Chemistry Topic 2

• when is something pure?

  when all the particles that make up the substance are the same
• what do all substances have at room temp?

  specific melting points
• how can you check the purity of a substance?

  comparing the actual melting point to the melting point at room temp
• what do impurities cause?

  the substance to melt at a different temperature
• what are formulations?

  mixtures that have been carefully designed to have specific properties e.g. alloys
• filtration

  a solid is separated from a liquid
• crystallisation

  a solvent is evaporated off to leave behind a solute in crystal form
• distillation

  two liquids with significantly different boiling points are separated, when heated the liquid with the lowest boiling point evporates first and the vapour is condensed and collected
• fractional distillation


  a mixture of liquids with different boiling points are separated
• what is chromatography?

  substances in a mixture are separated using a stationary phase and a mobile phase
• what is paper chromatography useful for?

  separating mixtures of dyes in solution
• what is thin layer chromatography?

  more accurate than paper chromatography and uses a thin layer of inert solid for the stationary phase
• what is gas chromatography?

  separates gas mixtures by passing them through a solid stationary phase
• how do you calculate rf values?

  distance travelled by substance/ distance travelled by solvent
• who developed the modern periodic table?

  Mendeleev
• describe Mendeleevs periodic table

  it was testable and predicted elements not yet discovered

  some put in wrong place as values used for their atomic masses weren't correct

  modern periodic table is a modified version of his

  it takes into account the arrangement of electrons, the number of electrons in outer shell, and atomic number
• why are metals useful materials?

  - they are lustrous

  - hard and have high density

  - high tensile strength

  - high melting an boiling points

  - good conductors of heat and electricity
• what can metals react with non metals to form?

  ionic compounds
• what is a chemical bond?

  transfer or sharing of electrons which leads to atoms involved becoming more stable
• what is an ionic bond?

  the electrostatic forces of attraction that hold the ions together

  
  

  formed when one or more electrons are donated by one atom or molecule and received by another atom or molecule
• when is a covalent bond formed?

  formed when atoms share electrons to complete their outermost shell
• what happens when an ionic compound is in solution or in a molten state?

  the ions move freely
• what happens when an ionic compound is solid?

  ions are arranged in a way to cancel out the charges
• what do ball and stick models show?

  the 3D shape of a molecule or compound
• limitations of models

  - scale of nucleus to the electrons is wrong

  - models show bonds as physical structures

  - don't give an accurate idea of the 3D shape of a molecule

  - bond lengths are not in proportion to the size of the atoms

  - aid our understanding, but arent the real thing
• what are simple molecules?

  when non metals or non ionic molecules join together, the atoms share electrons and form a covalent bond
• how are giant covalent structures formed?

  when the atoms of a substance form repeated covalent bonds
• how is a polymer formed?

  when repeated units are covalently bonded together
• how are metal atoms held together?

  strong metallic bonds
• what happens to metals in a metallic bond?

  they lose their outermost electrons and become positively charged, the electrons can move freely from one metal ion to another, causing a sea of delocalised electrons to be formed
• how many atoms can carbon atoms bond with?

  four other carbon atoms
• what are allotropes of carbon?

  different structures that are formed depending on how many carbon atoms bond together
• when is graphite formed?

  when each carbon atom bonds with three other carbon atoms

  
  

  graphite has free electrons so it can conduct electricity

  layers are held together by weak bonds, so they can break off easily
• what is graphene?

  a single layer of graphite
• properties of graphene

  has free electrons so it can conduct electricity

  used in electronics and solar panels
• when is diamond formed?

  when each carbon atom bonds with four other carbon atoms
• properties of diamond

  - can't conduct electricity as all its outermost electrons are involved in bonding

  - very hard, used in drill bits and polished diamonds are used in jewellery

  - extremely strong because each atom forms the full number of covalent bonds
• what can carbon form?

  long chains of atoms and rings
• what are organic compounds?

  naturally occurring and man made (synthetic) carbon based compounds
• what are fullerenes? uses?

  tubes and spherical structures formed using only carbon atoms, used as superconductors, for reinforcing carbon-fibre structures, and as containers for drugs being introduced into the body