Practicals Flashcards
Potential errors in using a gas syringe
- gas escapes before bung inserted
- syringe sticks
- some gases (CO2 and SO2) are soluble in water so true amount not measured
Why is the burette rinsed in titration?
because the acid or alkali may be diluted by residual water in it or may react with substances left from before
Why is only water used to rinse conical flasks between titrations?
because it doesn’t add any extra moles of reagents
What is considered concordant titre results?
those within 0.10 cm3
Sensitivity uncertainty of balance
+/- 0.001g
Sensitivity uncertainty of volumetric flask
+/- 0.1 cm3
Sensitivity uncertainty of 25cm3 pipette
+/- 0.1 cm3
Sensitivity uncertainty of burette
+/- 0.15 cm3
Equation for percentage uncertainty
+/- (uncertainty/ measurement made) x 100
How to reduce uncertainties in titration- apparatus
- replace measuring cylinders for pipettes or burettes
How to reduce uncertainty in burette reading
Make the titre a larger volume by increasing the vol and conc of a substance in the conical flask or decreasing the conc of the substance in the burette
Reducing uncertainties in measuring mass
use a balance that measures to more dp or use a larger mass of a solid
In a calorimetry experiment, what is an issue if the reaction is slow?
the exact temp rise is difficult to obtain as cooling is occurring simultaneously
What do we do to counteract this?
take readings at regular intervals and extrapolate the temp curve/line back to the time the reactants were added together
Errors in the calorimetry method
-energy transfer to surroundings
-assumes all solutions have same specific heat cap/density of H2O
-neglecting specific heat cap of the calorimeter
-reaction may be incomplete/slow
-
General method to calculate the enthalpy change of reaction
1- use q=mcAT
2- work out moles of reactants used
3- divide q by the no of moles of reactant not in excess to get AH
4- add sign and unit
How to measure reaction rate in sodium thiosulfate and HCl experiment
1/time
the time is the time taken for the cross to disappear
Why does the cross disappear in sodium thiosulfate and HCl experiment?
due to cloudiness of sulfur
InK= constant -Ea/RT
k is proportional to rate. What adjustments can we make?
Replace InK for In(rate_
What is magnesium hydroxide classed as in water? How does it appear?
insoluble- a white ppt, slightly alkaline (pH 9) due to very slight dissolving
Mg2+ (aq) + 2OH-(aq) –> Mg(OH)2
(s).
What is calcium hydroxide classed as in water? How does it appear?
partially soluble- white ppt, more alkaline (pH 11) than magnesium
What is the test for group 2 ions?
add dilute sodium hydroxide
Test for ammonia gas
turns red litmus paper blue
Test for sulfate ion
-BaCl2 solution acidified with HCl used as the reagent
-Added to sample
-When added to a solution with sulfate ions, a white ppt of Barium sulfate forms
Ba2+ (aq) + SO4
2-(aq) –> BaSO4
(s)
Why is HCl needed for test for sulfate ions?
to react with carbonate impurities often found in salts which would form a white barium carbonate ppt
Why is H2SO4 not use in the test for sulfate ions?
it contains sulfate ions- false positive
Test for halide ions with silver nitrate
- test solution is made acidic with nitric acid (react with carbonates to avoid formation of Ag2CO3 ppt)
- then silver nitrate added dropwise
Test for halide ions with silver nitrate- results
-Fluorides produce no precipitate
-Chlorides produce a white precipitate
Ag+
(aq) + Cl- (aq) –> AgCl(s)
-Bromides produce a cream precipitate
Ag+
(aq) + Br- (aq) –> AgBr(s)
-Iodides produce a pale yellow precipitate
Ag+
(aq) + I- (aq) –> AgI(
Test for carbonate ions
- add any dilute acid- effervescence
- bubble gas through limewater to test for CO2 (limewater turns cloudy)
Why does the reducing power of halides increase going down the group? What reaction is used to test this?
The ions get bigger so it is easier to give away outer electrons as the pull from the nucleus gets weaker.
-Reaction with conc sulfuric acid
Fluoride and Chloride with conc sulfuric acid
Not strong enough reducing agents to reduce the S in H2SO4 so redox
reactions occur. Only acid-base reactions occur. (H2SO4 is an acid)
Fluoride and conc H2SO4- equ and obs
NaF(s) + H2SO4
(l) –>NaHSO4
(s) + HF(g)
Observations: White steamy fumes of HF are evolved
Chloride and conc H2SO4- equ and obs
NaCl(s) + H2SO4
(l) –> NaHSO4
(s) + HCl(g)
Observations: White steamy fumes of HCl are evolved.
Bromide and conc. H2SO4- equ and obs
NaBr(s) + H2SO4 (l) --> NaHSO4 (s) + HBr(g) Observations: White steamy fumes of HBr are evolved.
Partial oxidation of primary alcohols- product, reagent, conditions and observations
P= aldehyde
R= potassium dichromate solution and dilute sulfuric acid
C= limited amount of dichromate, warm gently and distil out the aldehyde
O= : the orange
dichromate ion (Cr2O7
2-) reduces
to the green Cr3+
Full oxidation of primary alcohols- product, reagent, conditions and observations
P= carboxylic acid
R=potassium/sodium dichromate solution and sulfuric acid
C= excess of dichromate, heat under reflux
O= orange
dichromate ion (Cr2O7
2-) reduces
to the green Cr3+
Why are anti-bumping granules used?
to prevent vigorous, uneven boiling by making small bubbles to form instead of large ones
How is tollen’s reagent made?
mixing aqueous ammonia and silver nitrate
On a conc vs time graph, how can you calculate the initial rate?
take the gradient at time= 0
Recrystallisation: causes of loss of yield
- crystals lost when filtering or washing
- some product stays in solution after recrystallisation
- other side reactions occurring
