Chemistry

4. Oxidation-Reduction reactions are increasingly important as a source of energy. 1: Explain the displacement of metals from solution in terms of transfer of electrons. - When an active metal is placed in a solution containing ions of a less active metal, the active metal displaces the less active metal from solution. - This occurs because a more active metal atom loses one or more electrons and becomes a positive ion. - The electrons lost are transferred to the ions of the less active metal, resulting in them becoming metal atoms. - Oxidation is loss of electrons (or gain in oxidation number) - Reduction is gain of electrons (or loss in oxidation number) - Oxidation-reduction reactions are also called redox or electron transfer reactions. If an Iron nail is placed in a solution of blue copper (II) salt, some of the iron nail dissolves [pic] At the same time, the blue colour of Cu2+ ions disappears and a dark copper coating appears on the nail surface [pic] The overall reaction is: [pic] The electrons lost by iron atoms undergoing oxidation are used to reduce copper (II) ions to copper atoms. PRAC 1 : Perform a first-hand investigation to identify the conditions under which a galvanic cell is produced. Galvanic cells require: - The transfer of electrons between two dissimilar electrodes. - An electrolyte (a salt solution) - Salt bridge to allow transfer of ions between half-cells. - Conductive wire between two electrodes. - Each electrode must be in a 1M salt solution containing its own ion. 2: Identify the relationship between displacement of metal ions in solution by other metals to the relative activity of metals. In reacting the more active metal atom (M) changes to a metal ion (M+) by losing one or more electrons to form a cation. The ability of metal to lose electrons is measured on the electrochemical series (see Standard Reduction Potentials table.) The elements are arranged in order of their readiness to release electrons and form cations. This series is useful because we can then predict which metals will react with the ions of other metals. In displacement reactions, a more reactive metal will displace the ions of a less reactive metal from solution. Displacement reactions can be explained in terms of electron transfer reactions. Metals oxidise, or lose electrons, in displacement reactions and become metal ions. The original metal ions reduce, or gain electrons and become metal atoms. When a metal is dipped into a solution of one of its salts, the ions go into solution and the electrons cling to the metal. The more reactive a metal is, the greater the force with which the electrons will move. PRAC 2: Perform a first-hand investigation and gather first-hand information to measure the difference in potential of different combinations of metals in electrolyte solution. Aim: To investigate the link between using different pairs of metal electrodes and the resultant voltage they produce. Method: 1. The following apparatus was set up. For trial 1, beaker 1 contained a zinc electrode immersed in zinc sulfate solution and beaker 2 contained a copper electrode immersed in copper sulfate solution. The salt bridge was made from a strip of filter paper soaked in saturated KNO3 solution. 2. Different combinations of electrodes were used. 3. The polarity of electrodes and net voltage were recorded for each of the trials. Results: |Trial |Beaker 1 |Beaker 2 |Net voltage (V) | |1 |Zn + 1 molL-1 ZnSO4 |Cu + 1 molL-1 CuSO4 |0.35 | |2 |Zn + 1 molL-1 ZnSO4 |Pb + 1 molL-1 Pb(NO3)2 |0.1 | |3 |Cu + 1 molL-1 CuSO4 |Pb + 1 molL-1 Pb(NO3)2 |0.15 | Zn(s) → Zn2+(aq) + 2e- 0.76 V anode Cu2+(aq) + 2e- → Cu(s) 0.34 V cathode Pb2+(aq) + 2e– → Pb(s) -0.13 V cathode Zn(s) → Zn2+(aq) + 2e- 0.76 V anode Cu2+(aq) + 2e- → Cu(s) 0.34 V cathode Pb(s) → Pb2+(aq) + 2e– 0.13 V anode By doing this, the reactivity of the metals tested can be worked out and placed in order of strongest electron donor to weakest electron donor. 3: Account for changes in the oxidation state of species in terms of their loss or gain of electrons. Metal and metal ions change their oxidation states during displacement reactions. Metals are assigned a zero oxidation state. The oxidation state of a metal ion is equal to the charge on the metal ion. During displacement reactions, metals increase their oxidation states (oxidise) and metal ions decrease their oxidation states (reduce). The oxidation number (ON) of an atom is the “charge” it would have if it occurred as an ion. Oxidation numbers are written as roman numerals generally with a positive or negative sign. [pic] Increase in oxidation number (0→+II): Oxidation. Decrease in oxidation number (+II →0): Reduction Oxidation State rules 1. Oxygen is -2 in a molecule 2. Hydrogen is +1 in a molecule (except hydrides) 3. Simple ions have the same oxidation number as the valency 4. Complex molecules add up to give zero. 5. The oxidation number of an element is zero Changes in oxidation state result from a loss or gain of electrons. If given a number of equations and asked which one is a redox reaction, look for a change in valency or an element being formed or entering into a compound. 4: Describe and explain galvanic cells in terms of oxidation/reduction reactions. A galvanic cell is a device which makes a chemical reaction occur in such a way that it produces electricity. It is constructed so that a reductant and oxidant are physically separated, but connected by an external circuit made of a conductor (to carry electrons) and a salt bridge (to carry charged ions in solution). A galvanic cell is thus composed of two half-cells, a reductant half-cell and an oxidant half-cell. This arrangement ensures that electrons cannot go directly from the reductant to the oxidant, but they will move through the external circuit. The reaction is always a redox reaction - more specifically a displacement reaction. The anode performs oxidation, separating electrons from the electrode, while the cathode performs reduction, combining the electrons with the ions in the solution to form metal. This is only for metals with active electrodes – electrolytes can be made from non-metals (such as halides) and electrodes can be made from inert conductive substances such as graphite. 5: Outline the construction of galvanic cells and trace the direction of electron flow. A galvanic cell is a device constructed so that a reductant and oxidant are physically separated, but connected by an external circuit made of a conductor (to carry electrons) and a salt bridge (to carry charged ions in solution) This arrangement ensure that electrons cannot go directly from the reductant to the oxidant, but they will move through the external circuit Net ionic equation: Zn(s) + Cu2+(aq) ( Zn2+(aq) + Cu(s) A typical galvanic cell is made as shown in the diagram. Copper and zinc electrodes are used here. By displacement, based on their respective reactivities, zinc atoms will release electrons (oxidation) and become zinc ions into its solution. The electrons will follow through the wire (being conductive) and the copper electrode and be attracted to the copper ions in the solution. The copper ions become copper metal (reduction) which is deposited on the electrode. The electron flow is electrical energy. 6: Define the terms anode, cathode, electrode and electrolyte to describe galvanic cells. - Cathode: electrode where reduction occurs in a galvanic cell. It receives the electrons and is the least reactive of the pair. Is positive. – Anode: the electrode where oxidation occurs in a galvanic cell. Is the more reactive of the two electrodes. Is negative. – Electrolyte: Any substance that conducts electricity when dissolved or in liquid form (e.g. KNO3) – Electrode: solid conducting rod in an electric cell where electrons are exchanged via the external circuit. Electrodes can also be part of the reaction. PRAC 3: Gather and present information on the structure and chemistry of a lead acid cell and evaluate it in comparison to a fuel cell in terms of: - Chemistry. - Cost and practicality. - Impact on society. - Environmental impact. | |Type Of Cell | |Cell Feature |Lead Acid |Lithium Ion | |Anode |Pb+(s) + SO42- ( PbSO4(s) + 2e - |LiC6 ( Li+ + C6 + e- | |Cathode |PbO2(s) + 4H+(aq) + SO42-(aq) + 2e- |MO2 + Li+ + e- ( LiMO2 | | |( | | | |PbSO4(s) + 2H2O(l) | | |Electrolyte |Dilute H2SO4 |Solid-electrolyte interphase | |Cost & Practicality |Good value for money. Very large and heavy making it |Most expensive, very light. | | |impractical. |Its tiny size makes it very practical. | |Impact on Society |First portable heavy duty battery. Used in almost all |Its high specific energy makes it a huge | | |vehicles. |success for mobile applications. | | |Has allowed inventions that require high energy to be |Very high impact on society. | | |portable and mobile thus having a major impact. | | |Environmental Impact |97% of lead recycled and reused. |Fully recyclable. | | |Lead is heavier then other metals and is toxic. The |Therefore this Electrochemical cell has a low | | |dilute H2SO4 must be disposed of individually in |impact. | | |special containers. Overall this cell has a low | | | |impact. | | |Advantages |Highest energy density for price. Very well recycled. |High specific energy excellent discharge and | | | |storage characteristics. | |Disadvantages |Very heavy. Long recharge (6hrs). Must check |More expensive then lead-acid cells. | | |electrolyte regularly. | | PRAC 4: Solve problems and analyse information to calculate the potential [pic] requirement of named electrochemical processes using tables of standard potentials and half-equations. The equations show reduction (gain) when read from left to right, and reduction (loss) when read from right to left. The Eø value given in the table is the value for the equation as it is written from left to right. If you want the value for the reverse process, simply change the sign of the Eø value. Cells are often represented in a shorthand fashion. 1. Cu | Cu2+ || Zn2+ | Zn Because the reduction of Zinc (Zn2+ + 2e- ( Zn) has the potential Eø equal to -0.76 V, then the oxidation of Zinc (Zn ( Zn2+ + 2e-) has the potential Eø value equal to +0.76V From the table, the half equation, Cu2+ + 2e- ( Cu, has Eø = +0.34V In this galvanic cell, the changes are: [pic] Thus, this galvanic cell is predicted to produce 1.10 volts The single line denotes a change of phase in Zn (s) and Zn2+ ions in solution. The double line represents the salt bridge. 2. Ag|Ag+||Cl-, Cl2|Pt This has Cl2 and Cl- in solution with a Pt electrode. 3. Ag|Ag+||Cl-|Cl2|Pt This represents bubbles of Cl2 (g) passing over a Pt electrode in a solution of Cl- (Note: and are in different phase, Pt is an inert electrode.) 5. Nuclear Chemistry provides a range of materials 1: Distinguish between stable and radioactive isotopes and describe the conditions under which a nucleus is unstable. Conditions for an unstable nucleus are: - Too many protons compared to neutrons (α) - Too many neutrons compared to protons (β) (e- come out of neutron) - Too big (α) (>83 electrostatic repulsion outweighs strong nuclear force) Changes in temperature, pressure, state, electron shell state, concentration do not affect rate of decay. |Radiation |Type |Penetrating Power |Ionising ability | |Alpha α |4He2 particle |Lowest |Most | |Beta β |0e-1 particle | |Gamma / Photon γ |High frequency electromagnetic radiation |Highest |Least | A stable isotope of an element will not emit radiation, whilst a radioactive isotope will emit radiation and decay. The emission of radiation will continue until the nucleus becomes sable. Elements with atomic numbers greater than 83 are naturally radioactive. The stability of an isotope depends on several factors: Neutron to proton ratio is too high (excess neutrons): When there are too many neutrons compared with protons then a neutron decays to form a proton and a beta particle (a fast electron) which is emitted from the nucleus. If the nucleus has too many neutrons, the nucleus has too much weight to keep together and the nucleus is not held as strongly together. This is beta negative decay. Example: 13153I → 13154Xe + 0–1e Neutron to proton ratio is too low (excess protons): When there are too many protons compared with neutrons then a proton decays to form a neutron and a positron (same mass as an electron with a positive charge). If the nucleus has too few neutrons, the force of repulsion between the protons is too strong and thenucleus is not held as strongly together. This is beta positive decay. Example: 2211Na → 2210Ne + 01e Too many nucleons (nucleus too heavy): When there are too many nucleons then alpha decay occurs. The loss of an alpha particle reduces the nucleus by 2 protons and 2 neutrons: Example: 23090Th → 22688Ra + 42He An isotope will radiate gamma rays if the protons and neutrons rearrange in the nucleus. 2: Describe how transuranic elements are produced. Transuranic elements are elements with atomic number of greater than 92 (Uranium) Neptunium and plutonium can both be found naturally, but the rest are not and are exclusively synthesized. Some isotopes do not undergo fission (splitting the atom) when hit by neutrons. Instead they absorb the neutron and thus create new elements. Transuranic elements are usually made by bombarding elements with particles or small nuclei (He, B, C, etc.) in a particle accelerator, or with neutrons in nuclear reactors e.g. to get Np, Pu or Am. Accelerators can be linear or circular, and work by accelerating particles with a series of electromagnets that repel and attract the particle as it moves through the accelerator so that it moves forward at a rapid pace. Linear accelerators can be up to 2-3km long. When U-238 is bombarded with neutrons it can be converted to U-239 that undergoes beta decays to produce neptunium and plutonium.  [pic] Then Pu-239 is changed to americium by neutron bombardment. [pic] Note: mass (the upper superscript numbers) is conserved as well as the charge (shown by the lower subscript numbers) is conserved. E.g. [pic] PRAC 1: Process information from secondary sources to describe recent discoveries of elements. Transuranic elements do not occur in nature. All of these elements with a higher atomic numbers then 92 have had to be produced artificially and are all radioactive. The twenty transuranic elements with the atomic numbers above 95 (between 96 and 118) require high-energy particle accelerators to be produced. They began to be discovered in 1937 with the discovery of the element Technetium and into the 1940's, with others being made from high speed particles in the 50's and 60's. Element 106 was made in 1974, and elements continue to be discovered into the 80's and 90's 3. Describe how commercial radioisotopes are produced. Many commercially used radioisotopes are created at nuclear reactors. When the uranium nucleus breaks up into two nuclei, many different isotopes are formed. Differences in chemical properties of the elements produced can be used to chemically separate the different radioisotopes. The high-speed neutrons emitted can be used to bombard atoms of various elements to produce useful neutron rich isotopes 4: Identify instruments and processes that can be used to detect radiation Photographic Film Nuclear technicians wear sealed badges with photographic film; the radiation changes the silver salts’ colour. The amount of radiation can be measured by the degree of film darkening. Geiger-Muller Tube and Counter Geiger-Muller uses a tube of low-pressure gas (such as argon) which can be easily ionized. A wire in the centre of the tube is the positive electrode, with the casing of the tube being the negative electrode. At the front of the tube, a mica window lets the radiation in but not the gas out. The ions and loose electrons produced by ionising radiation hit the gas molecules create an electrical current (much like electrolysing salts.) The pulses in current are counted electronically, giving a radiation strength reading. Wilson Cloud Chamber A cloud chamber is a sealed environment containing a supercooled, supersaturated water or alcohol vapour. When an alpha particle or beta particle interacts with the mixture, it ionises it. The resulting ions act as condensation nuclei, around which a mist will form (because the mixture is on the point of condensation). The high energies of alpha and beta particles mean that a trail is left, due to many ions being produced along the path of the charged particle. These tracks have distinctive shapes (for example, an alpha particle's track is broad and straight, while an electron's is thinner and shows more evidence of deflection). When a vertical magnetic field is applied, positively and negatively charged particles will curve in opposite directions. Scintillation counter A scintillation counter measures ionizing radiation. The sensor, called a scintillator, consists of a transparent crystal, usually phosphor, plastic, or organic liquid that fluoresces when struck by ionizing radiation. Gamma rays hit the atoms, which excites the electrons and makes them jump shells. When the electron falls back, the radiation is emitted again in the form of visible light. A sensitive photomultiplier tube (PMT) measures the light from the crystal. The PMT is attached to an electronic amplifier and other electronic equipment to count and possibly quantify the amplitude of the signals produced by the photomultiplier. 5: Identify one use of a named radioisotope in medicine. 6: Describe the way in which the above named radioisotope is used and explain its use in terms of its chemical properties. Medical: Technetium-99m (Tc-99m) is a gamma ray emitting isotope used in radioactive isotope medical tests, for example as a radioactive tracer that medical equipment can detect in the body. It is created from Molybdenum-99, which is a radioactive isotope of Molybdenum (average atomic mass is