A level Chemsitry OCR A Practicals

How are gas volumes measured?

With a gas syringe Measure at room temperature as the volume will change with different temperatures

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Why is gas collection method a continuous rate?

We are measuring the rate of reaction CONSTANTLY from beginning to the end

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Why do we calculate Initial Rate

Because it tells us the KNOWN conc of the acid and the KNOWN mass of Mg

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Describe the method for gas collection?

1) Add reactants to flask and USE STOP WATCH and start the timer
2) Record volume of gas until reaction completes at SET TIME INTERVALS (0 to 10 to 20.....)
3) Repeat and change conc or SA or Temp

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What is the units for gas collection method

cm^3/s

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State three sources of error in gas collection method

1) Gas escaping due to Gaps in apparatus
2) Gas escapes when replacing bung
3) Some gases are water soluble and not reach syringe

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Method of Purification of Organic Solid

1) Recrystallize impure solid in minimum volume of hot solvent
2) Cool solution and filter under pressure
3) Scratch with glass rod
4) Wash with cold solvent/solvent and dry
5) Measure Melting Point

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What is the units for Loss in mass method?

g/s

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Describe method for loss in mass?

1) Place conical flask known conc/ vol HCl and calibrated mass balance and with CaCO3 and it's side.
2) Combine reactants and replace with cotton wool and START TIMER
3) Record at regular time intervals

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State three errors in loss in mass method?

1) Liquid escaping
2) Balance Errors
3) Water soluble gases REMAIN in flask

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In Standard Solution, Why do we use a glass stirring rod

To minimise contamination

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In Standard Solution, Why do we use minimum volume of distilled water

Preventing diluting the solution too much

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In Standard Solution, Why do we add Distilled water to our beaker after we add it to the volumetric flask?

To minimise the loss of the solution's molecules

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Describe the method of Standard Solution (3 marks)

1) Weigh a KNOWN mass of solid by difference using mass balance
2) Dissolve in minimum volume of distilled water in a beaker with the solid in beaker too by using a glass stirring rod.
3) Add the solution to 250 cm^3 volumetric flask with a FUNNEL

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Describe the method of Standard Solution (3 marks) FINAL PART!!!

4) Rinse beaker with Distilled Water and the washings to the V.Flask.
5) Make up to the mark with more Distilled H20 6) Bottom of meniscus should be on mark
7) Invert to mix solution which makes conc HOMOGENOUS!!!

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State 4 things to do when carrying out a titration?

1) Repeat until 2 concordant results achieved (0.1 cm^3 )
2) Calculate mean using concordant results.
3) All readings must be 2 D.P with last number being 5 or 0
4) Use Phenolph or Methyl Orange Indicator

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State what colour change occurs in phenolphthalein and methyl orange?

1) Phenolphthalein (In acid colourless and pink in alkali)
2) Methyl Orange (Red in acid and Yellow in alkali)

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In titrations what thing can we consider?

1) Titre value to be around 15-25 cm^3 range

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In titration what should we do if titre value is too low?

Use a more dilute solution

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In titration what should we do if titre value is too high?

Use a more concentrated solution

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Suggest why using more dilute or concentrated solutions in titration is helpful?

1) Gives better burette readings
2) Deceases % uncertainty

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How to prevent an increase in % uncertainty?

1) Use a more accurate apparatus
2) Round to higher D.P
3) Increase Measurements

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In Redox what is the colour change from MnO4- to Mn2+

Purple to Pale Pink

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In titration what do air bubbles do

Causes titration to be larger than expected

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State what indicator can be used for the thiosulfate test and iodine test

Starch is used as an indicator as it Iodine forms a blue/black solution

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What happens when Iodine runs out (when end point is reached)

It turns blue/black into colourless

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In Iodometry what is the formula equation when Cu2+ reacts with excess Iodide ions

2Cu2+ + 4I- -----> 2CUI + I2

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In Iodometry what is the formula equation when Thiosulfate ions react with Iodine

2S203^2- + I2 ------> S4O6^2- + 2I-

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In Qualitative Analysis, Describe the method for CO32-

1) Add Acid
2) Fizzing observed
3) Bubble gas through limewater
4) Limewater goes cloudy

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For CO32- test state the..... 1) Reagent 2) Observation 3) Equation

1) HNO3 2) Fizzing 3) CO32- + 2H+ -----> CO2 + H2O

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For SO42- test state the..... 1) Reagent 2) Observation 3) Equation

1) Ba(NO3)2 or BaCl2
2) White ppt
3) SO42-(aq) + Ba2+(aq) -------> BaSO4(s)

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For Halide test state the reagent

AgNO3 with Ammonia(NH3)

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In Qualitative Analysis, Describe the method for NH4+

1) Add OH- to sample
2) Heat to produce NH3(g)
3) Hold Damp Red Litmus paper
4) Litmus paper should go blue due to NH3

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For NH4+ test state the..... 1) Reagent 2) Observation 3) Equation

1) NaOH(aq) with heat
2) Damp red litmus turns blue
3) NH4+ OH- ------> NH3 + H20

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Suggest why we do the Qualitative Analysis in the order Carbonate to Sulfate to Halide

1) Neither sulfate or halide produce with dilute acic to make a gas so we use Carbonate test to produce gas
2) CO32- ions also react with Ba2+ = white ppt so sulfate test carried out later
3) Both Carbonate and Sulfate give white ppt with Ag+

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State the formulas when Cr3+ and [Cr(H20)6]2+ reacts with NaOH

1) Cr3+ 3OH- ------> Cr(OH)3
2) [Cr(H20)6]2+ +6OH- ------> [Cr(OH)6]3- + 6H20

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In Enthalpy of Neutralisation describe the method for it

1) Using measuring cylinder measure out a known volume of a known conc of your acid and base
2) Allow solutions to reach same temperature by leaving them for 3 mins
3) Measure temp of both solutions using thermometer
4) Place acid in Poly cup

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In Enthalpy of Neutralisation describe the FINAL STEP FOR IT!!!

5) Carefully add base to acid in polystyrene cup 6) Record Max Temp reached

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Describe the steps for plotting Enthalpy of Neutralisation graph

1) Plot Time Vs Graph
2) Extrapolate on both sides in T mix line to find Temp change
3) Apply = Q=-mcAT
4) Calc n(H20)
5) Do Enthalpy change = (Q divided by mol of H20)

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State the units for specific heat capacity 2) What sign do we add in the equation to help calc be easier 3) What do we do at the end of the calc of Q = AH/n

1) c = J/K/g 2) DO Q = MINUS mass times Delta Temp 3) Divide by 1000 to make units from J/mol into KJ/mol

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State the sources of error for Enthalpy change practicals

1) Heat lost to surroundings
2) NOT under Standard Conditions
3) Incomplete Combustion

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State what Reflux?

Continuous evaporation and condensation

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State what does Distillation do?

1) Separates out liquid with lowest bp
2) After using separating funnel re-distil the purified liquid
3) Collect at the Bp of your desired product

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Suggest why do we draw the thermometer in distillation at the T-Junction (where gas product collected)

Allows to know the bp and collect at specific temperature

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In Reflux answer the following :- 1) Why do we use anti bumping granules 2) Why do we use heat mantle 3) Why is the top of liebig condenser

1) Prevent large bubbles from forming
2) Reactants are flammable so can be dangerous 3) Prevent pressure building up

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In Distillation what Decimal Place does the reading need to be and what number at the end?

One Decimal Place and either 0 or 5

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Describe the steps for Purification of an ORGANIC LIQUID

1) Place organic and aqueous layer in Separating funnel
2) Add Na2CO3 to remove acid impurities
3) Remove Organic layer
4) Add drying agent like CaCl2
5) Re-distill collecting at the boiling point of the desired product

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State three types of methods of checking purity of a product (3 marks)

1) Measure melting point and compare with known values
2) Run a TLC to calculate Rf value
3) Compare to known data and if accurate the values should be similar 4) Run a NMR and compare with spectra of known sample and should have similar peaks

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Describe the method of Water Crystallisation (3 marks)

1) Heat hydrated solid for 2 mins
2) Cool and then weigh
3) Repeat till heated to a constant mass when all water has evaporated
4) Subtract mass of Anhydrous solid (after heating) from hydrated solid (before heating) to get mass of water

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How do we calc XH20 (2 marks)

4) Subtract mass of Anhydrous solid (after heating) from hydrated solid (before heating) to get mass of water 5) Divide by Mr of H20 to get number for x

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Describe the method for the method for pH measurement? (3 marks)

1) Add Indicator e.g like phenolphthalein.
2) Mesure pH of solutions using a pH meter
3) Measure change of pH of solutions on addition of small amount of acid/alkali using a pH meter

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In Electrochemical cells use the following equation as an example
Cu2+ -----> Cu
Zn-------> Zn2+
1) What will happen if we increase the concentration of Cu2+ ions
2) What will happen if we decrease the concentration of Cu2+ ions

1)The Cell Potential would increase
2) The Cell Potential would decrease
(Remember Reduction side causes increase in Cell Potential just remember opposite style)

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In Electrochemical cells what happens if we increase the temperature

Cell Potential Decreases

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Describe the practical for Iodine Clock Test

1) Use burette to measure out volumes of metal Iodide (e.g Potassium Iodide) and Thiosulfate and starch indicator and place in test tube
2) Measure out volume of Metal Peroxydisulfate
3) Add Metal Peroxydisulfate to other test tube
4) Use a stop watch t

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State the units that would be used for the reaction if drawn on a graph

X axis - Conc of H20
Y- axis - 1/T

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For Continuous Method describe what would u do with the results when finished continuous monitoring method

1) Plot a conc time graph
2) Use half life to determine order with respect to the reactant
3) Don't forget to include both the stop watch and which time interval you would record the mass/volume
4) This can be done using gas syringes

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When calculating half life what is one important thing that u MUST DO

Work out order given a conc/time graph by calculating the half-life : should be constant and FIRST ORDER!!!.
MUST SHOW AT LEAST TWO HALF LIVES on graph and state the value

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A level Chemsitry OCR A Practicals

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