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Questions and Answers
(a) Explain, in terms of intermolecular forces, the differing solubilities of SO2 and I2 in water. (4 marks)
-iodine is NON-POLAR and contains DISPERSION forces -sulfur dioxide is POLAR and contains dispersion forces and dipole-dipole forces -water is HIGHLY POLAR and contains dispersion forces, dipole-dipole forces and hydrogen bonding -the type and strength of intermolecular forces within water and sulfur dioxide are more similar, therefore, the intermolecular forces of sulfur dioxide can disrupt the hydrogen bonding of the water to a GREATER EXTENT THAN IODINE
As suggested by the shape of the graph, no matter how high the pressure is increased, the volume of the gas will never become zero.
(a) Explain why this is so, referring to the difference between ‘ideal’ and ‘real’ gases in your answer.
-particles of an ideal gas have negligible volume whereas particles of a real gas do have volume -therefore, volume of a gas would never be 0 (under high pressures the gas would condense to form a liquid)
W is covalent molecular bonding, why
-covalent molecular substances exhibit weak intermolecular forces -therefore only a small amount of heat energy is required to overcome the intermolecular forces, resulting in a low boiling point (therefore most likely to be a liquid or gas at room temperature, rather than a solid)
Why is X metallic (is not brittle)
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describe how Calcium dihydrogenphosphate, Ca(H2PO4)2, and ammonium hydrogenphosphate, (NH4)2HPO4, could be distinguished using a flame test. Your answer should include an explanation of how an emission spectrum is produced in a flame test.
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One process used to increase the shelf life of milk is pasteurisation. The process of pasteurisation kills many of the bacteria that convert the lactose to lactic acid. (a) Explain, in terms of the collision theory, how the pasteurising process increases the shelf life of milk.
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Explain, in terms of the collision theory, how the refrigeration process increases the shelf life of milk.
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briefly explain the difference between addition and substitution reactions
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Describe how the process of distillation can be used to separate a mixture. (3 marks)
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(a) Explain why the dispersion forces in water and methane are of similar strength. (2 marks)
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(b) Explain why the boiling point of water and methane differ so greatly. (4 marks)
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High performance liquid chromatography (HPLC) can be used to test for the presence of different amino acids in food. This is generally done using ‘reverse-phase HPLC’ which uses a non-polar stationary phase in combination with a polar mobile phase.
(a) Explain how reverse-phase HPLC is able to separate the components of a substance. Your answer should refer to the role of both the stationary and mobile phases and the effect of component polarity on retention / elution time.
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What is an enzyme? Explain the function of an enzyme in terms of the collision theory.
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Study Notes
Intermolecular forces and Solubility
- SO2 is soluble in water due to its ability to form hydrogen bonds with water molecules. This occurs because sulfur dioxide forms sulfurous acid (H2SO3) when dissolved in water.
- I2 is only slightly soluble in water because it is a non-polar molecule and cannot form hydrogen bonds with water molecules. The only intermolecular forces present between iodine and water are weak London dispersion forces.
Ideal vs Real Gases
- Ideal gases are theoretical and assumed to have no intermolecular forces and occupy no volume.
- Real gases exhibit intermolecular forces, therefore they occupy a finite volume.
- As pressure increases, the gas particles are pushed closer together. At very high pressures, the intermolecular forces become significant and the gas deviates from ideal behaviour.
- Volume will never become zero because real gas molecules always occupy some space, no matter how high the pressure.
Covalent Molecular Bonding
- Covalent bonds are formed by the sharing of electrons between atoms.
- In covalent molecular substances, the molecules are held together by relatively weak intermolecular forces.
- This explains why W is covalent molecular bonding because it is brittle and its constituent molecules are weakly attracted to each other.
Metallic Bonding
- Metallic bonds are formed by the delocalisation of electrons within a lattice of positively charged metal ions.
- This delocalisation results in high electrical conductivity and explains why X is not brittle.
Flame Tests
- Flame tests involve heating a sample of the substance in a flame. The colour of the flame is due to the emission of light by excited electrons as they return to their ground state.
- Emission spectrum is produced when a substance is heated in a flame and the excited electrons emit light at specific wavelengths, producing a characteristic flame colour.
- Calcium dihydrogenphosphate (Ca(H2PO4)2) will produce a brick red flame.
- Ammonium hydrogenphosphate ((NH4)2HPO4) will produce a yellow flame.
Collision Theory and Milk Shelf Life
- Collision theory states that for a reaction to occur, particles must collide with sufficient energy (activation energy) and in the correct orientation.
- Pasteurization involves heating milk to a high temperature, which increases the rate of collisions between bacteria and heat, effectively killing the microorganisms.
- Refrigeration slows down the rate of reactions (including bacterial growth) by reducing the kinetic energy of the molecules involved, thereby lowering the frequency and effectiveness of collisions.
Addition and Substitution Reactions
- Addition reactions are the reaction where two or more molecules combine to form a single molecule.
- Substitution reactions are the reaction where an atom or group of atoms is replaced by another atom or group of atoms.
Distillation
- Distillation involves heating a mixture to evaporate the component with the lower boiling point.
- The vapour is then cooled and condensed, separating it from the remaining components in the mixture.
- This process relies on the difference in boiling points of the components in the mixture.
Dispersion Forces and Boiling Point
- Dispersion forces are weak intermolecular forces that arise from temporary fluctuations in electron distribution within molecules.
- Water and methane have similar dispersion forces because they have similar molecular sizes and masses.
- Water has hydrogen bonding, a much stronger intermolecular force than the dispersion forces present in methane.
- The stronger hydrogen bonding in water requires more energy to overcome, hence the higher boiling point compared to methane.
Reverse-Phase HPLC
- Reverse-phase HPLC uses a non-polar stationary phase and a polar mobile phase.
- Polar components in the sample will interact more strongly with the polar mobile phase, resulting in faster elution times.
- Non-polar components will interact more strongly with the non-polar stationary phase, resulting in slower elution times.
- The difference in retention (elution) times allows for the separation of the components.
Enzymes
- Enzymes are biological catalysts that speed up the rate of specific biochemical reactions without being consumed in the process.
- Enzymes work by lowering the activation energy required for a reaction to occur.
- Enzymes achieve this by providing an alternative pathway for the reaction with a lower activation energy, increasing the frequency and effectiveness of collisions between reactants.
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Description
solubilities, HPLC chromatography, types of bonding, addition/substitution reactions, ideal gases, STP, dispersion forces
