required practicals Flashcards
1
Q
what is the aim of the titration practical?
A
- make up a volumetric solution
- carry out a simple acid-base titration
2
Q
outline a 10 step method for making up a volumetric solution (titration practical)
A
- STEP 1: calculate the mass of required substance needed to produce 250cm cubes of a 0.100 moldm-3 solution
- STEP 2: weigh a clean dry weighing boat on a 2dp balance
- STEP 3: place the weighing boat on the pan of a digital balance & zero the balance. Using a spatula, place approximately the calculated mass of required substance into the bottle
- STEP 4: remove from the balance, set to zero & re-weigh the weighing boat & its contents. Record the value
- STEP 5: pour the contents of the weighing boat into a clean glass beaker & re-weigh the weighing boat, recording the value
- STEP 6: calculate the mass of required substance that you have transferred to the beaker (difference in two mass values)
- STEP 7: add approximately 100cm cubed of deionised water to the beaker containing the solid. Use a glass rod to stir the contents of the beaker until all of the solid has dissolved
- STEP 8: using a funnel, pour the contents of the beaker into a 250cm cubed volumetric flask. Using more deionised water in a wash bottle. Wash out the beaker & funnel, transferring the washings into the volumetric flask
- STEP 9: make the volumetric flask up to the graduated mark by carefully adding deionised water from a wash bottle
- STEP 10: put a stopper on the flask & invert it to combine thoroughly
3
Q
what kind of errors may be involved with making up a volumetric solution? (4 things) (titration practical)
A
- if using anhydrous sodium hydrogen sulfate make sure it isn’t too old as it will have picked up water so mass values may not be as accurate
- balance usually gives to 0.01g - systematic error as it is the same for each value recorded
- flask may be filled so that the bottom of the meniscus is not on the line on the neck of the flask
- alternatively the known mass of the solid in the weighing boat could be transferred to the beaker, washed & washings added to the beaker
4
Q
how do you reduce uncertainties in measuring mass for the volumetric solution? (titration practical)
A
- use a more accurate balance/larger mass to reduce the uncertainty in weighing a solid
- weighing the sample before & after addition then calculating the difference will ensure a more accurate measurement of the mass added
5
Q
give an 8 step method for the titration of the titration practical
A
- STEP 1: pour approximately 100cm cubed of the standard solution of known concentration into a beaker
- STEP 2: fill the butter with the standard solution of known concentration into
- STEP 3: pour approximately 100cm cubed of t the solution with unknown concentration into a second beaker
- STEP 4: using a pipette filler & pipette transfer exactly 25cm cubed of solution into a 250cm cubed conical flask
- STEP 5: add 2-3 drops of phenolphthalein indicator to the solution in the conical flask & note the initial colour of the indicator
- STEP 6: record the initial burette reading
- STEP 7: titration the contents of the conical flask by adding solution to it from the burette until the indicator undergoes a definite, permanent colour change. Record the final burette reading in a table of results. Calculate the titre volume (change in volume in the burette)
- STEP 8: repeat, calculate & record the volume of solution used in the titration in a table. Repeat until two concordant results are obtained. Record all of the results that you obtain
6
Q
what safety precautions are there in the titration practical?
A
- acids & alkalis are corrosive (at low concentrations acid are irritants) so wear eye protection & gloves. If spilled, immediately wash affected parts after the spillage
- if a substance is unknown, treat is as potentially toxic & wear gloves
7
Q
what needs to be considered if you are titration a mixture? (RP1)
A
- if it has other substances that have acid-base properties they could affect the reaction
8
Q
what type of uncertainty may be encounter when performing a titration? (RP1)
A
- uncertainty of a measurement from a burette
- if the burette used has an uncertainty of +/- 0.05 can cubed for each reading, two readings are taken to make the overall uncertainty +/- 0.10 cm cubed
9
Q
how can we reduce uncertainties when performing a titration? (RP1)
A
- replacing measuring cylinders with pipettes/burettes which have lower apparatus uncertainty will lower the overall error
- to reduce the uncertainty in a burette reading, the titre volume needs to be made larger. This could be done by increasing the volume & concentration of the substance in the conical flask, or by decreasing the concentration of the substance in the burette
- leaving NaOH in the burette will cause damage to the apparatus, which could lead to errors
10
Q
what is the aim for RP3? (rates of reaction)
A
- investigation of how the rate of reaction changes with temperature
- disappearing cross: change in rate of the reaction of sodium thiosulfate with hydrochloric acid as temperature is changed
- Na2S2O3 + 2HCl —> 2NaCk + SO2 + S + H2O
11
Q
outline a 9 step method for RP3 (rates of reaction)
A
- STEP 1: add about 10 cm of 1 moldm-3 hydrochloric acid to the ‘acid’ tube. Place this tube into a plastic container (without a cross under it)
- STEP 2: use a measuring cylinder to add 10.0cm of 0.05 moldm-3 sodium thiosulfate solution to the second tube. Place this tube into the plastic container with with the cross under it & carefully place a thermometer in the tube
- STEP 3: record the start temperature & then add 1 cm of the acid to the thiosulfate solution & start timing
- STEP 4: look down through the tube from above & record the time for the cross to disappear from view
- STEP 5: record the final temperature of the reaction mixture, then pour the cloudy contents of the tube into the sodium carbonate solution (this acts as a stop bath)
- STEP 6: now add water from a kettle to the plastic container. The water should be no hotter than 55˚C. Add cold water if necessary
- STEP 7: measure another 10.0cm of 0.05 moldm-3 sodium thiosulfate solution into a clean tube. Insert this tube into the correct hole in the plastic container (one with the cross under it)
- STEP 8: leave the tube to warm up for about 3 minutes
- STEP 9: repeat steps 3-6 in order to obtain results for at least 5 different temperatures in total
12
Q
what are some safety precautions that need to be taken in RP3? (rates of reaction)
A
- to minimise the escape of toxic sulfur dioxide during the experiment using s lid is advised. Two holes should be made in the lid using a hot cork borer. The holes should securely hold the glass vertically in the container. The experiment could also be carried out in a fume cupboard
- wear eye protection & handle HCl with care as it as an irritant
- ensure that the experiment is carried out in a well ventilated room & appropriate measures are taken to dispose of waste solutions
13
Q
how are stop baths used in RP3? (rates of reaction)
A
- containers of sodium carbonate solution & phenolphthalein should be available so that the acid & sulfur dioxide can be neutralised at any point during the experiment
- once the colour of the stop bath changes, the sodium carbonate has been used up & the stop bath will need to be replenished
- the stop bath should be places in a fume cupboard, if available
14
Q
outline the AS analysis for RP3 (5 things) (rates of reaction)
A
- in these experiments at different temperatures the concentration of all reactants are the same
- the time taken to produce the same amount of sulfur at different temperatures is an indication of rate of reaction
- a graph of the amount of sulfur produced against time can be plotted
- the initial rate of reaction = (amount of sulfur)/time so that the initial rate of reaction is proportional to 1/time
- this is an approximation for the rate of reaction as it des not include the concentration. This can be used because it is assumed that the amount of sulfur produced is fixed & constant
15
Q
what is the A-level analysis for RP3? (7 things) (rates of reaction)
A
- the rate constant for a reaction varies with temperature, according to the equation k = Ae -Ea/RT, where T is the temperature in kelvins
- taking natural logarithms —> In k = - Ea/RT (1/T) + In A
- in this experiment, the rate constant is directly proportional to 1/t therefore (In 1/t = - Ea/RT (1/T) + constant)
- plot a graph of In 1/t on the y axis against 1/T
- the graph should be a straight line with gradient - Ea/RT so measure the gradient
- calculate a value for the activation energy & give answer in kJ mol-1
- R - 8.31 JK-1 mol-1
