- Created by: beccihoward
- Created on: 21-06-17 15:47
Required Practical 1: Acid-Base Titrations
- Substance of unknown conc. goes in conical flask.
- Use a conical flask because it is easier to swirl without spilling.
- Use 25cm^3 pipette/pipette filler to accurately measure amount to put in conical flask.
- Standard solution is known mass of substance of unknown conc. being made up to 250cm^3 using distilled water. Measure in volumetric flask.
- Ensure meniscus is on line.
- Rinse equipment with distilled water, then substance that will be put in it.
- Readings to 2 d.p, with last digit a 0 or 5.
- Ensure air bubbles are removed.
- Phenolphthalein is pink in alkali and colourless in acid.
- Methyl orange is yellow in alkali and red in acid.
- Do a rough run, then repeats until 2 concordant titres found (within 0.1 of eachother).
- Use average titre in calculations.
- Distilled water can be used to wash edges of conical flask to ensure all acid and base react. This will not affect amount reacting.
- Corrosive reactants: Wear gloves and goggles. Wash spilt substances immediately.
- Unknown substances: treat as toxic.
- Each piece of apparatus has a sensitivity error.
Error of equipment is equal to sensitivity error/measurement made x100.
- Remember to times sensitivity error by two if two measurements made. For example, in titrations.
- Overall error is all the equipment errors added up.
- When reducing errors, think about things that lower sensitivity errors, like making bigger measurements, or making bigger measurements. Use the equation to help.
- Sensitivity errors: cause each reading to be different from the true value by the same amount - equipment error.
- Random errors: readings different to true value in an unpredictable way - human error.
Converting mol dm^-3 to g dm^-3
- Multiply concentration by Mr.
Required Practical 2: Enthalpy change
- Could be measurement of enthalpy of combustion using calorimeter method - burning fuel.
- This can be inaccurate because of heat loss to surroundings, and loss of volatile, flammable liquids by evaporation.
- Could also be measurement of enthalpy change of solution. Using a polysterene cup and creating a graph with two lines of best fit to get accurate temperature change.
- Energy change = -q/n
Required Practical 3: Rate of reaction & temp
- Eg. 1. Timing how long a precipitate takes to form (however this is subjective so not reliable).
- Eg. 2. Measuring decrease in mass as an indication of rate.
- Eg. 3. Measuring gas given off using a gas syringe. This is an indication of rate. Very accurate measurements.
- Eg. 4. Cross method using sodium thiosulphate and hydrochloric acid. Time how long it takes for precipitate to form so cross underneath is no longer seen.
- Carry these out at different temperatures to see how rate is affected.
Required Practical 4: Tests for cations and anions
- Acidify with dilute nitric acid.
- Test with silver nitrate, dilute ammonia and concentrated ammonia (use fume cupboard).
- Add ammonium ions. Ammonia gas will be produced.
- Test for ammonia gas; distinct smell, red litmus goes blue, dip hydrochloric acid, using a glass rod, into test tube and a white smoke is formed.
- Dilute hydrochloric acid
- Pipette gas formed into test tube of lime water and bubble it through.
- White precipitate formed.
Required Practical 4: Tests for cations and anions
- Add dilute hydrochloric acid.
- Add barium chloride.
- White precipitate formed.
Notes: acids often added to stop other ions interfering in the test. Sulphate ion test links to solubility of the group 2 metals.
- Add sodium hydroxide.
- Ammonia produced.
Required Practical 5: Distillation
- Eg. Oxidation of ethanol to ethanal. Product must be collected before it is oxidised to ethanoic acid.
- Use anti bumping granules.
- Oxidising agent: acidified potassium dichromate.
- Solution colour change from orange to green.
- Fruity smell.
Required Practical 6: More tests...
- Positive test: gentle fizzing
- Negative test: no fizzing or violent fizzing
- Safety - ensure no water or acid in sample.
Distinguis primary and secondary alcohols from tertiary alcohols
- Acidified potassium dichromate.
- Primary/secondary alcohols: will be oxidised so colour change of orange to green.
- Tertiary alcohols: will not be oxidised so no colour change.
- Tollen's reagent: silver mirror
- Fehling's solution: blue to brick red colour change
Required Practical 6: More tests... (2)
- Bromine water
- Positive test: pale yellow to clear solution
- Solid sodium hydrogencarbonate
- Positive result: fizzing and CO2 produced.
- Stopping a reaction from continuing any further.
- Useful if measure rate of reaction at a set time.
- One way can be to remove the reagent. For example if it is an acid, react it with a base, or vice versa.
- Quickly cooling the mixture to stop reaction.
- Discrete: certain values on a scale.
- Continuous: any value on a scale.
- Categoric: values can be sorted into categories.
- Ordered: when categories can be put in order.
Seperates solids from liquids.
Method 1: funnel and filter paper
Method 2: filtration under reduced pressure
- uses Buchner apparatus
Method 3: decanting
- allows solid and liquid or two immiscible liquids to be seperated
- allows heavier substance to sink to the bottom and lighter to be drained off
- Centrifuge can speed up process
Method 4: evaporation
- evaporating substances off
- eg. water
Heating under reflux
So substances can be fully oxidised.
- You can react an unknown solid in excess acid
- React 25cm^3 of acid with base to work out number of moles used.
- Multiply up to get moles that react with solid
If experimental error is greater than the total apparatus error, then the results are inaccurate.
Testing the purity of a product
- See what temperature the product boils or melts at.
- Compare to data book value.
- Closer to correct value, and the smaller the range the product melts/boils at. the more pure the product.
Safety when using potassium cyanide
- Extremely dangerous if ingested or inhaled
- It can react with moisture to produce toxic HCN gas
- Wear gloves, goggles and a lab coat
- Use a fume cupboard
Purifying organic compounds
- Add an anhydrous salt
- Eg. Calcium chloride, Magnesium sulphate
Remove impurities by washing
- Product contaminated with left over reagents or side products.
- Eg. use aqueous sodium hydrogencarbonate to remove acid
- Dissolve impure solid in just enough hot solvent so that solution is saturated
- Filter through heated filter to remove insoluble impurities (so it doesn't crystalise too early)
- Leave to cool. Crystals of product form.
- Filter under reduced pressure to remove soluble impurities in liquid.
- Wash with ice cold solvent to remove any soluble impurities on surface (cold so crystals don't dissolve again).
- Leave to dry. Can use paper towel to dry quicker.
Required Practical 7: Rate of reaction
- Meausure change in conc. over a period of time early on in the reaction
- Plot graph and measure gradient of tangent to get close approximation to initial rate
- Repeat this with different conditions that you are investigating (eg. temp)
- Can use this to work out order of reaction/rate equation
- Measuring conc. of product/reactant at different time intervals
- Make sure to quench reaction
- Carry out a titration so conc. can be calcultated
- Can also track a reaction where a colour change occurs with a colorimeter
- Concentration is proportional to light absorbed.
Required Practical 8: Electrochemical cell EMF
- Standard electrode potentials measured in: 100kPa, 298K, zero current and concentrations are 1.0 mol dm^-3 for all ions
- Voltmeter with a very high resistance
- Salt bridge to allow ions to flow and complete circuit:
- Soak chromatography paper in saturated solution of potassium nitrate
- Pick different solution if potassium nitrate would react with anything in cells
- Ecell = Eright - Eleft
- Ecell = Ereduced - Eoxidised
- When you read the voltmeter, if the value is positive the cells are in standard arrangement.
- If voltage value is negative, cell is not in standard arrangement.
Required Practical 9: pH changes in titrations
Investigating how pH changes when a weak acid reacts with a strong base and when a strong acid reacts with a weak base
- Remember the graphs learnt in theory lessons.
- Using a pH meter to measure pH at varying volumes, using rough run as a guide.
- Carry on beyond the end point until the volume at the end point is approximately doubled.
Required Practical 11: Transition metal ions
See table to learn
Required Practical 12: Thin Layer Chromatography
- Rf values
- Stationary phase: thin layer of silica
- Mobile phase: solvent
- Rf values are specific to conditions so ensure data book values you are comparing to were obtained under the same conditions your Rf value was obtained under
- Watch glass prevents evaporation of solvent
- Dry the chromatography paper in the fume cupboard
- Can use UV light to look at spots if they are hard to see.
Required Practical 10: Preparations
Preparation of a pure organic solid and test of its purity (eg. aspirin)
- React substances to produce the product and filter off using Buchner funnel under reduced pressure
- Recrystallisation technique to purify solid
- Test its purity by...
- Melting substance and seeing how close it is to the data book value for the melting point. Also noting if it occurs over a narrow or wide range of temperatures.
- Infrared spectrum would show impurities in the fingerprint region compared to known sample of aspirin.
Preparation of a pure organic liquid (eg. ethyl ethanoate)
- Know difference between reflux and distillation apparatus.
- For ethyl ethanoate - heat under reflux.
- Safety: using heating mantle instead of bunsen as ethanol is flammable.
- Seperating funnel technique for two immiscible liquids.
- Distill product.