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1.13 SOME CONCEPTS IN CHEMISTRY

1.13 SOME CONCEPTS IN CHEMISTRY. 1.13.1 The components of the atom 1.13.2 Chemical reactions 1.13.3 Acids and bases 1.13.4 Oxidising and reducing agents 1.13.5 The lime cycle 1.13.6 The gypsum cycle. Protons.

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1.13 SOME CONCEPTS IN CHEMISTRY

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  1. 1.13 SOME CONCEPTS IN CHEMISTRY • 1.13.1 The components of the atom • 1.13.2 Chemical reactions • 1.13.3 Acids and bases • 1.13.4 Oxidising and reducing agents • 1.13.5 The lime cycle • 1.13.6 The gypsum cycle

  2. Protons • Protons. These have a mass of 1.67  10-27 kg and a charge of 1.6  10-19 Coulomb. The number of protons in an (the atomic number) atom determines what element it is.        

  3. Neutrons • These have the same mass as a proton but no charge. They normally occur in approximately equal numbers to the protons in an atom. The number of neutrons determines the isotope of the element. Most elements have a few stable isotopes but if the number of neutrons is outside the range of the stable isotopes (higher or lower) the atom will be unstable and will spontaneously decay and emit radiation.

  4. Electrons. • These have a mass of 9.11  10-31 kg and a charge equal and opposite to the proton. The number of electrons determines the charge of the atom, if it is equal to the number of protons there is no net charge but if it is not the atom is a charged ion. The amount of energy required to remove an electron (the ionisation energy) determines the chemical properties of the element.

  5. Anions and Cations In construction many materials (e.g. in concrete) are in solution in water and the atoms may be present as charged ions. Anions are negative (i.e. they are found at the anode) and cations are positive.

  6. Atomic number Symbol Name 1 H Hydrogen 6 C Carbon 7 N Nitrogen 8 O Oxygen 11 Na Sodium 12 Mg Magnesium 13 Al Aluminium 14 Si Silicon 17 Cl Chlorine 19 K Potassium 20 Ca Calcium 26 Fe Iron 29 Cu Copper 82 Pb Lead

  7. The convention for describing an atom 63 2+ 29Cu. In this the Cu means that the element is copper, the 29 means that it has 29 protons, the 63 means that the total number of protons + neutrons (the atomic mass) is 63 and the 2+ means that it has two less electrons than protons (i.e. a positive charge equal to two electrons).

  8. Molecules A molecule is a number of atoms which are held together by bonds between them. For example H2 is a hydrogen molecule with two hydrogen atoms in it. Hydrogen atoms in hydrogen gas always form this molecule.

  9. One mol of a gas One "mol" of a material is defined 6.02 1023 molecules. The mass of 1 mol of a material with an atomic mass of m is m grammes If the material is a gas the formula: PV = nRT applies (for ideal gasses) where: n is the number of mols of gas present , R = 8.31 J/mol/oK, and P,V,T are the pressure, volume and temperature (oK) Thus at a given temperature and pressure one mol of any gas will occupy the same volume

  10. 1.13 SOME CONCEPTS IN CHEMISTRY • 1.13.1 The components of the atom • 1.13.2 Chemical reactions • 1.13.3 Acids and bases • 1.13.4 Oxidising and reducing agents • 1.13.5 The lime cycle • 1.13.6 The gypsum cycle

  11. Heat in reactions • Exothermic reactions: These reactions proceed as soon as the components come into contact and give off energy (heat). An example is the mixing of the components of an epoxy adhesive. • Endothermic reactions: These reactions require heat to make them proceed. An example is the manufacture of cement in kilns where the components would not react at all if not heated.

  12. Measuring plaster exotherm

  13. Rules of Reactions • The number of atoms of each element must be the same on both sides of the equation. • If the reaction only proceeds in one direction the symbol  is used but if it can proceed in both directions  is used.

  14. Reaction Rates • The temperature. A rough rule is that an increase of 20oC doubles the rate of reaction. • The pressure, high pressures will increase reaction rates. • The energy released by the reaction. • The physical consistency of the materials. A fine powder will react faster than a coarse one • The presence of a catalyst. This is a material which is not used up by the reaction but makes is proceed faster.

  15. 1.13 SOME CONCEPTS IN CHEMISTRY • 1.13.1 The components of the atom • 1.13.2 Chemical reactions • 1.13.3 Acids and bases • 1.13.4 Oxidising and reducing agents • 1.13.5 The lime cycle • 1.13.6 The gypsum cycle

  16. pH pH = log(1/H+) where H+ is the number of grammes of hydrogen ions per litre. In pure water the following equilibrium reaction takes place: H2O  H+ + OH- and there are 10-7 grammes of hydrogen ions per litre. Thus the pH of water is 7 and is defined as neutral. Acids have pH below 7 and alkalis (bases) have pH above 7. Concrete has a pH of 12.5.

  17. Dissolution of Metals • Metals dissolve in acidic waters. • This is a major problem because most metals are highly toxic in solution. • When they are not in solution, i.e. in solid form (or liquid for mercury) they are not very toxic because they are not absorbed by the body. • Many metals, e.g. lead, aluminium and mercury can build up in the body over a period of time.

  18. Hard and Soft water • Tap water is usually described as "hard" or "soft" depending on its acidity. • Hard water is alkaline (pH > 7), • Soft water is acidic (pH < 7). • Acidic air pollution causes acid rain which is tending to "soften" water supplies. • Large scale water transfer schemes are also changing water supplies

  19. Effect of water types • Hard water will not dissolve metals and will often leave carbonate deposits on them. • Soft water will dissolve lead from pipes and even lead solder on copper pipes.   • Concrete has the potential to remove metals from solution because it is alkaline and has a high capacity for adsorption.

  20. 1.13 SOME CONCEPTS IN CHEMISTRY • 1.13.1 The components of the atom • 1.13.2 Chemical reactions • 1.13.3 Acids and bases • 1.13.4 Oxidising and reducing agents • 1.13.5 The lime cycle • 1.13.6 The gypsum cycle

  21. Oxidising and reducing agents • The term "oxidising agent" originally applied to compounds which would add oxygen to substances but it is now generally applied to all compounds which lose electrons in reactions. • A reaction which falls within this wider definition is: 2Na + Cl2 2NaCl in which sodium gives up an electron to chlorine to form common salt.

  22. Eh • The tendency of a given element or compound to give electrons depends on the energy level of the "outermost" electrons in it. • This energy is called the "redox potential" or "Eh". • The difference in Eh between two samples may often be measured electrically by putting them in a conducting solution and measuring the voltage between them.

  23. 1.13 SOME CONCEPTS IN CHEMISTRY • 1.13.1 The components of the atom • 1.13.2 Chemical reactions • 1.13.3 Acids and bases • 1.13.4 Oxidising and reducing agents • 1.13.5 The lime cycle • 1.13.6 The gypsum cycle

  24. The lime cycle Calcium carbonate (limestone)  Calcium oxide (quicklime) + Carbon dioxide CaCO3 CaO + CO2 Quicklime + water  Hydrated (slaked) lime CaO + H2O  Ca(OH)2 Hydrated lime + Carbon dioxide  Calcium carbonate + water Ca(OH)2 + CO2 CaCO3 + H2O

  25. Quicklime The quicklime is highly reactive. Some of it is used directly (e.g. in concrete block production) but most of it is slaked in the hydration reaction which is quite violent and gives off a lot of heat.

  26. Hydrated Lime • Hydrated lime is sold at builders merchants for use in bricklaying mortar. Traditional mortars set by the carbonation reaction. • In modern construction cement is used in the mortar to achieve a rapid strength gain.

  27. 1.13 SOME CONCEPTS IN CHEMISTRY • 1.13.1 The components of the atom • 1.13.2 Chemical reactions • 1.13.3 Acids and bases • 1.13.4 Oxidising and reducing agents • 1.13.5 The lime cycle • 1.13.6 The gypsum cycle

  28. The Gypsum Cycle • If gypsum is heated it will calcine to produce hemihydrate (stucco): • 2CaSO4.2H2O  2CaSO4.½H2O + 3H2O  • Hemihydrate can be calcined further to produce anhydrite: • 2CaSO4.½H2O  2CaSO4 + H2O

  29. Hemihydrate • This material is used for plaster and plasterboard. • When mixed with water it will revert back to gypsum, i.e the plaster will set. • It can also be used for block manufacture (for internal use) and floor screeds, although this is not common in the UK.

  30. Sources of Gypsum • In the UK plaster and plasterboard is produced by calcining both natural and by-product gypsum (mainly from coal burning power station flue-gas scrubbers). • The cement industry also uses both sources.

  31. Flue Gas Scrubber Installation

  32. Hemihydrate feed

  33. Paper feed for plasterboard

  34. Plasterboard Forming Plates

  35. Plasterboard line

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