6.7 Where Does Colour Come From?
- A transparent object/solution -> all wavelengths of visible light are transmitted -> colourless
- An opaque object -> all wavelengths of visible light are reflected -> appears white
- For coloured objects, certain wavelengths are absorbed and some are reflected. When absorption occurs, wavelengths corresponding to one colour are removed from the white light and you see the complementary colour.
- When an atom absorbs visible radiation, an electron moves to a higher energy.
- lf a molecule absorbs visible radiation, it moves to an electronically excited state. An electron is promoted and the molecule goes into one of several possible excited electron levels
- The energy absorbed is re-emitted. The molecule may emit a small amount of energy and fall into an intermediate energy level.
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9.2 Redox Reactions and Electrode Potentials
- Redox reactions involve electron transfer and can be split into two half-reactions. For example Zn (s) + Cu ^2+ (aq) -> Cu (s) + Zn ^ 2+ (aq) can be split into Zn (s) -> Zn^ 2+ (aq) + 2e- (oxidation) and Cu^ 2+ (aq) + 2e- -> Cu (s) (reduction)
- Two half reactions can be arranged to occur separately with electrons flowing through an external wire between reactions. The two parts are called half-cells and when combined make an electrochemical cell.
- Cells and half cells are compared using voltages. The potential difference is measured between the terminals of the cells when no current flows, it is given the symbol Ecell (or called electromotive force/emf). It is measured using a high resistance voltmeter (no current flows)
- A simple half-cell is made up by dipping a strip of metal into a metal ion solution- each half cell has it's own electrode potential.
- When we put two half-cells together, the one with the most positive potential will be the positive terminal of the cell, and the other is the negative
- A connection is needed between the solutions (but the solutions cannot mix)- a strip of filter paper soaked in potassium nitrate serves as a junction called a salt bridge or ion bridge. The ions carry the current so there is electrical contact. The circuit is completed with a metal wire connecting the strips of metal
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9.2 Redox Reactions and Electrode Potentials 2
- A standard hydrogen half-cell is used as a common reference point- 2H+ (aq) + 2e- -> H2 (g)
- Half-cells must be done under standard conditions, as electrode potentials vary with temperature. The standard electrode potential of a half-cell is defined as the potential difference between it and a standard hydrogen half-cell.
- EƟ values have a sign depending on whether the half-cell has a higher or lower potential than the standard hydrogen half-cell. An electrochemical series is a list (in order) of EƟ values
- The cell at the bottom (most positive) is most likely to accept electrons
- Rusting is an electrochemical redox reaction. The two half-reactions are
- Fe^ 2+ (aq) + 2e- -> Fe (s) EƟ= -0.44V
- 1/2 O2 (g) + H2O (l) + 2e- -> 2OH- (aq) EƟ= +0.40V
- In the water droplet, water is reduced into hydroxide ions. On the surface of the iron or steel, iron is oxidised and Fe^ 2+ ions pass into solution. The hydroxide and Fe^ 2+ ions react to form iron oxide (after further reacting with water).
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9.3 Predicting the Direction of Redox Reactions
- Electrode potential charts can be used to make predictions about the direction a redox reaction will take
- 1. Construct an electrode potential chart
- 2. Use it to make predictions about whether a reaction could occur
- 3. Use the half-equations to give an overall equation
- Electrons flow from the more negative half-cell to the more positive half-cell
- Electrode potentials can be used to make predictions about the feasibility of redox reactions
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11.5 The d Block: Characteristics of Transition Me
- All transition metals have and incomplete 3d sub-shell. Chromium and copper are not typical transition metals in that they have incomplete 4s outer shells; all other transition metals have full outer sub-shells
- The characteristic properties (coloured compounds and variable oxidation states) of transition metals are due to the inner incomplete d sub-shell. Zinc has a full outer d sub-shell, so a transition metal is defined as 'an element which forms at least one ion with a partially filled sub-shell of d electrons'
- Transition elements are dense metals with high melting and boiling points. They are hard and durable, with high tensile strength. These properties are due to strong metallic bonding. They are often put into alloys.
- When force is applied to a metal crystal, the layers of atoms can slide- slip, why they are malleable and ductile. In an alloy, the regular arrangement is interrupted by differently sized atoms, making it more difficult for slip to occur. Alloyed metals are less ductile and malleable
- Variable oxidation states- due to successive ionisation enthalpy changes. The change from one oxidation state to another involves a redox reaction, so the relative stability of different oxidation states can be predicted by looking at standard electrode potentials. Higher= more stable, and more likely to be an oxidising agent
- Are good catalysts, as the availability of 3d and 4s electrons make them intermediate comp
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11.6 The d Block: Complex Formation
- A complex consists of a central metal atom/ion surrounded by a number of -vely charged ions or neutral molecules possessing a lone pair of electrons. The surrounding anions or molecules are called ligands, and they donate electrons to the central atom/ion
- If a complex is charged, it is called a complex ion.
- The number of bonds from the central ion to ligands is the coordination number. 6- octahedral 4- tetrahedral 2-linear
- Naming 1. Give number of ligands with Greek prefix (mono, di) 2. Identify ligands (-o ending for anions) 3. Name central metal ion (english for +ve or neutral, latinised for -ve) 4. Indicate oxidation number of central metal in brackets
- Ligand which can bond to central metal through one atom/ion= monodenate. Two= bidentate, many= polydentate.
- The metal ion is held in a five-membered ring= chelate ring.
- Reactions can occur when one ligan displaces another- ligand substitution. Will only occur if the proposed complex is more stable than the original one
- d-block compounds are often coloured. Light is absorbed only if the energy of the light matches the gap between two energy states in the atom. The ligands around the metal atom/ion cause the orbitals to split unevenly. The difference between the two levels is now such that the light absorbed falls in the visible part of the spectrum
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