Chemical changes and structure

• Rates of reaction

  Can be increased by:

  • Increasing the temperature
  • Increasing the concentration of a reactant
  • Increasing surface area/decreasing particle size
  • Through the use of a catalyst
• Catalyst

  Substances that speed up chemical reactions but can be recovered chemically unchanged at the end of the reaction
• Calculating the average rate of a chemical reaction

  rate = Quantity / Time
• The rate of a reaction can vary at different stages of the reaction
• Covalent bonds form between non-metal atoms
• Covalent bond

  Forms when two positive nuclei are held together by their common attraction for a shared pair of electrons
• Diagrams can be drawn to show how outer electrons are shared to form the covalent bond(s) in a molecule
• Elements that exist as diatomic molecules through the formation of covalent bonds

  • H₂, N₂, O₂, F₂, Cl₂, Br₂, I₂
• Shape of simple covalent molecules

  • Depends on the number of bonds and the orientation of these bonds around the central atom. Can be described as linear, angular, trigonal pyramidal or tetrahedral
• More than one bond can be formed between atoms leading to double and triple covalent bonds
• Covalent substances

  Can form either discrete molecular or giant network structures
• Ions

  Formed when atoms lose or gain electrons to obtain the stable electron arrangement of a noble gas
• Ionic bond

  The electrostatic attraction between positive and negative ions
• Ionic compounds

  • Form lattice structures of oppositely charged ions with each positive ion surrounded by negative ions and each negative ion surrounded by positive ions
• Isotopes

  Atoms with the same atomic number but different mass numbers, or atoms with the same number of protons but different numbers of neutrons
• Most elements have two or more isotopes
• Covalent molecular substances

  • Have strong covalent bonds within the molecules and only weak attractions between the molecules
  • Have low melting and boiling points as only weak forces of attraction between the molecules are broken when a substance changes state
  • Do not conduct electricity because they do not have charged particles which are free to move
• Covalent network structures

  • Have a network of strong covalent bonds within one giant structure
  • Have very high melting and boiling points because the network of strong covalent bonds is not easily broken
  • Do not dissolve
• In general, covalent network substances do not conduct electricity because they do not have charged particles which are free to move
• Ionic compounds

  • Have high melting and boiling points because strong ionic bonds must be broken in order to break up the lattice
  • Many are soluble in water as the lattice structure breaks up allowing water molecules to surround the separated ions
  • Conduct electricity only when molten or in solution as the lattice structure breaks up allowing the ions to be free to move
• Conduction in ionic compounds can be explained by the movement of ions towards oppositely charged electrodes
• Chemical equations, using formulae and state symbols, can be written and balanced
• pH scale

  Indication of the hydrogen ion concentration, runs from below 0 to above 14
• A neutral solution has equal concentrations of H(aq) and OH(aq) ions
• Water dissociation

  1. H₂O()

  2. H(aq) + OH(aq)
• Only a few water molecules are dissociated into free ions at any time
• The symbol indicates that a reaction is reversible and occurs in both directions
• Acidic solutions

  Have a higher concentration of H(aq) ions than OH(aq) ions, have a pH below 7
• Alkaline solutions

  Have a higher concentration of OH(aq) ions than H(aq) ions, have a pH above 7
• Dilution of an acidic solution with water

  Decreases the concentration of H(aq) and the pH will increase towards 7
• Dilution of an alkaline solution with water

  Decreases the concentration of OH(aq) and the pH will decrease towards 7
• Soluble non-metal oxides dissolve in water forming acidic solutions
• Soluble metal oxides dissolve in water to form alkaline solutions
• Bases

  Metal oxides, metal hydroxides, metal carbonates and ammonia, neutralise acids
• Bases that dissolve in water form alkaline solutions
• Neutralisation reaction

  A base reacts with an acid to form water and a salt
• Neutralisation reactions

  1. metal oxide + acid

  2. metal hydroxide + acid

  3. metal carbonate + acid
• Salt

  Formed in a neutralisation reaction, depends on the acid and base used
• Hydrochloric acid produces chlorides, sulfuric acid produces sulfates and nitric acid produces nitrates
• Spectator ions

  Ions that remain unchanged by the reaction