on water
-heat mixture at 90 degrees using water bath for 10 mins
Distillation
-set up condenser for d
-heat mixture using water bath at 44 degrees (as boiling point of cyclopentene is 44 degrees) until a small quantity of product has been collected (alkene) so cyclopentene would collect in ice water mixture beaker through receiver while the alcohol is left behind in pear shaped flask
Purification:
-add neutralising agent- na2co3
-pour distillate into separating funnel
-add small quantity of distilled water to remove water soluble impurities and allow layers to separate (alkene will be upper layer as its less dense than water so impurities in lower aqueous layer)
-run out aqueous layer and discard.
-collect organic layer in small conical flask
-add small quantity of anhydrous CaCl2 to flask and swirl to remove small trace of water as any water traces will be absorbed by it and leave organic product
-now have solid and liquid cyclopentene in flask so filter and collect in small container
- redistillation to further purify, control temp to exact 44 degrees so you know youre just getting across your product that you want.
Aspirin can be made by refluxing 2-hydroxybenzoic acid with ethanoic anhydride in the presence of a concentrated acid catalyst according to the equation below
(Acid) + (CH3CO)2O -> aspirin + CH3COOH
Info:
Aspirin is more soluble in hot than cold water
PY of aspirin is 85.5 in this method
Outline a plan to prepare 8.40g of pure aspirin, include mass of 2-hydroxybenzoic acid needed and the purification steps from hot reaction mixture
Mass= 7.53
Purification from hot reaction mixture:
-cool reaction mixture and filter under reduced pressure using buchner apparatus
-recrystallise impure aspirin by dissolving the minimum amount of hot water
-cool to allow crystallisation to occur then filter again under reduced pressure. Crystals that reform are purer as the impurity stays in solution
-wash the purified aspirin using pure water (as we may lose product with hot water)
To test purity of aspirin, a student did a melting point determination
Melting point range for aspirin pure= 139-140
Outline steps for this procedure and explain how results can give an indication of the purity of the sample
-Put small amount in melting point tube
-set ramping temperature of melting point apparatus several degrees below the melting point of aspirin. It will heat quickly then very gradually increases around ramping temp
-insert sample into melting point apparatus
-record range that sample melts
A student tested purity of aspirin using TLC, outline steps for this procedure and explain how the results can give an indication of the purity of the sample
Rf value of 2-hydroxybenzoic acid=0.30
Rf value aspirin=0.75
-draw pencil line near bottom of TLC plate
-run chromotograms for 2-hydroxybenzoic acid, pure acid, impure acid
-mark position solvent reaches (solvent front)
-compare rf values of spots for impure aspirin with pure substances
Impurities are likely from hydroxybenzoic acid
-dot in line with pure aspirin indicates some aspirin youve made, not in line shows impurities, faint dot here= small amount of impurities.
A student had to investigate the effect of changing the concentration of acid on the rate of reaction between Mg and HCl. He was given 2.00moldm3 and a Mg ribbon
Mg + 2HCl -> MgCl2 (aq) + H2
Outline a method using continuous monitoring the student could use to show that the reaction is first order with respect to HCl. show how the results could be used to confirm the reaction order
Method= gas collection
-Measure out excess quantity of 2moldm3 HCl eg: 50cm3 using a measuring cylinder and transfer to a conical flask/ boiling tube (0.1mol acid)
-measure out a mass of Mg ribbon eg: 0.1g using a 2dp balance (0.004 mol mg), so we need 0.008 mol of acid as equation is 1:2. As we are using 0.1 mol of acid, its in excess
-add Mg ribbon to acid and quickly fit a delivery tube connected to a gas syringe
-record volume of H2 produced at regular intervals eg: every 20 secs until the reaction stops (all Mg reacts as acid in excess)
-Dilute 2.0moldm3 to 1moldm3 by measuring 25cm3 of the acid and adding 25cm3 of water (0.05mol)
-repeat using this new 1moldm3
-Dilute the 2.0moldm3 HCl to 0.5 moldm3 by measuring out 12.5cm3 of the acid and adding 37.5cm3 of water (0.025mol)
-repeat using this new 0.5moldm3 of HCl
*logic- when u take a certain volume from the starting volume, the conc will be relative to the factor you decreased the volume by. Eg: start with 2mol dm3 of 50cm HCL, if you then take 25cm3 of HCL, you have 1mol dm3. Then u just add water to make volume up to 50cm3.
How would u analyse results of rates of reaction pag with HCl and Mg, proving its first order?
-Plot graphs on same axes of Volume of H2 against time
-Calculate initial rate of reaction for each concentration of HCl by drawing tangents to curves at time=0
-compare 3 gradients of 3 tangents; if reaction is 1st order with respect to HCl, the results should show that when the conc of HCl is halved, the rate approximately halves.
A student plans to prepare 250cm3 pf 0.4moldm3 solution of MgNO3, starting from Mg(NO3)2•6H2O crystals.
Describe how the student would prepare the solution, giving full details of quantities, apparatus and method.
Mol of crystal= 0.4 x 0.25= 0.1
Mass = 0.1 x 256.3 =25.63g
Method:
-Using a 2dp balance, weigh out 25.63g of crystals in a small beaker
-Dissolve in a small amount of distilled water (less than 250cm3) in breaker
-Transfer solution to a 250cm3 volumetric flask
-Rinse beaker and transfer washings to volumetric flask from beaker
-make solution up to the mark with distilled water so that the base of the meniscus sits on the line. Use a dropper (pasteur pipette) when close to the line
-insert stopper and invert several times to ensure mixing
Using KI, K2S2O8, Na2S2O3, starch indicator and distilled water, plan an experiment to find the order of the reaction with respect to I-. In your answer quote suitable volumes of solutions and explain how the results can be used to show that the reaction is first order with respect to I-.
How would u analyse these results?
-Need to vary the concentration of I- by diluting with water while keeping the concentrations of everything else the same. total volume for the reaction mixture will be 10cm3.
-using burettes measure 5cm3 of KI, 2cm3 of Na2S2O3 and 1cm3 of starch indicator into a test tube. Can be in same test tube as reaction wont start until K2S2O8 is added
-in a separate test tube, measure 2cm3 of K2S2O8
-add this to other test tube
-using stop watch record time when blue black colour disappears
-repeat the procedure using 4cm3 of KI and 1cm3 of H2O
-repeat procedure for 3:2 2:3 1:4 cm3 of KI:H2
Analysis
Calculate the conc of I- for each experiment
-mol I= 1 x 0.005
Conc I- = 0.005/0.01 (as total 10cm3 vol) = 0.5
-other concs= 0.4,0.3,0.2,0.1
Initial rate expressed using 1/time
Graph of initial rate (y axis) against I- (x axis)
Straight line through origin= first order with respect to I-
