Transition Metals

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General Properties

A transition metal is a metal that can form one or more stable ions with a partially filled d-subshell. Transiton metals form positive ions, when this happens the s electrons are removed first and then the d electrons.

PHYSICAL PROPERTIES: They all have similar physical properties                                           -They all have a high density                                                                                                             -They all have high melting and boiling points                                                                                    -They have very similar ionic radii.

CHEMICAL PROPERTIES: They all have different chemical properties                                             -They can form complex ions. e.g iron and water ---> [Fe(H2O)6]2+                                                  -They form coloured ions                                                                                                                     -They are good catalysts. e.g iron is used in the haber process                                                         -They can exist in variable oxidation states. e.g. Fe2+ and Fe3+  (this is possible as 4s and 3d energy levels so close together so different numbers of electrons can be gained or lost using same amounts of energy)

INCOMPLETE D-SUBSHELL: This causes the special properties.

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Complex Ion/Ligands

Complex ion is a metal ion surronded by coordinately bonded ligands. LIGAND= is an atom, ion or molecule that donates a pair of electrons to a central metal ion.

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A ligand must have at least one lone pair, or it can't form a coordinate bond. Different ligands can have different numbers of lone pairs and can form different numbers of coordinate bonds. Ligands that form 1 coordinate bond are called unidentate.

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Ligands that form more that 1 coordinate bond are called multidentate

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Complex Ion/Ligands

Multidentate ligands that can form 2 coordinate bonds are called bidentate.

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OXIDATION STATES OF COMPLEX IONS:

Overall charge on a complex ion is its total oxidation state. It is put outside the square brackets. Can work out the oxidation state of the metal ion by....

The oxidation state  =   The total oxidation              -           The sum of the oxidation states

of the metal ion             state of the complex                             of the ligands

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Haem and Haemoglobin

Haemoglobin is a protein found in the blood, helps transport O2. Contains Fe2+ ions, are hexa-coordinated- 6 lone pairs are donated to them to form 6 coordinate bonds. 4 of the lone pairs come from nitrogen atoms, form a circle around the Fe2+ --> this is called the haem. The molecule that the 4 nitrogen atoms are part of is a multidentate ligand. A protein called globin and either an oxygen or a water will binf to the Fe2+ to form an octehedral structure.

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Shapes of complex ions

COORDINATION NUMBER: number of coordinate bonds that are formed with the central metal ion. (usually 6 & 4).  If ligands are small (e.g. H2O or NH3) 6 can fit around the central metal ion. But if they are big (e.g. Cl-) only 4 can fit around.

SIX COORDINATE BONDS: Have an octahedral shape...

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Different types of bond arrows show complex is 3D.

FOUR COORDINATE BONDS: Tetrahedral shape....

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Shapes of complex ions

But in a few complexes four coordinate bonds form a square planar shape...

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TWO COORDINATE BONDS: Some silver complexes have 2 coordinate bonds and form a linear shape.

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Formation of Coloured Ions

Normally 3d orbitals of transition element ions all have the same energy but when ligands come along and bond to the ions, some orbitals are given more energy than others. Splitting the 3d orbitals into 2 different energy levels.

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Electrons jump from ground state to higher orbitals by using energy from light.

Energy absorbed when electrons jump can be worked out using....

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Amount of energy needed depends on the central metal ion and its oxidation state, Ligands and coordination number. These effect the size of the energy gap.

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Colours of Compounds

When light hits a transition metal ion, some frequencies are absorbed as electrons jump to higher orbitals and the rest are reflected, these make the colour you see. If there are no 3d electrons or the 3d shell is full, no electrons will jump and all light will be reflected making the compoun white or colourless.

IDENTIFYING TRANSITION METAL IONS

Changes in oxidation state; if the oxidation state of a transition metal ion in a complex ion changes, then the colour of the complex ion may also change.

Changes in coordination number; May also result in a colour change, always involves a change in ligand too.

Changes in ligand; Changing the ligand can cause a colour change even if the oxidation state and coordination number stay the same.

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Spectrometry

Spectrometry can be used to determine the concentration of a solution by measuring how much light it absorbs. White light is shone through a filter, which is chosen to only let the colour of light in that the sample absorbs.Light then passes through the sample to a colorimeter, shows how much light was absorbed by the sample. More concentrated the sample the more light it absorbs.

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Variable Oxidation States

CHROMIUM

Most commonly exists in +3 and +6 oxidation state. Can also exist in the +2, much less stable.

CrO7^2-    -----> +6  -----> ORANGE     /     Cr^3+   -----> +3  ----->  GREEN 

CrO4^-     ----->  +6  -----> YELLOW     /     Cr^2+  ----->  +2  -----> BLUE

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Variable Oxidation States

COBALT

Can exist as =2 and +3. Prefers +2. There are 2 ways of oxidising Co2+ ions to Co3+ ions. Can be done by oxidising Co2+ with H2O2 in alkaline conditions:   2Co^2+  + H2O2  ---> 2Co^3+  + 2OH-

Or you can oxidise Co2+ with air in an ammoniacal solution. Start with pink solution [Co(H2O)6]2+ ions. Add small amount of NH3... [Co(H2O)6]2+   +  2NH3  ---> [Co(H2O)4(OH)2]  +  2NH4^+      [Co(H2O)4(OH)2] is a blue precipitate. If you then add excess ammonia to solution [Co(NH3)6]2+ ions form, producing a straw coloured solution. If they are left to stand in air, oxygen oxidises them to [Co(NH3)6]3+ which is a dark brown colour ////.

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Titration Reactions

Manganate (7) ions (MnO4-) in aqueous Potassium manganate (7) (KMnO4). Solution turns purple at the end and the equation is:

MnO4^-     +    8H+     +    5Fe^2+      ------>     Mn^2+     +     4H2O     +     5Fe^3+

Dichromate (6) ions (CrO7^2-) in aqueous potassium dichromate (6) (K2Cr2O7). Solution turns orange at the end point and the equation is:

Cr2O7^2-     +     14H+     6Fe^2+     ------>     2Cr^3+     +     7H2O     +     6Fe^3+

CALCULATING THE CONCENTRATION OF A REAGENT

1. Write out redox equation thats occuring in the conical flask.  2. Decide what you know and what you need to know.   3.Reagent know both conc. and vol. calculate moles                                      Moles = Conc. x (Vol. / 1000)                                                                                                               4.Use molar ratios to find how many moles of other reagent were present in the solution.                 5. Calculate unknow conc. using :  Conc. = (Moles x 1000) / Vol.

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Catalysts Intro.

Transition metals and their compounds make good catalysts as they can change oxidation states by gaining or losing electrons (they have variable oxidation states). This means they can transfer electrons to speed up reactions.

Elements in other blocks in the periodic table generally don't make good catalysts as they dont have a incomplete 3d-subshell and don't have variable oxidation states.

2 Types of catalyst need to know about...

Heterogeneous

Homogeneous

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Heterogeneous Catalysts

A Heterogenous Catalyst is one that is in a different phase from the reactants. (e.g. different state) and the recation occurs at the surface.

IRON: used in the haber process for making ammonia:  N2 (g)  +  3H2 (g)  --Fe(s)-->  2NH3 (g)

VANADIUM (5) OXIDE: used in the contact process to make sulfuric acid:                                    SO2 (g)  +  1/2O2 (g)  --V2O5(s)--> SO3.    It catalyses it in 2 steps:                                                  1st is catalyses SO2 to SO3 and itself is reduced :   V2O5  +  SO2  ---->  V2O4  +  SO3                    The reduced catalyst is then oxidised by the oxygen to original state : V2O4  +  1/2O2  ---->  V2O5

CHROMIUM (3) OXIDE: used in the manufacture if methanol from carbon monoxide and hydrogen: CO (g)  +  2H2(g)  --Cr2O3-->  CH3OH (g)

USE OF SUPPORT MEDIUMS                                                                                                       When a hetrogeneous catalyst is used the reaction takes place on the surface of the catalyst. Increasing the surface area of the catalyst increases the number of molecules that can react at the same time, increasing rate of reaction. Support medium often used to make the surface area of the catalyst as large as possible. Catalytic converters contain a ceramic lattice coated in a thin layer of rhodium, which acts as a catalyst to convert harmfull waste into less harmful products.

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Heterogeneous Catalyst

CATALYST POISONING

During a reaction, reactants are adsorbed onto active sites on the surfaces of heterogeneous catalysts. Impurities in the reaction mixture may also bind to the catalysts surface and block reactants from been adsorbed ---> Catalyst poisoning.

It reduces the surface area of the catalyst avaliable to reactants, slowing down the reaction. Increases the cost of the chemical process as there can be less product made in a shorter amount of time or a certain amount of energy. Catalyst may even need replacing or regenerating, which also costs money.

LEAD POISONS: Catalytic converters reduce harmful emissions from car engines. Lead can coat the surface of the catalyst, so the vehicles that have them fitted must only be run on unleaded petrol

SULFUR POISONS: Hydrogen in Haber process produces Methane. This methane is obtained from natural gas, which contains impurities including sulfur compounds. Any sulfur not removed is adsorbed onto the iron, forming iron sulfide, and stopping the Fe from catalysing the reaction properly.

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Homogeneous Catalyst

Homogeneous Catalysts are in the same physical state as the reactants.

A homoegeneous catalyst works by forming an intermediate species. The reacts combine with the catalyst to make an intermediate species, which then reacts to form the products and reform the catalyst. Causing the enthalpy profile for a homogeneously catalysed reaction to have 2 humps in it, corresponding to the 2 reactions. The activation energy needed to form the intermediates is lower than needed to make the products directly from the reactants.

FE2+ CATALYSING REACTION BETWEEN S2O8^2- AND I- :                                                S2O8^2-(aq)  +  2I- (aq)  --->  I2 (aq)  +  2SO4^2- (aq).          Slow reaction as bothe ions are - charged, they repel each other so unlikely they will collide and react.

If Fe2+ ions are added the reaction speeds up :                                                                            S2O8^2- (aq)  +  2Fe2+ (aq)  ---->  2Fe3+ (aq)  +  2SO4^2- (aq).  

  The newly formed Fe3+ ions now easily oxidise I- ions to iodine and the catalyst is regenerated.   2Fe3+ (aq)  +  2I- (aq)  ---->  I2 (aq)  +  2Fe2+ (aq)                                                                              You can test fot iodine by adding starch solution. If iodine is present, it will turn blue/black

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Homogeneous Catalyst

Mn2+ AUTOCATALYSING THE REACTION BETWEEN MnO4- AND C2O4^2-

It is an autocatalysis reaction because Mn2+ is a product of the reaction and acts as the catalyst for the reaction. This means that as the reaction progresses and the amount of product increases, the rate of reaction speeds up :                                                                                                      2MnO4^- (aq)  +  16H+ (aq)  +  5C2O4^2- (aq)  ---->  2Mn^2+ (aq)  +  8H2O (l)  +  10CO2 (g)

No Mn2+ at the start so the initial reaction is very slow, during this part the activation energy is very high.                                                                                                                                                   But once a little more Mn2+ catalyst has been made it reacts with the MnO4^- ions to make Mn3+ ions :                                                                                                                                            4Mn2+ (aq)  +  MnO4^- (aq)  +  8H+ (aq)  ---->  5Mn3+ (aq)  + 4H2O (l)                                               The Mn3+ ions are the intermediate. They then react with the C2O4^2- ions to make Co2 and reform the Mn2+ catalyst. :                                                                                                                2Mn3+ (aq)  +  C2O4^2- (aq)  ---->  2Mn2+ (aq)  +  2CO2 (g)

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Other Uses of Transition Metals

HAEMOGLOBIN : Contains complexes formed from Fe2+ ions. Both water and oxygen will bind to the Fe2+ ions as ligands, so the complex can transport oxygen where it is needed, then swap it for a water molecule.

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CARBON MONOXIDE POISONING : Process of oxygen transportation can be disrupted if carbon monoxide (CO) is inhaled. CO produced by incomplete combustion. When it is inhaled the haemoglobin swaps its water ligands for CO ligands, forming carbohyhaemoglobin. This is bad as CO is a strong ligand and won't readily change with oxygen or water ligands, so can no loger transport oxygen --> causing dizziness, unconsciousness and even death.

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Other Uses of Transition Metals

CISPLATIN : Is a complex of platinum (2) with 2 chloride ions and 2 ammonia molecules in a square planar shape.

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Can be used to treat some types of cancer, by preventing cancer cells from reproducing.  Cisplatin forms coordinate bonds with nitrogen atomsin the DNA molecule and prevents the DNA strands from unwinding. So the cell can't replicate it's DNA so it can't divide. HOWEVER... cisplatin can prevent normal body cells from reproducing, including blood and hair cells. Can cause hair loss and supress the immune system. Can also cause damage to the kidneys.

TOLLENS REAGENT : Prepared by adding just enought ammonia solution to silver nitrate solution to form a colourless solution containing the complex ion [Ag(NH3)2]+.                                          Tollens reagent is used to distinguish between aldehydes and ketones. Aldehydes give a silver mirror. :  RCHO + 2[Ag(NH3)2]+  + 3OH-  ---> RCOO-  + 2Ag  + 4NH3  + 2H2O                                 Ketones don't react with tollens

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