Chem P1

• why are ionic compounds usually solid at room temperature

  strong electrostatic forces between oppositely charged ions which require a lot of energy to overcome
• covalent bonds

  they do not conduct electricity when dissolved in water
• why are small covalent molecules gas at room temperature

  the intermolecular forces between molecules are weak, a small amount of energy is required to break the bonds, so at room temperature molecules can move freely, so are gaseous
• why are strong acids more reactive than weak acids

  strong acids have a higher concentration of H+ ions, so more of them will react in a given time. This increases the rate of the reaction
• In the electrolysis of aqueous solutions, what is the rule for determining which ion will be reduced at the cathode

  Always be H+, unless there is a less reactive metal ion present e.g. always the ion of the least reactive element
• why were elements often placed in the wrong group of the periodic table in earlier versions

  elements were placed in order of atomic weight rather than atomic mass, and didn't take into account the elements chemical properties
• as you go down group 7 halogens, why does the reactivity decrease

  atomic radius and shielding increase, this means there are weaker electrostatic forces of attraction between the nucleus and the electron its trying to gain. This means more energy is required for a reaction to occur
• properties of group 1 metals

  soft, low density, very reactive, weak, relatively low melting points
• problems with low atom economy

  more expensive as more reactants required, less sustainable as it uses more resources, there will be more waste products to be disposed of which causes damage to the environment
• why do ionic compounds have high melting and boiling points

  strong ionic bonds (electrostatic forces of attraction) between oppositely charged ions require a lot of energy (high temperature) to break
• what is meant by covalent bonding

  the electrostatic attraction between a shared pair of electrons and the nuclei of the 2 bonded atoms. The 2 atoms share electrons to gain a full outer shell
• structure and bonding of graphite

  each carbon is covalently bonded to 3 other carbon atoms, forming a sheet of hexagonal rings. The separate sheets are held together by intermolecular forces. It also has delocalised electrons that can flow through the structure and carry and electrical current.
• what are nano particles

  small particles with a diameter between 1 and 100nm
• law of conservation of mass

  no atoms are lost or gained during a chemical reaction
• properties of graphite

  high melting and boiling points, soft & brittle, good conductor of heat & electricity
• what happens to negative ions during electrolysis

  move towards anode, they lose electrons, become oxidised
• what three types of substances can covalent bonds make

  polymers, giant covalent structures, simple molecular structures
• why do the electrodes need to be inert in electrolysis

  don't react with electrolyte and dissolve away, requiring replacement
• why wouldn't you use universal indicator for titrations

  doesn't provide a sudden colour change, so it would be hard to see the end point
• properties of graphene

  high melting and boiling points, strong, conducts electricity, very light
• describe the bonding in simple molecular substances

  strong covalent bonds between the atoms of each molecule, weak intermolecular forces between the different molecules
• why is cryolite used the the electrolysis of aluminium oxide

  aluminium oxide has a high mp so it would be expensive. cryolite lowers the mp and reduces the cost of the process
• how are atoms in the current periodic table measured

  increasing atomic number, patterns of chemical and physical groups
• what is a fuel cell

  an electrical cell supplied with fuel and oxygen, uses energy from the reaction to produce electricity
• advantages of fuel cell in cars

  dont produce as many pollutants, rechargeable (there is a limit to the number of times they can recharge), store more energy than batteries
• disadvantages of hydrogen fuel cells

  hydrogen takes up alot of space as it is a gas, hydrogen is very explosive when mixed with water so it is had to store safely, making hydrogen requires electricity which is generated through burning fossil fuels
• reaction at cathode of a hydrogen-oxygen fuel cell

  4H+ + O2 + 4e- -> 2H2O
• reaction at anode of hydrogen- oxygen fuel cell

  H2 -> 2e- + 2H+
• commonly used electrolyte for hydrogen fuel cells

  phosphoric acid
• overall reaction in hydrogen fuel cell

  2H2 + O2 -> 2H2O
• what are electrodes made from

  carbon