C7

• Endothermic reactions:

  • energy from surroundings is transferred to reacting chemicals, causing temp. of surroundings to dec.
• Examples of endothermic:

  • thermal decomposition
  • reaction happening when citric acid combined with Na hydrogen carbonate (Na bicarbonate)
  • sports injury packs - when squeezed, ammonium nitrate and water mix in pack, resulting in instant cooling; speed of reaction makes them ideal for scenarios where ice won’t be available immediately
• Exothermic reactions:

  • energy from reacting chemicals transferred to surroundings, often inc. temp. as a result
• Examples of exothermic

  • combustion
  • oxidation
  • neutralisation
  • hand warmers - use energy released by iron oxidation; reusable ones source energy from crystallisation of salt solutions; boiling pack re-dissolves crystals so it can be activated again
• Activation energy = minimum amount of energy needed for a reaction to take place

  Catalyst lowers activation energy to increase reaction rate
• Activation energy = reactants → peak

  Energy change = reactants → products
• In exothermic reaction: reactants higher than products

  In endothermic reaction: products higher than reactants
• Bond calculations:

  • overall energy transferred = energy required to break bonds - energy required to make bonds
  • positive number means endothermic reaction
  • negative number means exothermic reaction
• • batteries and cells contain chemicals that react to produce electricity
  • can make simple cell by connecting 2 different electrodes and putting them in liquid electrolyte (difference in reactivity of metals produces voltage)
• • chemical cell produces p.d. until reactants are used up
  • changing electrolyte changes reactions happening at electrodes, changing voltage output of cell
• greater reactivity difference between electrodes will result in cell having greater voltage
• Batteries:

  • battery is one or more cell joined in series
  • voltage of battery is calculated as sum of voltages of individual cell components
  • when chemicals in battery run out, battery can’t be used anymore
• Rechargeable

  • connection to electric current reverses reactions happening at electrodes
  • means electricity can continue to be produced if there’s access to this external electric current
• Non-rechargeable

  • e.g. alkaline batteries, irreversible reaction takes place at electrodes
  • mean electricity can’t be produced as soon as one of reacting chemicals has run out
• Fuel cells:

  • electrodes are switched (anode = negative, cathode = positive)
  • produce electrical energy using reaction between external fuel source (usually hydrogen) and oxygen: fuel added to cell then there’s constant supply of oxygen - fuel is oxidised creating a voltage
  • advantages - reliable, high efficiency, compact and lightweight, less pollution than traditional methods
• Hydrogen fuel cell:

  • disadvantages - H’s explosive; H must be stored at high pressure
  • oxidation of hydrogen generates voltage and only waste product is water
• H fuel cells:

  • anode - hydrogen pumped at anode; hydrogen ions formed
  • 2H2 → 4H+ + 4e-
• H fuel cells:

  • cathode - oxygen pumped at anode; water formed as O reacts with H ions moving through electrolyte from anode
  • O2 + 4H+ + 4e- → 2H2O
• H fuel cells:

  overall equation - 2H2 + O2 → 2H2O
• Hydrogen fuel cell vs Battery:

  • toxicity - H f.c. use H and O and only waste product is water; Battery made from toxic compounds and their disposal causes pollution
  • lifetime - H f.c. can be reused by adding more H and O; Battery just be replaced after certain number of recharges
  • capacity - H f.c. produce more energy per gram of H relative to petrol; Battery have smaller capacity than H f.c. so must be charged more regularly