3 - Periodic Table & Energy

• Periodicity

  Trends or recurring variations in element properties with increasing atomic number.
• Structure of the Periodic Table

  Elements are arranged by increasing atomic number.

  
  

  All elements within a period have the same number of electron shells, thus there are repeating physical and chemical trends.

  
  

  All elements within a group have the same number of electrons in their outer shell, thus they have similar chemical properties.
• First Ionisation Energy

  The enthalpy required to remove one mole of electrons from one mole of gaseous atoms, to form one mole of gaseous 1+ ions.
• Factors Affecting Ionisation Energy

  Nuclear Charge - measure of the ability of protons in the nucleus to attract negative electrons; the more protons in the nucleus, the stronger the attraction.

  
  

  Atomic Radius - size of the atom, measured from the nucleus to the electron cloud; a greater distance reduces the nuclear attraction.

  
  

  Electron Shielding - the reduction of the attraction between the nucleus and outer electrons; the inner electrons repel the outer electrons and reduce the nuclear attraction.
• Trends in Ionisation Energies

  Ionisation energy decreases down a group - this is due to extra electron shells increasing the atomic radius and increasing electron shielding, therefore reducing nuclear attraction.

  
  

  Ionisation energy increases across a period - number of protons increases thus increasing the nuclear attraction.

  This causes the electrons to be pulled closer, reducing atomic radius.
• Trends in Ionisation Energies - Exceptions

  Group 2 and 3:

  - Ionisation decreases slightly.

  - Outer electron in group 3 is in a p-orbital rather than s-orbital.

  - This increases distance from the nucleus and electron shielding, thus reducing nuclear attraction.

  
  

  Group 5 and 6:

  - Ionisation energy decreases.

  - in group 5, the electron is being removed from a singly occupied orbital.

  - In group 6, the electron is being removed from an orbital containing two electrons.

  - The repulsion between the two electrons means it is easier to removed from the shared orbital.
• Successive Ionisation Energies

  Measure of the energy required to remove each electron in turn.

  
  

  Each successive ionisation energy is greater than the last as there is less repulsion allowing them to be drawn closer to the nucleus.

  The decreasing atomic radius increases the nuclear attraction.
• Giant Covalent Lattices

  Huge networks of covalently bonded atoms.

  
  

  Diamond:

  - Covalently bonded to four other carbon atoms.

  - Forms a tetrahedral structure.

  - Cannot conduct electricity

  
  

  Graphite:

  - Sheets can slide over each other due to weak IMFs between layers.

  - Delocalised electrons allow it to conduct a charge.

  - Layers are far apart, so it is less dense than diamond.
• Giant Metallic Structures

  Metallic Bonding - strong electrostatic attraction between cations and delocalised electrons.

  
  

  Number of delocalised electrons per atom affects the melting point; the more there are the stronger the bonding, therefore a higher melting point.

  
  

  Malleable and ductile as there are no bonds holding specific ions together.

  
  

  Insoluble.

  
  

  Conductors - delocalised electrons can move and carry a charge.
• Group Two

  Reactivity increases down group two - ionisation energies decrease due to increasing atomic radius and electron shielding.

  
  

  React with water to produce hydroxides.

  
  

  Burn in oxygen to form solid white oxides.

  
  

  React with dilute acids to produce a salt and hydrogen.
• Group Two - Uses

  Group two oxides and hydroxides are bases, and most are soluble in water, so are also alkalis.

  
  

  Calcium hydroxide is used in agriculture to neutralise acidic soils.

  
  

  Magnesium hydroxide and calcium carbonate are used as antacids.
• Group Seven

  Melting and boiling points increase down the group as London forces increase, due to the increase in electrons.

  
  

  Reactivity decreases down the group due to increased atomic radius and electron shielding, which reduces the nuclear attraction.
• Group Seven - Displacement Reactions

  Mix the halogen water with a (more reactive) potassium halide solution.

  Will cause a colour change.

  If bromine is displaced - it will turn orange.

  If iodine is displaced - it will turn purple.

  Hexane makes the colour change easier to see.

  Cl2 will displace Br- and I- ions.

  Br2 will displace I- ions.
• Group Seven - Uses

  Chlorine and NaOH form bleach - used in water treatment, bleaching paper and textiles, and household cleaning.

  
  

  Chlorine and water forms HCl and HClO - HClO ionises to form chlorate(I) ions, which kill bacteria in water (making it safe to drink and swim in).

  
  

  + Prevents growth of algae and reinfection.

  + Kills micro-organisms.

  
  

  - Chlorine gas is harmful if inhaled.

  - Liquid chlorine causes chemical burns.

  - Chlorine reacts with organic compounds to form chlorinated hydrocarbons - these are carcinogens.
• Tests for Ions - Carbonates

  Add dilute acid.

  
  

  If a carbonate is present, carbon dioxide is released.

  
  

  Carbon dioxide can be detected with limewater.
• Tests for Ions - Sulfates

  Add dilute HCl, then BaCl2

  
  

  A white precipitate will form (this is insoluble barium sulfate).
• Tests for Ions - Halides

  Add dilute nitric acid, then silver nitrate.

  
  

  White precipitate - chlorine

  
  

  Cream precipitate - bromine

  
  

  Yellow precipitate - iodine
• Tests for Ions - Ammonium

  Add NaOH and warm.

  
  

  Test with RED litmus paper - it will turn BLUE.
• Tests for Ions - Order of Tests

  Should first do carbonate test - sulfate test - halide test
• Enthalpy Change of Formation


  One mole of a compound is formed from its constituent elements under standard states and in standard conditions.
• Enthalpy Change of Combustion

  One mole of a substance reacts completely with oxygen under standard states and conditions.
• Enthalpy Change of Neutralisation


  One mole of water is formed in a reaction between an acid and an alkali under standard conditions and in standard states.
• Calculating Enthalpy Changes


  Q = mcΔT

  
  

  Q = energy change (J)

  m = mass (g)

  c = specific heat capacity

  ΔT = temperature change (K)

  
  

  - Calculate Q

  - Convert Q from J to kJ

  - Calculate the moles of the fuel.

  - ΔH = -Q/n
• Average Bond Enthalpy

  The energy required to break one mole of a specified type of bond in a gaseous molecule.
• Hess' Law

  States that the total enthalpy change of a reaction is always the same, no matter the route taken, as long as the initial and final conditions are the same.

  
  

  Formation Cycles = both arrows upwards

  (therefore, ΔHr = ΔH2 - ΔH1)

  
  

  Combustion Cycles = both arrows downwards

  (therefore, ΔHr = ΔH1 - ΔH2)
• Collision Theory

  Increasing concentration - the particles are close together, meaning they'll collide more frequently, increasing the number of successful collisions.

  
  

  Increasing pressure - at higher pressures the particles are closer together, increasing the rate of successful collisions.

  
  

  Catalysts - lower activation energy by providing an alternative route, increasing the rate of successful collisions.
• Investigating Rate of Reaction - Change in Mass

  Used when the product is a gas.

  
  

  - Take mass measurements at regular intervals.

  - Very accurate, but can be dangerous if the gas released is flammable or toxic.

  
  

  rate = amount of reactant used/time
• Investigating Rate of Reaction - Volume of Gas Produced

  Use a gas syringe.

  
  

  - Measure the volume of gas in the syringe.

  - Accurate, but vigorous reactions can blow the plunger out of the syringe.

  
  

  rate = product formed/time
• Catalyst

  Substance that increases the rate of a chemical reaction by providing an alternative reaction pathway with a lower activation energy, without undergoing any permanent change itself.
• Heterogenous Catalyst

  One that is a different phase to the reactants.

  
  

  The reaction occurs on the surface of the catalyst, where the reactants are adsorbed.
• Homogenous Catalyst

  In the same phase as the reactants.

  
  

  Forms an intermediate species, which then reacts to form the products and reform the catalyst.
• Benefits of Catalysts

  Lowers cost - alternative pathway of lower temperature and/or pressure.

  
  

  Environmental Sustainability - less energy required, reducing the amount of carbon dioxide released from fossil fuels.

  
  

  Catalytic Converters - reduce pollution released by cars.
• Boltzmann Distribution

  Area beneath the curve represents total number of particles present.

  
  

  Area after Ea represents the proportion of molecules greater than or equal to activation energy.
• Boltzmann Distribution - Increasing Temperature

  Peak of the graph is lower and shifted to the right.

  
  

  There is a greater proportion of particles that have an energy greater than or equal to Ea, therefore there are more successful collisions.
• Boltzmann Distribution - Catalysts

  Changes the position of Ea, as catalysts provide an alternative pathway with lower activation energy.

  
  

  Lowering Ea means a greater proportion of molecules have sufficient energy for an effective collision.
• Dynamic Equilibrium

  Exists in a closed system when the rate of the forward reaction is equal to the rate of the reverse reaction, and the concentrations of reactants and products do not change.
• Le Chatelier's Principle

  If a change is made to a system in dynamic equilibrium, the position of equilibrium moves to counteract the change.

  
  

  Pressure - shifts in the direction that produces the smaller number of molecules to counteract the change.

  
  

  Temperature - an increase causes the position of equilibrium to move in the endothermic direction to counteract the change (and vice versa).

  
  

  Catalyst - does not affect the position of equilibrium, only the rate of reaction (but do mean equilibrium is reached faster).
• Equilibrium Constant (Kc)

  Kc = [products]/[reactants]

  
  

  Only TEMPERATURE affects Kc
• Using the Equilibrium Constant

  - Write the expression for Kc

  - Calculate the concentration of reactants and products.

  - Substitute the concentrations

  - Deduce the units for Kc - if there are no units write "no units"
• Estimating the Position of Equilibrium Using Kc

  Larger value of Kc - further to the right, more product.

  
  

  Smaller value of Kc - further to the left, more reactant.