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AS and A Level: Physical Chemistry

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The dynamic equilibrium and 'Le Chatelier Principle'

  1. 1 Le Chatelier Principle states “If a system that is in dynamic equilibrium is subjected to a change, the position of equilibrium will shift to minimise that change”. Put simply- if you do something, the system will try and reverse what you’ve done!
  2. 2 Adding a catalyst does not change the concentration, pressure or temperature of a system. Therefore adding a catalyst has no effect on the position of equilibrium.
  3. 3 If you increase the concentration of one side of an equation, the position of equilibrium will shift to the other side (to try and return the concentration back to its original value).
  4. 4 If you increase the pressure of the system, the position of equilibrium will shift to the side of the reaction with less moles of gas (to try and reduce the pressure back to its original value).
  5. 5 If you increase the temperature the position of equilibrium will shift in the direction of the endothermic reaction (to try and reduce the temperature back to its original value).

Top equations for acid / base chemistry (A level only)

  1. 1 For a strong acid the acid concentration is equal to the H+ concentration. This is because strong acids fully dissociate their H+ ions. [acid] = [H+]
  2. 2 For a weak acid, because they only partially dissociate their H+ ions, to find the H+ concentration we must use the following equation: [H+] = √ka[acid]
  3. 3 For a buffer, we calculate the value of H+ by using: [H+] = Ka[acid] / [salt] (where Ka is the acid dissociation constant)
  4. 4 For a strong base, we calculate the H+ value by using: [H+] = Kw / [base] (where Kw is the ionic product of water = 1 x 10-14)
  5. 5 To convert [H+] into pH, we would use the equation: pH = -log[H+]

Top tips for ionisation energy

  1. 1 One of the factors which will affect ionisation energy is electron shielding. This is how many inner shell electrons an atom has. The more electron shielding, the lower the ionisation energy. Electron shielding stays the same across a period and increases down a group.
  2. 2 The second factor affecting ionisation energy is the proton number / nuclear charge. The higher the nuclear charge the higher the ionisation energy. Nuclear charge increases across a period and down a group.
  3. 3 The third factor affecting ionisation energy is the atomic radius (size of the atom). The higher the atomic radius the lower the ionisation energy. Atomic radius decreases across a period (as the increased number of protons pulls the electron shells closer) and increases down a group.
  4. 4 All three of these factors combine to have an effect of increasing the ionisation energy as we go across a period (eg F has a higher ionisation energy than O)
  5. 5 All three of these factors combine to have an effect of decreasing ionisation energy as we go down a group (eg K has a lower ionisation energy than Na)

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  1. The aim of the experiment is to determine the percentage of nickel present in hydrated nickel sulphate by Complex metric titration with EDTA

    The weighing bottle was reweighed to ensure that all the nickel salt was transferred to the beaker and any mass need to be deducted for calculation purpose. The solution was stirred until the solid dissolved and the solution was transferred to a 100cm³ flasks. the beaker was rinsed several times to ensure that all the solution had been transferred to the flask and the solution was made up to the graduation mark by adding deionised water, the solution was inverted several time to ensure a proper mix.

    • Word count: 1564
  2. Acids and Bases in the World - acid rain, chlorine in pools and cleaning products.

    Acid rain causes the pH of the water to become lower, a majority of fish eggs won't hatch if the pH of the water is lower than 5 and adult fish can die if the pH becomes lower than this 2. As rivers and lakes become more acidic due to the acid rain, the less lifeforms that are able to live in those conditions, substantially reducing biodiversity 1. Acid rain can have huge effects on soil, the hydronium ions of acid rain mobilise toxins such as aluminium and leach away essential nutrients and minerals such as magnesium 1.

    • Word count: 2082
  3. Titration lab report - determine the percentage of acetic acid present in vinegar

    Add 2 drops of phenolphthalein indicator. 4. Record initial buret reading (2 d.p) 5. Titrate to the end point. Take it carefully near the end by adding NaOH drop by drop. 6. Record the final buret reading. 7. Repeat those steps above once more or until get consistent results. 8. Find the average molarity of the NaOH solution base on calculations of each trial. 1. Use hot water to clean a 5 mL pipet: 2. Place a rubber propipette bulb on the end of the pipet and squeeze ?A? to push the air out.

    • Word count: 598

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