Explain how you would make up a volumetric solution?
- Weigh out a precise amount of solid
- Add to a small volume of distilled water until it is all dissolved
- Transfer to a volumetric flask
- Wash beaker, stirring road and filter funnel with distilled water, pouring this into the volumetric flask
- Fill the remainder of the flask (to the mark) with dsitilled water
- Add stopper and invert flask to mix contents
What is the calculation for percentage uncertainty?
% uncertainty = uncertainty / measured value x 100%
In what situations is % uncertainty doubled?
When you are adding or subtracting two measurements:
- Balance to measure the initial and final mass
- Thermometer for the measurement of ΔT
- Burette to find the initial reading and final reading
Describe how you would carry out an experiment to determine the enthalpy of solution
of anhydrous magnesium chloride.
Explain how your results could be used to calculate the enthalpy of solution.
- Measure 100cm3 of water using a measuring cylinder
- Pour into ploystyrene cup with lid
- Record initial temperature of the cup for 3 minutes
- Then at the 4th minute add a known mass of MgCl2, measuring the mass of the weighing bottle before and after adding the solid
- Then record T at regular intervals until a trend is observed
- Plot a graph or time against temperature
- Extrapolate the lines to when the solid was added at 4 mins
- Find the temperature change by taking the initial temperature from the final temperature
- Then use q = mcΔT and ΔH = q/n to find the enthalp of solution
How is data processed in an enthalpy change for reactions in solution experiment?
- Temperature correction graph plotted (temperature-time)
- Extrapolate line to get ΔT
What are the main sources of error in a calorimetry experiment?
- Heat loss to the environment
- Incomplete combustion
Describe the reaction of group 2 elements Berylium to Barium with oxygen.
think: general equation, flame
2M (s) + O2 (g) → 2MO (s)
Ca, Sr = red flames
Ba = green flame
Mg, Be = white flame
(Mg only one to burn easily, bright white flame)
Describe the reaction of group 2 elements Berylium to Barium with water.
think: general equation and trend
M (s) + 2H2O (l) → M(OH)2 (s) + H2 (g)
Reactivity increases down group:
- Be does not react with water
- Mg only reacts with steam
- Ca, Sr, Ba react with water
Describe the reaction of group 2 elements Berylium to Barium with dilute HCl.
think: general equation and reactivity
M (s) + 2HCl (aq) → MCl2 (aq) + H2 (g)
All react vigorously with HCl
Describe the reaction of group 2 elements Berylium to Barium with dilute H2SO4.
think: general equation and reactivity
M (s) + H2SO4 (aq) → MSO4 (aq) + H2 (g)
- Be, Mg react vigorously
- Reaction with Ca slowly stopped by formation of sparingly soluble sulfate
- Reaction with Sr and Ba quickly stopped by formation of insoluble sulfates
Describe how to test for group 2 ions.
NH3 solution:
Mg = white ppt
Excess NaOH:
Mg, Ca = white ppt
Excess H2SO4:
Sr, Ba = white ppt
Ca = slight white ppt
How do you test for ammonium ions?
- Add NaOH
- Heat and then test gas with damp red litmus which turns blue
How do you test for the halide ions?
Add acidified AgNO3:
White ppt = AgCl
Cream ppt = AgBa
Yellow ppt = AgI
Then add dilte/conc NH3 (aq)
Dilute = AgCl dissolves
Conc. = AgBr dissolves
How do you test for carbonate ions?
- Add HCl
- Test any gas produced using Ca(OH)2, limewater turns cloudy in presence of CO2
How do you test for sulfate ions?
- Acidify with dilute HCl then add a few drops of BaCl2 (aq)
- White ppt forms
Describe how you would measure the rate of reaction by:
a) an initial rate method.
b) a continuous monitoring method.
a) Gas syringe
- plot a graph of change in volume over time
- draw tangenet at t=0 and calculate gradient
b) Quenching
- quench the samples of reaction mixtures at regular intervals (neutralise in acid/base catalyst, cool or dilute)
- titrate each solution that is quenched to calculate concentration
Suggest briefly a practical procedure that a student could use to obtain data from which the curve in a graph of decreasing pH could be plotted.
- Add a fixed volume of alkali to a conical flask using a graduated pipette
- Then add a controlled amount of acid to the flask bit by bit using a burette
- Stir and use a pH meter to record the pH after every addition
What is the midpoint of the inflection of an acid-base curve called?
Equivalence/stochiometric point
How do you purify an organic solid by recrystallisation?
- Dissolve the solid in the minimum volume of hot solvent
- Filter whilst hot
- Allow filtrate to cool and crystals to form
- Filter crystals under reduced pressure using a buchner funnel
- Rinse crystals with a small volume of cold solvent
- Dry crystals between filter paper
Why are the following steps done in a recrystallisation practical?
a) The minimum quantity of hot solvent used
b) The flask was cooled to room temperature before the crystals were filtered off
c) The crystals were compressed in the funnel
d) A little cold water was poured through the crystals
a) Ensures the whole of the hot solution is saturated, allowing crystals to form on cooling
b) Yield of crystals is lower if the temperature is warmer as solubility is higher
c) Forces air through the crystal sample to enable better drying
d) Dissolves soluble impurities
How can you test the purity of an organic solid?
- Measure melting point using melting point apparatus
- Melting point is sharper with a more pure organic product (smaller range of temperatures for which it melts)
- Melting point is closer to the actual value (found in data book) for more pure sample
How do you seperate an organic liquid?
- Mixture added to seperating funnel with sodium carbonate (neutralises any unreacted acid and helps seperation)
- Add stopper and invert, open to release pressure
- Two layers allowed to seperate over time
- Denser layer (usually aqueous) on bottom, less dense layer (usually organic) on top
- Sodium sulfate added to absorb water
What are the main parts of the TLC paper?
- Stationary phase
- Mobile phase
- Baseline
- Sample compound
- Solvent front
-
3.1.1 Atomic structure20
-
3.1.2 Amount of substance4
-
3.1.3 Bonding26
-
3.1.4 Energetics8
-
3.1.5 Kinetics15
-
3.1.6 Chemical equilibria and Kc11
-
3.1.7 Redox6
-
3.1.8 Thermodynamics19
-
3.1.9 Rate equations8
-
3.1.10 Equilibrium constant Kp3
-
3.1.11 Electrode potentials and electrochemical cells24
-
3.1.12 Acids and bases19
-
3.2.1 Classification7
-
3.2.2 Group 29
-
3.2.3 Group 718
-
3.2.4 Period 3 elements and their oxides12
-
3.2.5 Transition metals43
-
3.2.6 Reactions of ions in aqueous solution12
-
3.3.1 Introduction to organic chemistry, 3.3.7 Optical isomers8
-
3.3.2-4 Alkanes, Haloalkanes and Alkenes20
-
3.2.5 Alcohols11
-
3.3.6 Organic analysis8
-
3.3.8-9 Ketones, aldehydes, carboxylic acids and derivatives21
-
3.3.10-11 Aromatics and amines25
-
3.3.12-13 Polymers, DNA, proteins and amino acids25
-
3.3.14 Organic synthesis25
-
3.3.15-16 NMR and chromatography14
-
Required practicals23
