Higher Chemistry Quality Control NEW LEARNING Chromatography Back titration REVISION Titration Separating techniques Colorimetry (flame tests)

Starter Questions S3 Revision

Starter Questions S3 Revision

Starter Questions S3 Revision

Starter Questions S3 Revision

Lesson 1: Chromatography Today we will learn to Identify the components of a mixture using chromatography. We will do this by Explaining how different types of chromatography work and using a card game to learn key terms. We will have succeeded if We can define retention time, stationary phase, mobile phase and explain their role in analysis.

Paper Chromatography p39 During paper chromatography a small sample of the mixture being tested is spotted onto the base line (a straight line usually drawn in pencil above the level of the solvent) on the filter paper. The filter paper is then placed in a solvent. Components can move quickly or slowly up the paper depending on the solvent used, polarity and therefore solubility and the attraction for the paper. The solvent is the mobile phase in paper chromatography and the wet paper is the stationary phase.

Retention factor Value p39 The Rf factor can be used to compare the different components found in a sample. The Rf values of a mixture being tested can be compared with known samples. Note: if two substances on chromatography paper have the same Rf value it is likely that they are the same compound. If they do not have the same Rf value then they are definitely different compounds. Rf = distance from the base line to the spot distance from the base line to the solvent front The Rf value of the red, green and blue particles can be measured and calculated to prove that certain molecules are present in the mixture.

GC, TLC and HPLC!! p39 GC : Gas chromatography. Gaseous samples are carried over a solid in a tube or column by another ‘solvent’ or carrier gas. TLC : Thin Layer Chromatography. Similar to paper chromatography, but the stationary phase is a thin film on top of a glass or silica plate. HPLC : High pressure liquid chromatography. Liquid samples in a solvent passed over a silica solid in a tube or column. The principles of phase and Rf are the same as in paper chromatography.

ConsolidationTask Match the correct answers to the questions on the cards. S3 Revision

Starter Questions C(graphite) + O2(g) → CO2(g) ΔH = −394 kJ mol−1 C(diamond) + O2(g) → CO2(g) ΔH = −395 kJ mol−1 What is the enthalpy change, in kJ mol−1, for the conversion of one mole of graphite into one mole of diamond? A +789 B +1 C −1 D −789

Starter Questions A 0·10 mol l−1 solution could be prepared most accurately from a 1·0 mol l−1 solution using A a 1 cm3 dropping pipette and a 10 cm3 measuring cylinder B a 10 cm3 measuring cylinder and a 100 cm3 volumetric flask C a 25 cm3 pipette and a 250 cm3 volumetric flask D a 50 cm3 burette and a 500 cm3 measuring cylinder. [

Lesson 2: Determination of Aspirin Today we will learn to Use a back titration to measure purity. We will do this by Reacting aspirin with an alkali to hydrolyse and neutralise it, then back titrating with acid. We will have succeeded if We can calculate the purity of the aspirin tablet.

Aspirin Titration p40 Aspirin has the following formula Since it is insoluble in water, it cannot be directly titrated using an alkali, which would react in the ratio one mole to one mole, with the COOH group on the molecule. A back titration has to be done. This involves treating the aspirin with an excess of NaOH solution of accurately known concentration. The alkali catalyses the breakdown of aspirin to salicylic acid and ethanoic acid. It then neutralises these acids, which are present in equal numbers of moles. The amount of alkali remaining after the reaction is found by titration with standard sulphuric acid solution.

Aspirin Titration p40 A known mass of aspirin is added to a known volume of 1.0 mol l-1 NaOH. Water is added and the mixture is heated for 30 minutes. The mixture is then made up to 250 cm3 in a standard flask. Samples of this alkaline solution are titrated with 0.05 mol l-1 sulphuric acid. Phenolphthalein is used as an indicator. The volume and concentration of acid allow you to calculate the concentration of the NaOH remaining. This lets you find the moles NaOH left in the 250 cm3. This allows calculation of the number of moles of NaOH which have reacted. This lets you calculate the number of moles of aspirin which were present. Assume that one mole of aspirin is equivalent to 2 moles of NaOH.

Calculation p40 1. Use all the CONCORDANT results to calculate your average titre. 2. Calculate how many moles of acid you have used (the acid was 0.05moles per litre). 3. Calculate how many moles of alkali this reacted with. 4 Subtract from your initial moles of NaOH used – this will tell you how much NaOH reacted with the aspirin. 5 Multiply by 0.5 as 1 mole aspirin reacts with 2 moles NaOH. 6 Convert to grams of aspirin to find out how much was in a tablet. 7 Calculate the % aspirin in a tablet.

Definition Choose three new words you have learnt in this topic and write dictionary definitions.