chemistry

• Chemical reactions involve the change and production of new chemical substances
• Reactants are the substances involved in a chemical reaction and are changed by it
• Products are the substances produced by a chemical reaction
• Indicators of a chemical reaction:

  • Colour change
  • Effervescence (bubbles of gas produced)
  • Precipitation (formation of an insoluble solid)
  • Energy (temperature) change
• Colour change in chemical reactions:

  • Can occur when two substances react or when a compound is broken down by heating (thermal decomposition)
• Effervescence in chemical reactions:

  • Results in bubbles of gas being produced
  • Indicator of a chemical reaction
• Precipitation and chemical reactions:

  • Some liquids react to produce an insoluble solid (precipitate)
  • Precipitation reaction involves the formation of a solid precipitate
• Energy change in chemical reactions:

  • Different substances hold different amounts of energy
  • Bonds must be broken and new bonds formed during a reaction
  • Exothermic reactions release energy (temperature rise)
  • Endothermic reactions absorb energy (decrease in temperature)
• Atoms are made up of three smaller particles: electrons, protons, and neutrons
• Electrons:

  • Negatively charged particles that spin around the positive centre of the atom in circles called energy levels
  • Their mass is nearly zero
• Protons:

  • Positively charged particles that are contained in the nucleus of the atom
  • Have a mass of 1 amu (atomic mass unit)
• Neutrons:

  • Particles with no charge that are also contained in the nucleus of the atom
  • Have a mass of 1 amu
• A titration experiment is used to accurately measure the volume of substances that react in chemical reactions
• titrations are Commonly used in neutralisation reactions
• Titration Can be used to calculate an unknown concentration of either the acid or alkali
• Essential to measure things as accurately as possible during titration
• Steps of the experiment:

  • If the sample is a solid, it is weighed using an accurate balance and dissolved to make up a known volume of solution (usually 100 cm3)
  • A pipette is used to measure accurately a volume of this solution, for example, 10 cm3, and emptied into a conical flask
  • A few drops of an indicator may be added to the conical flask to show a change of colour when the titration is complete
  • A second chemical is placed in a burette, often an acid of a precise, known concentration
• • The solution from the burette is added drop by drop into the conical flask with swirling to mix the solutions as the end-point is approached
  • A colour change indicates that the correct amount has been added to react completely with the sample in the conical flask
  • The volume of solution added from the burette is noted for calculating the concentration of the sample in the conical flask
• Homologous series are families of hydrocarbons with similar chemical properties and the same general formula
• Three hydrocarbon series: alkanes, alkenes, and cycloalkanes
• Hydrocarbons are compounds containing only hydrogen and carbon
• Alkanes:

  • Names end in -ane
  • Uses include:
  • Methane (natural gas): cooking, heating
  • Propane: used in gas cylinders for BBQ, etc
  • Octane: used in petrol for cars
• General formula of alkanes: CnH2n+2
• Alkanes are insoluble in water
• Alkenes

  The second homologous series is the alkenes. Their names all end in –ene, for example ethene.

  Alkenes all contain a carbon to carbon double bond which makes them more reactive than the alkanes. The alkenes have the general formula

  CnH2n.
• Cycloalkanes

  The third homologous series is the cycloalkanes. Their names all end in –ane and begin with cyclo-, for example cyclopropane. The cycloalkanes have the general formula CnH2n
• Isomers

  Isomers are compounds with the same molecular formulae but different structural formulae.

  They have the same number of each type of atom but may have different physical and chemical properties.
• Combustion reactions

  All hydrocarbons (including alkanes, alkenes and cycloalkanes) can undergo combustion reactions with oxygen to give the same two products. Hydrocarbon fuels burn when they react with oxygen in the air. As all hydrocarbons only contain the elements carbon and hydrogen, the only products will be oxides of these elements. So as long as enough oxygen is present for complete combustion, the two products formed will be carbon dioxide (CO2) and water (H2O). The word equation for this reaction is shown below.
• Burning methane gas produces carbon dioxide and water vapour
• A covalent bond is a shared pair of electrons between atoms of two non-metal elements
• Covalent bond happens when the positive nuclei from two different atoms are held together by their common attraction for the shared pair of electrons held between them
• Covalent bonds are strong bonds
• A molecule is a group of atoms held together by covalent bonds
• A diatomic molecule is a molecule containing only two atoms
• There are seven diatomic elements: Iodine, Bromine, Chlorine, Fluorine, Oxygen, Nitrogen, and Hydrogen
• Mnemonic to remember the seven diatomic elements: "I Bring Clay For Our New House"
• Diagrams can be used to show how the outer electrons are shared to form the covalent bonds in a molecule
• Ionic bonds are formed between a metal and non-metal
• The ionic bond is the electrostatic force of attraction between a positively charged metal ion and a negatively charged non-metal ion
• Metals form positive ions because they lose electrons to become stable