H2 Chemistry Practical Notes & Theory Flashcards

chem prac notes docs ver. theory flashcards: https://algoapp.ai/g/RuxnsIjgEOorkckc CHEMISTRY PRACTICAL NOTES General Notes : ● ≥3 points are needed for a BFL ● For graphs, one x-value to at most ONE y-value ● Include units for gradients when possible ● Include M r of H 2 O when calculating if the compound has water of crystallisation ● Number of moles of e - should be whole numbers ● Role of XXX in a reaction is usually a catalyst/oxidising agent/ reducing agent/acid/base ● During titration, rinse the inner side of the conical flask with deionised water near endpoint so that V total measured by the burette reacts with analyte in the conical flask (will NOT affect titre) ● ∆H neut = -57.3 kJ mol -1 for strong acid + strong alkali ● The purple fumes observed is iodine vapour which appears due to low solubility in water ● Do NOT record “ppt formed” if excess solid was used ● For I 2 /S 2 O 3 2- titrations, add starch indicator only when the solution is pale yellow (otherwise, titre will be very high) ● Remember to write “colourless, odourless…” for QA ● When H 2 O 2 is used, O 2 (g) is usually evolved as H 2 O 2 decomposes ● Starch + Iodine = blue- black solution ● NH 3 (g) and SO 2 (g) are soluble and do not effervescence ● Effervescence is fast, bubbles of gas is slow ● Water bath should be used for reflux, NOT bunsen burner Number of Significant Figures / Decimal Places : ● Burette — 2d.p. ○ Readings should end with .00 or .05! ● Pipette — 1d.p. ● Thermometer — 1d.p. (half of 0.2 °C) ● M r — 1d.p. (NO units!) Washing of Apparatus : ● Burette — tap water → deionised water → titrant ● Pipette — tap water → deionised water → analyte

● Conical/Volumetric Flask — tap water → deionised water Apparatus & Capacities Measuring Cylinder 10 cm 3 , 50 cm 3 , 100 cm 3 Beaker 100 cm 3 , 250 cm 3 , 500 cm 3 Conical Flask 50 cm 3 , 100 cm 3 , 250 cm 3 , 500 cm 3 Pipette 10.0 cm 3 , 25.0 cm 3 Common Indicators : Indicator Working pH range Colour of indicator in acidic medium end-point alkaline medium methyl orange pH 3 — 5 red orange yellow thymolphthalein pH 8 — 10 colourless pale blue blue Common Equations : ● 2S 2 O 3 2- → S 4 O 6 2- + 2e - ● Cr 2 O 7 2- + 14H + + 6e - → 2Cr 3+ + 7H 2 O ● MnO 4 - + 8H + + 5e - → Mn 2+ + 4H 2 O (acidic medium) ● MnO 4 - + 4H + + 3e - → MnO 2 + 2H 2 O (neutral/slightly alkaline medium) ● H 2 O 2 + 2H + + 2e - → 2H 2 O (O.A.) ● H 2 O 2 → O 2 + 2H + + 2e - (R.A.) ● CO 2 (g) in limewater: Ca(OH) 2 (aq) + CO 2 (g) → CaCO 3 (s) + H 2 O(l) ● Formation of NH 3 (g): NH 4 + (aq) + OH - (aq) → NH 3 (g) + H 2 O(l) Reaction Kinetics : ● Use rate = 1/t only when specified by the question ● Otherwise, rate = |gradient| of the graph at t = 0 ● Total volume should be constant so V reactant ∝ [reactant] Quenching Agents : ● Na 2 CO 3 /NaHCO 3 — bases quench reactions by neutralising the acidic catalyst, causing reaction to slow down

● Cold Water — ↓ temperature so ↓ average KE so ↓ rate of effective collisions and ↓ rate of reaction Things to Record for Qualitative Analysis Questions : 1. Colour of solid/solution before & after heating/dissolving 2. Colour of precipitate formed 3. Effervescence observed 4. Colour + Smell of the gas + Test & Identification of gas 5. Colour of filtrate & residue Note: When testing for anions, need to remove cations (jic they interfere) CAQs : Q Explain how you would ensure that all the acid is no longer present. A ● Add sodium carbonate until no further effervescence is observed ● Add alkali until indicator shows that the reaction mixture is neutral or not acidic Q Suggest two properties of a good solvent for recrystallisation

A ● The boiling point of the solvent is lower than the melting point of the substance to be crystallised ● Solubility of the substance in the solvent is high at high temperatures but low at low temperatures Q Will experimental results be more accurate if an instrument of a better precision is used to measure the reactant used in excess? A ● No, the precision of the volume measured for the reactant does not matter if it is used in excess Q Why do we need to dilute some reactants before adding them to the conical flask? A ● If 25.0 cm³ of those reactants were used, they require >50.00 cm³ of the titrant ● This exceeds the burette capacity, so we need to refill which results in a higher percentage error Q Will replacing a (strong acid) with a (weak acid) affect the enthalpy change? A ● T max is lower. XXX is a weak acid that partially dissociates in water ● Some of the energy released from the neutralisation is absorbed to bring about complete dissociation of XXX ● Hence, the calculated heat evolved will be smaller and the enthalpy change will be less negative Q Will doubling the volume of both reactants affect the temperature change? A ● No, because double the heat produced will cancel out with the double in volumes. Q = mc∆T Q A student suggested that it would have been better to calculate the enthalpy change for each experiment separately. What would be the advantage of this suggestion? Explain your answer fully. A ● Able to check whether further experiments should be carried out Q Why is the graphical method to determine ∆T max more accurate than a direct measurement of initial and maximum temperature reached? A ● The graphical method accounts for heat lost to the surroundings Q Why is water added to the reaction mixtures? A ● To keep the total reaction volume constant across all experiments

● This allows the concentration of reactants used to be directly proportional to its volume added Q Why is 1/t used as a measure of the rate of reaction? A ● Since the same end point is reached, it means that the same fixed amount of product is formed in each reaction ● Hence, 1/t is directly proportional to the rate of reaction Q (heat reactants before reaction experiment) An extra procedural step in the method is required to be able to determine an