Cramming Chemistry Paper 3
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Errors in calorimetry method (5 marks)
• Energy transfer from surroundings (usually loss) • Approximation in specific heat capacity of solution (and the assumption that all solutions have the heat capacity of water) • Neglecting the specific heat capacity of the calorimeter- energy absorbed by apparatus is ignored • Reaction or dissolving may be incomplete or slow • Density of solution is taken to be the same as water
Errors in spirit burner calorimetry experiment (5 marks)
• Energy losses from calorimeter • Incomplete combustion of fuel • Incomplete transfer of energy • Evaporation of fuel after weighing • Heat capacity of calorimeter not included
Test for group 2 metals (3 marks)
Sodium hydroxide in excess: • Hydroxides become more soluble down the group so ppt formation –> no ppt • Also becomes more alkaline down the group OR Sulfuric acid: - no ppt –> white ppt formation
Tests for aluminium, copper and iron (4 marks)
Sodium hydroxide: • Aluminium: white precipitate which dissolves in excess NaOH • Copper: blue precipitate • Iron: green precipitate • Iron: brown precipitate
Test for ammonium ions (3 marks)
• Add NaOH, shake, then warm in a water bath • Damp red litmus paper turns blue • Due to formation of alkaline ammonia gas
Test for sulphates (2 marks)
• Barium chloride acidified with dilute HCl - White precipitate forms
During the test for sulfates, what would be observed if carbonate impurities were present? Give an equation (2 marks)
• Fizzing due to CO₂ - 2HCl + Na₂CO₃ –> 2NaCl + H₂O + CO₂
Test for carbonate ions (2 marks)
• Any dilute acid - Effervescence
Test for hydroxide ions (1 mark)
• Red litmus paper turns blue
Test for ethanal (3 marks)
• Tollen’s Reagent • Warm • Silver mirror ppt
In TLC, why should the solvent be below the origin line (1 mark)
• If it were above the origin line, it would dissolve the mixture / amino acids
Explain why amino acids have different Rf values (2 marks)
• Different polarities - So different retention on the stationary phase OR different solubility in the developing solvent
Explain the difference in the strengths of ethanoic acid and ethanedioic acid (6 marks)
Difference in structure of the two acids • The acids are of the form RCOOH • but in ethanoic acid R = CH3 • whilst in ethanedioic acid R = COOH The inductive effect • The unionised COOH group contains two very electronegative oxygen atoms • Therefore has a negative inductive (electron withdrawing) effect • The CH3 group has a positive inductive (electron pushing) effect How the polarity of OH affects acid strength • The O–H bond in the ethanedioic acid is more polarised / H becomes more δ+ • More dissociation into H+ ions • Ethanedioic acid is stronger than ethanoic acid
The initial rate of the reaction between sodium thiosulfate and HCl can be monitored by measuring the time taken for a fixed amount of sulfur to be produced. Describe and experiment to investigate the effect of temperature on the initial rate of reaction.
Method • Idea of using disappearing cross or colorimetry • Puts acid or thiosulfate into container on/with cross or in colorimeter • Add second reactant and start timing Measurements • Repeat at different temperatures (if number of temperatures stated, must be more than two) • Record time, t, for cross to disappear / defined reading on colorimeter • Idea of ensuring other variables (cross, volumes, concentrations) kept constant (apart from T) Use of Results • 1/t (or 1000/time, etc) is a measure of rate • Plot of rate (or 1/t etc) (y-axis) against T (xaxis) (can come from labelled axes on sketch)