OCR 21st Century C7.4,5

Reversible reactions and equilibria & Analysis
1. Nitrogen fixation
  1. A significant amount of nitrogen is fixed by the Haber process which converts hydrogen and nitrogen into ammonia
    1. Nitrogen fixing bacteria turns unreactive nitrogen from the atmosphere into useful compounds such as nitrates, ammonia and nitrogen dioxide. The growth of all organisms is dependent of nitrogen as they are used to make amino acids. However biological fixation cannot sustain a growing population
    2. The feedstocks are nitrogen which is obtained easily from the air
      1. as well as hydrogen which comes from the cracking of chemicals in natural gas (methane-CH4) using steam. Methane is a fossil fuel and so it's non-renewable. Also it produces Co2 and CO3
        Enzymes can work at room temperature and normal atmospheric pressure so don't require the burning of fossil fuels, strong and expensive equipment and workers are safer
        Although expensive and don't always have a high atom economy
        requires very specific conditions
        Fossil fuels to extract methane, burnt to produce high temperatures, transportation
        ammonia can cause eutrophication. It's a dangerous gas that causes health problems and can cause explosions
        More than 1% of all the energy consumed in the world is used for ammonia production
Dynamic Equilibrium
1. Reversible reactions
  1. Reversible reactions are where the products can react together into the origional reactants, depending on the conditions
    1. If they take place within a closed container (none of the producst/reactants escape), reversible reactions will eventually reach a state of dynamic equilibrium.
2. At equilibrium, the forwards and backwrads reactions of a reversible reaction are still continuing but at equal rates so that there is no overall change (in a closed system)
  1. At dynamic equilibrium, half of the copper sulfate would be blue and half would be white
    1. The position of an equilibrium can be changed by changing the conditions under which a reaction happens
The Haber Process
1. N2(g)+3H2(g) 2NH3(g)
  1. the pressure favours the side with fewer gas molecules (forward gas molecule), therefore the pressure is at 200 atmospheres
    1. Increasing the temperature always favours the endothermic reaction (backwards reaction) hence a relatively low temperature
    2. If the concentrations of the reactants are higher, then more products will be produced to reform the dynamic equilibrium. Therefore ammonia is tapped off to increase yield
  2. the gases do not stay in the reactor long enough to reach equilibrium. But the yield is further increased by recycling the unreacted hydrogen and nitrogen after they have been separated from the ammonia (when the mixture leaves the reactor).
  3. Choosing the pressure Increasing the pressure causes the yield to increase. However, the energy costs also increase and the equipment used for the process must be strong to withstand the high pressure, which makes it expensive. The pressure chosen is a compromise between rate, cost, and safety.
    1. Choosing the temperature Increasing the temperature decreases the yield but increases the rate. The temperature chosen is a compromise between rate and yield.
      1. Selecting the catalyst The catalyst can be expensive to develop and to buy - but it does not need to be replaced, because catalysts are not used up or chemically changed in a reaction. Care must be taken to ensure that the catalyst is not poisoned by impurities in the reactants.
        The catalyst does not affect the yield, but it does increase the rate of the reaction. This allows a lower the temperature to be used (increasing temperature is one the way of speeding up a reaction) and using a lower temperature allows the yield to be maximised as well
2. An iron catalyst is used to increase the rate of the reaction. The catalyst does not affect the yield of ammonia. Scientists are constantly trying to find new catalysts that will speed up the reaction even more, and therefore, make the Haber process more efficient.
  1. Ruthenium- pressure 40 times atmospheric pressure. yields of about 20%. Fitted to existing plants saving money. More expensive but outweighed by other cost saving
    1. Nitrogenase- enzymes (biosynthesis) room tempearures and normal pressure few resources burnt
    2. Unreacted gases are recycles so no wasteage
Analytical procedures
1. Qualitative analysis means any method of identifying the chemicals in a sample.
  1. Quantitative analysis means working out how much of a particular chemical there is in a sample.
    1. It can be used to work out the molecular formula of a sample. E.g. if you had a sample containing carbon and hydrogen you can know that it was a hydrocarbon but without quantitative analysis, you wouldn't know if it was methane, butane or other hydrocarbons
2. Any analysis must be carried out on a sample of a substance that is representative of the bulk of the substance. This means that the results from the analysis can be used to draw conclusions about the rest of the substance.
  1. Samples are usually tested in solution. A solution is made by dissolving the sample in a solvent. There are two types, aqueous (water) and nonaqueous (other e.g. ethanol). Which you use depends on the type of substance being tested.
3. Standard Procedures
  1. agreed methods of working- they are chosen because they are the safest, most effective and most accurate methods to use.
    1. Wherever or whenever the test is done, the result should be the same- it should give reliable results each time
      1. There are standard procedures for the collection, storage and preparation of the samples as well as how it should be analysed
Chromatography
1. An analytical method used to identify and separate the chemicals in a mixture; to check the purity of a chemical and to purify small samples of a chemical. In chromatography, substances are separated as they travel in a mobile phase which passes through a stationary phase.Different substances travel at different speeds, so some move further through the stationary phase than others in a given time.
  1. Mobile- where the molecules can move. This is always a liquid or a gas
  2. Stationary- where the molecules can't move. This can be a solid or a really viscous liquid
    1. Locating agent- identify colourless solute spots. UV light- with a TLC plate that contains fluorescers so he spots appear violet under UV light. Spray reagent- reacts with the substances to form a coloured compound)
  3. For each component/chemical in a sample/mixture, there is a dynamic equilibrium between the stationary and mobile phases. THE DYNAMIC EQUILIBRIUM DETERMINES THE DISTRIBUTION OF SAMPLE BETWEEN PHASES.
  4. The analyst adds a small sample of mixture to the stationary phase. As the mobile phase moves through the stationary phase, the chemicals in the sample distribute themselves according the dynamic equilibrium
2. Paper
  1. Paper-the stationary phase is paper. The mobile phase may either be an aqueous (water-based) liquid or a non-aqueous organic (carbon-based) solvent. An example of an organic solvent is propanone - which is the main chemical in nail varnish remover.
    1. Here the mobile phase (solvent) has moved through the stationary phase and the compounds of the sample have been distributed between the mobile and stationary phases. Each solute has a different distribution according to their affinities to the phases and the distribution is determined by the Dynamic equilibrium.
      1. The chemicals further up have a greater affinity to the mobile phase and so have been carried further and faster. Each solute is attracted differently so they move at different speeds and are separated.
3. Thin Layer Chromatography
  1. Thin layer chromatography (TLC) is similar to paper chromatography but instead of paper, the stationary phase is a thin layer of an inert substance (eg silica gel) supported on a flat, unreactive surface (e.g a glass plate or plastic)
    1. TLC has some advantages over paper chromatography. For example: the mobile phase moves more quickly through the stationary phase the mobile phase moves more evenly through the stationary phase there is a range of absorbencies for the stationary phase. It's easier to analyse.
4. Rf Value
  1. distance travelled by solute/ distance travelled by solvent
    1. ratio between distance travelled by the dissolved substance (solute) and the distance travelled by the solvent
      1. Chromatography is often carried out to see if a certain substance is present in a mixture. You run a pure, known sample of the substance alongside the unknown mixture. If the Rf values match then the substances may possibly be the same (not direct proof)
        Chemists use 'standard reference materials' (SRMs) to check the identities of substances. These have carefully controlled concentrations and purities
5. Running the chromatogram
  1. 1.The analyst adds the chosen solvent (mobile phase) to the chromatography tank and covers it. This ensures the solvent doesn't evaporate. After the tank has stood for a while, the atmosphere becomes saturated with solvent vapour.
    1. 2. The next step is to place he prepared paper or TLC plate into the tank, checking that the spots are above the level of the solvent.
      1. 3. The solvent immediately rises. The chemicals in the sample dissolve in the solvent and move between it and the paper/TLC plate (stationary phase). This sets up an equilibrium between the solvent and the paper. When the chemicals are in the mobile phase they move up the stationary phase with the solvent (mobile phase).
        4. The stationary phase is removed before the solvent reaches the top.
        5. The different chemicals in the sample form separate spots on the paper. Those further up have a greater affinity to the mobile phase
        6. The analyst then marks the solvent front
6. Gas
  1. Gas Chromatography (GC) used to analyse unknown substances and separate complex mixtures., The technique separates much better than paper or thin-layer chromatography. It can be used to identify how much of a chemical is present not just whether it is present. (Quantitative)
    1. Mobile- Unreactive carrier gas e.g. nitrogen or helium
      1. Stationary-very thin layer of an inert liquid on an inert solid support - such as beads of silica packed into a long thin tube (this flexible tube is coiled many times inside a thermostatically-controlled oven to keep it at a constant temperature).
        The unknown mixture is injected into a long tube coated with the stationary phase
        2. As it passes along the column (long thin tube) it separates into the different substances. Substances with a greater affinity (attraction) for the mobile phase reach the detector at the end of the column more quickly. Substances with a greater affinity for the stationary phase move more slowly through the column.
        3. The time it takes the chemical to travel through it is called the retention time. It is different for each chemical and is compared with standard reference data to identify it.
        the number of compounds in the mixture - represented by the number of peaks
        how much of each compound is present - represented by the height of the peak (higher = more)
        the retention time - indicated by the position of the peak
        The longer the retention time, the greater affinity the substance had to the stationary phase

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OCR 21st Century C7.4,5

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	Creado por Pritesh Patel hace más de 8 años