1.13 SOME CONCEPTS IN CHEMISTRY

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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

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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.        

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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.

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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.

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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.

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• 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

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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).

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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.

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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

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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

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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.

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Measuring plaster exotherm
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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.

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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.

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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

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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.

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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.

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Hard and Soft water

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

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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.

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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

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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.

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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.

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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

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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

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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.

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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.

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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

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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

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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.

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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.

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Flue Gas Scrubber Installation
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Hemihydrate feed
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Paper feed for plasterboard
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Plasterboard Forming Plates
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Plasterboard line