Chemistry - unit 2

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Rate of Reaction

Rate of Reaction = change in [...] / time                (moldm-3s-1)

On a graph of concentration v time the gradient of the curve is equal to the rate of reaction.

Rate = k[A]1[B]2

The rate constant k is the rate of reaction/concnetration of reactants, units are calculated in the same way; t-1(moldm-3)-(n-1)

Increasing the temperature increases the rate of reaction by increaseing the rate constant. 

The rate determining step is teh slowest step in a multistep reaction and controls the speed of the reaction. 

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Order

Zero order - rate is unaffected by changes in concentration, half life decreases with time (graph is a striaght line down)

First order - rate is directly proportional to changes in concentration, half life is constant over time

Second - rate increases exponentionally with time, half life increases with time

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Equilibrium

Dynamic equilibrium in a closed system is where the rate of the forward reaction is equal to the rate of the reverse reaction and there is no overall concentration change. 

Kc is the equilibrium constant = [product]/[reactant]

If Kc>1 then the reaction is product favoured 

Kc only permenatly changes with temperature and will change temporarily with changes in concentration and pressure. 

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pH and Acids

pH = -log[H+]

[H+] = 10-pH

When an acid is added to water it dissociates releasing protons into solution. Strong acids fully dissociate whereas weak acids only partially dissociate. 

Ka is the dissociation constant, if Ka is large it indicates a large extent of dissociation

Ka = [H+][A-]/[HA]

pKa = -logKa

For strong acids [H+]=[HA] so pH can be calculated from [HA]

For weak acids we assume that [H+]=[A-] and [HA - H+]=[HA] to give Ka = [H+]2/ [HA]

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Water and Bases

Water is able to act as an acid or as a base by accepting or donating a proton (H3O+ or OH-). 

Kw is the ionic product of water, Kw =[H+][OH-]. In pure water Kw = 1x10-14

The strength of a base is proprtional to it's ability to dissociate into OH- ions. To work out the pH of a strong base you need to work out [H+] by using the Kw value. 

Acid-base pairs are a pair of two species that are able to transform into each other by losing or gaining a proton.

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Buffers

A buffer solution is a mixture that minismises pH change on addition of a small amount of acid or base. They are made of a mixture of a weak acid (HA) and it's conjugate base (A-). It can be made by mixting a weak acid with the salt of a weak acid or a partially neutralised aqueous aklaline. 

The weak acid partially dissocaites and the salt fully dissocaites leading to a mixture containing a large [A-] and a large [HA]. 

When acid is added the position of equilbirum shifts to the left and the protons reaction with A- ions to remove H+ from the system. The opposite happens when a base is added. 

The pH of a buffer is dependent on its Ka value and the ration of weak acid to base. 

pH = pKa + log([A-]/[HA])

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Neutralisation

The equivalence point in a titration is the point at which the volume of one solution is completely reacted with the volume of a second solution. 

Indicators are often weak acids (HIn). The end point of an indicator is when there are equal amounts of weak acid and base. 

To choose an appropriate indicator for a titration you should choose the one where the end point of the indicator is as close as possible to the equivilance point.

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Enthalpy changes

Lattice enthalpy - formation of one mole of an ionic compounr from it's gaseous ions 

Formation - formation of one mole of a compound from it's constituent elements in their standard states

Ionisation - change when one electron is removed from every atom in one mole of gaseous ions

Electron affinity - one electron is added to every atom in one mole of gaseous ions

Atomisation - one mole of gaseous ions forms from the element in its standard state

Solution - one mole of a compound is completely dissolved in water

Hydration - one mole of gaseous ions dissolves in water forming one mole of aqueous ions 

Lattice enthalpy + solution = Hydration

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Entropy

Entropy is hte quantative measure of the degree of disorder in a system. Entropy is always positive. Entropy change = products - reactants. 

Free energy: ΔG = ΔH – TΔS

Temperature is given in kelvin (add 273 to celcius to get to kelvin)

  • If ΔG is negative the reaction is feasible
  • If ΔG is positive the reaction is never feasible 
  • If ΔH and ΔS are negative and ΔG is negative the reaction is feasible at low tempreatures 
  • If ΔH and ΔS are positive and ΔG is negative the reaciton is feasible at high temperatures 
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Cells Half Cells

Half cells exist where there is an equilibirum between an element in two oxidation states. 

  • Metal: solid metal in a solution of it's ions 
  • Non Metal: platinum electrode in contact with a solution of it's ions and non-metal gas
  • Metal ion: platinum in a solution of the same element in two different ionic oxidation states

A simple cell is made by connecting two half cells by connecting electrodes to a high-resistance voltmeter and the solutions with a salt bridge (KNO3)

The more negative the standard electrode potential the more readily it releases electrons, electrons flow from the more negative to more positive electrode. 

Emf is the voltage produced when no current flows. 

The larger the differen in emf of two half cells the more feasible a reaction is, for there to be a reaction emf difference muse be greater that 0.4 V

The more negative electrode will go from solude to ion (release electrons) the more positive will do the opposite

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Fuel Cells

Three types of cell:

  • Non-rechargeable - cell provides electrical energy until all the chemcials have reacted 
  • Rechargeable - the chemicals in the cell react providing electrical energy, cell reaction can be reversed during charging and chemcials are regenerated and used again
  • Fuel cell - cell reaction uses external supplies of a fuela nd an oxidant which are provided.

Fuel cell vehicles use hydrogen or hydrogen rich fuels, The hydrogen gas is reacted with oxygen gas to produce water and electricity. FCVs produce less pollution and CO2 and are more efficient than combustion engins (40-60% compared to 20%).

Hydrogen can be stored as a liquid under low temperatures and pressure or stored adsorbed onto a solid surface or absrobed within a solid material.

However large-scale storage and transport of hydrogen is problematic and expensive, adsorbers and absrobers have limited lifetimes and need to be replaces, fuel cells have limited lifetimes and require regular replacment and disposal, fuel cells use toxic chemicals in production.

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Transition Elements

Transition elements are d-block elements that can form ions with an incompleted d sub-shell. They lose electrons from the 4s subshell before the 3d sub shell to form positive ions. Chromium and copper both remove electrons from the 4s subshell and use them in 3d to create a more even spread of electrons.

They have the same properties as most metals (shiny, high density, high melting/boiling point, exist as giant metallic lattices) but also form ions with different oxidation states, form ions with different charges, form coloured solutions in water, and are good catalysts. 

They are good catalysts becayse they provide a surface for the reaction, can change oxidation states, and have empty d orbitals to gain electrons.

  • Haber process uses iron 
  • Contact process (make sulphur trioxide) uses vanadium V oxide
  • Hydrogenation of alkenes uses nickel
  • Decomposition of hydrogen peroxide uses manganese dioxide 
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Complex ions

Complex ions are transition metal ions bonded to one or more ligand by coordinate bonds.

Ligands are molecules or ions that can donate a pair of electrons to a transition metal.

The coordination number is the number of cordinate bonds on the central metal ion.

Complex ions can show cis/trans isomerism as well as optical isomerism.

EDTA bonds to transition metals ions on a 1:1 basis. It is a chelating agent. It is used in detergents, foods, to prevent blood clotting and to treat mercyrt poisoning. 

Haemoglobin is a complex ion fo Fe2+ bonded to  four nitrogen atoms, a protein and oxygen. 

Kstab is teh stability complex from complex ions, the larger Kstab value the more stable a complex ion is and the further the position of equilbirum is the the right. 

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Complex ion reactions

Copper

  •  [Cu(H2O)6]2+ + excess aqueous ammonia à [Cu(NH3)4(H2O)2]2+ (pale blue à deep blue
  • [Cu(H2O)6]2+ + concentrate hydrochloric acid à [CuCl4]2- (pale blue à green solution à yellow solution)

Cobalt

  • [Co(H2O)6]2+ + hydrochloric acid à [CoCl4]2- (pale pink àdark blue)
  • [Co(H2O)6]2+ + ammonia à [Co(H2O)4(OH2]2+ + NH4 (ammonia acts as a base)
  • [Co(H2O)6]2+ + excess ammonia à [Co(NH3)6]2+
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