Chemistry AQA A2 unit 5

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  • Created on: 10-06-13 13:44

Enthalpy Change

Enthalpy change: heat exchange in a system at constant temperaure.

Standard conditions: 1000Kpa , 293K , 1.0moldm^-3

Hess' law: enthalpy change is the same no matter the route taken fron reactants to products.

atomisation: enthalpy change when 1mol gaseous ions formed from elements in standard                            states.

1st IE: enthalpy change when 1mol gaseous atoms converted into 1 mol gaseous 1+ ions.

E- affinity: enthalpy change when 1mol gaseous atoms converted to 1mol gaseous 1- ions.

lattice formation: enthalpy change when 1mol solid ionic compound made from gaseous ions.

hydration: enthalpy change when water molecules surround 1mol gaseous ions.

solution: enthalpy change when 1mol of solute dissolves in sufficient solvent to form a solution                where all of the ions are far enough appart not to react.

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Born-Harber Cycles


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More Enthalpys

Solid Ionic Lattice:                                                                                                                  When a solid ionic lattice dissolves in water these two things happen:                                       1. The bonds between the ions break - this is endothermic. This enthalpy change is the lattice           enthalpy of dissociation.                                                                                                       2. Bonds between the ions and the water are made- this is exothermic. The enthalpy change             here is called the enthalpy change of hydration.                                                                        (Oxygen is more electronegative than hydrogen, so it draws the bonding electrons towards             itself, creating a dipole).                                                                                                    3. The enthalpy change of solution is the overall effect on the enthalpy of these two things.

Mean Bond Enthalpies:                                                                                                                   Mean bond enthalpies are the averages of these bond enthalpies. Only the bond enthalpies           of diatomic molecules, such as H2 and HCl will always be the same.                                           So calculations using mean bond enthalpies will never be perfectly accurate. You get more           exact results from experimental data obtained from specific compounds. 

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Entropy- measure of dissorder in a chemical system.

spontaneous (or feasible) change: one that will just happen by itself- you don't need to give it                                                          energy. You need to supply energy to endothermic                                                                    reactions, but some are spontaneous.                                            In some reactions, the entropy increases so much that the reaction will happen by itself. 

+ve entropy = products more dissorderd than reactants and so a reaction will happen.

-ve entropy = reactants more dissordered than products.

Gibbs free energy: /\G = /\H - T/\S                                                                                                                       if /\G<0 then a reaction is feasible.                                                                                           when G=0 a reaction is just feasible.                                                                           Some reactions, although feasible react so slowly you cannot see them taking place.                   Some reactions may only take place at a high temperature.

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Sodium(Na): Conducts electricity, tarnishes rapidly, silvery-white soft metal, reacts with acids to                      form salts and hydrogen gas.                                                                                                    2Na + 2H2O --> 2NaOH + H2 , this is a vigorous reaction with effervescence.                                burns with a yellow flame in air and a white flame in oxygen.                                                        Sodium oxides reacts with acids to give water only.

Magnesium(Mg): relatively soft silvery-white metal, conducts electricity and reacts with acids to                              form salts and hydrogen gas.                                                                                                    2Mg + O2 --> 2MgO , burns with a white flame.                                                                          Mg + 2H2O --> Mg(OH)2 + H2 , slow reaction, faste with steam, effervescence.                            Magnesium oxides react with acids to give water only.

Aluminium(Al): light weight, silvery metal which conducts electricity.It reacts with acids.                                     4Al + 3O2 --> 2Al2O3 , aluminium is always covered in a coat of aluminium oxide.                         Aluminium oxide is unreactive & is amphoteric(reacts with acids & bases).

Silicone(Si): A blue-grey solid which is in a giant covalent structure. it has a high melting point                         and only conducts electricity to some extent.                                                                             Silicone dioxide will react as a weak acid with strong bases.

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Periodicity Continued

Phosphorus(P): spontaneously combusts and doesnt conduct electricity. it has several allotropes                          and low melting and boiling points.                                                                                            4P + 5O2 --> P4O10  

Sulphur(S): yellow solid which doesnt conduct electricity, has a low melting and boiling point and                    forms a ring of 8 atoms.                                                                                                            S + O --> SO4 , Burns with a blue flame in oxygen.                                                                    If sulphur dioxide & sodium hydroxide are mixed, sodiun hydrogensulphate is formed.                    SO2 + NaOH --> NaHSO4 .

Chlorine(Cl): yellow-green gas, non-metal with low melting and boiling points.                                                  Cl2 + H2O --> HClO + HCL

Argon(Ar): colourless,odorless noble gas. chemically unreactive and monatomic.

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Half Cells

  • Electrons flow from a low to a high order of reactivity.
  • The potentail difference is a measure of how different two metals and their tendancies to donate electrons.


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Hydrogen electrode

Compares reactivity of metals. conditions: 1.0moldm^3 , 100kpa , 298K. Hydrogen gas is bubbled through a solution of H+ ions, and electrical contact is made through a platinum metal electrode. -ve e- means that the metal being tested is a better reducing agent than hydrogen.                                     (

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Predicting Direction of Redox Reactions

  • Electrons always move from a low voltage to a high voltage.
  • The electrode potential of a half cell is a measure ofelectromotive force (e.m.f) of the cell
  • The standard electrode potential has the symbol EѲ, When two half cells are combined the cell potential or standard e.m.f of the cell is given by:  EѲ cell = EѲ R.H.S - EѲ L.H.S
  • EѲ values are determined using a standard reference electrode, this is usually the standard hydrogen electrode (S.H.E).          e.g.                                                                                  (
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Electrochemical Cells

Non-rechargable Cells: Zinc-copper Cells- not practical for portable batteries, 1.1v emf.             Zinc-carbon Cells- Zn + 2NH4+ <--> Zn2+ + 2NH3 + H2 , used in most  ordinary batteries.

Rechargable Cells: Lead-acid batteries- used in car batteries, and contain six 2v cells.

Portable Cells: Nickel/cadmium- uses an alkaline electrolyte, can be recharged multiple time             and are cost efficient.                        2Ni(OH)2 + Cd(OH)2 <--> 2NiO(OH)2 + 2H2O + Cd

The Fuel Cell: 2H2 + O2 --> 2H2O , H+ ions flow from anode, through electrolyte to cathode, the electrodes are separated by a permiable polymer.                                                                                        

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Transition Metals

Transition Metals: d-block elements which have a part filled d-orbital & can form >1 stable ions.

Electron config: elements- d-block elements with the two 4s electrons then the 3d shell is filled.                                  ions-  lose the 3d electrons before the 4s.

Properties: Physical-Transition elements are all typical metals and have similar physical                                              properties: They all have a high density, they all have high melting and high                                  boiling points, their ionic radii are more or less the same.                                           Chemical- Form complex ions, form coloured ions, good catalysts, exist in variable                                      oxidation states.

Shapes: linear-2 bonds, tetrahederal-4 bonds, octahederal-6 bonds.

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Complex formation of ions

  • Transition metals form co-ordinate bonds with ligands.
  • ligands are Lewis bases and nucleophiles.
  • coordination number is the number of coordinate bonds formed in the complex ion.
  • if a ligand donates 1 one pair it is called unidiative, if it donates 2 it is called bidative and if it donates multiple lone pairs it is called multidentate.
  • complex ions with polydentate ligands are called chelates.

Haemoglobin: it is the red pigment in blood and has an iron ion centre. oxygen can bond                                    reversably to the ion, however some other ligand bond irreverseably to the                                    haemoglobin , rendering teh protein unusable.

Cis-Platin: it is a successful anti-cancer drug. it works by bonding to DNA and then prevents the                    replecation of cancerous cells.

Common Ligands:                                                                                                                                              H2O            OH-           NH3          Cl-,F-            EDTA4-                                                    -------------------------------------------------------------------------------------------------->                                                                         Increasing in strength.

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Coloured Ions

Why are they coloured?: Because the electrons in transition metal compounds can move                                                 orbitals and when this happens the electrons absorb visible light                                                 energy and these light wavelengths are not seen.

                                        Violet = high energy wavelengths                                                                                                  Red = low energy wavelengths

                                                 /\E = HV                                                                                                        Energy absorbed = Frequency x plancks constant

Colourimetry: Finds tha ration of metal ions to ligands in a complex                                                                 This is found by mixing different solutions of different proportions, when solutions                         are mixed in the same ratio as complex, maximum concentration is seen.                                                        

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Properties: not used up in a reaction, and provide a differnt route for the reaction to go down.

Homogeneous catalysts: In the same phase as the reactants and an intermediate species is                                            formed.                                                                                                                                  Auto catalysts: where one of the products in a reaction is the catalyst                                                                for the reaction itself.

Heterogenerous catalysts: transition metals, their partially filled d-orbitals can hold reactants                                               together and weaken the bonds within them, this is called                                                           adsorbsion. Ater this the products are then released (desorbsion).                                               They are expensive.                                                                                                                 These catalysts can be made more efficient by making them into                                                 powders so they have a smaller surface area and spreading them on                                           a support, however they can then be poisoned by impurities. 

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Lewis Acids and Bases

Lewis Acid: lone pair acceptor

Lewis base: lone pair donator

Test for Iron ions: add dilute alkali, this precipitates the hydroxides whose colours are differnet. 

Amphoteric: has both basic and acidic properties

reactions with CO32-: sodium carbonate acts as a base with M+3 aqua ions to produce a                                             hydrated metal oxide.

Anionic transition compounds: only transition matals with high oxidation states, bonding is                                                        essentially covalent. 

aqueous transition ions: water molecules surround the salt ofrming a complex ion with an                                                  octahederal shape.

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Ligand Substitution Reactions

Chelation: formation of a complex ion with one or more multidentate ligands. these are normally                    very stable due to entropy.

Change in Co-ord number: when copper and hydrochloric acids react both the co-ord number                                                and the charge change. There is also a colour change from bue to                                              yellow.

replacing water as a ligand: water can be replaced by charged ligands like Cl-. these may be                                                replaced in part or by multidentate ligands.

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Acid-Base reactions of metal ions

M 3+ ions are more acidic than M 2+ ions because they have a smaller mass to charge ratio and are therefore more strongly polarised.

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The standard condition temperature on slide 1 should be 298k and the pressure 100kpa.Thank you for this great resource.

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