C5 Energy Changes

• Exothermic reactions

  • Energy transferred from reaction into surroundings - energy released
  • Temperature rises
  • eg combustion, neutralisation, oxidation
  • EXothermic = energy EXits
• Endothermic reactions

  • Energy is transferred into reaction from surroundings
  • Temperature decrease
  • eg thermal decomposition
  • ENdothermic = energy ENters
• Old bonds are broken and new bonds are made in chemical reactions
• Breaking bonds requires more energy
• Making bonds releases energy
• Endothermic = requires energy
• Exothermic = releases energy
• Endothermic reactions in real life

  • Sports injury cooling packs
  • Photosynthesis
• Exothermic reactions in real life

  • Handwarmers
  • Selfheating cans
• Activation energy = amount of energy required for particles to collide and react
• Reaction profiles

  • Activation energy goes from reactants to the peak
  • Energy change goes from reactants to products
• Reaction profiles

  • Exothermic - goes downwards because energy is LOST
  • Endothermic - goes up because energy is NEEDED

  
• Summary of reaction profiles - EXOthermic

  • Reaction gives off heat energy
  • Energy leaves reaction and enters surroundings
  • Temp of surrounding increases
  • Products have less energy because energy has left reaction
• Summary of reaction profiles - ENDOthermic

  • Reaction takes in energy
  • Energy enters reaction from surroundings
  • Temp of surrounding decreases
  • Products have more energy as energy goes into reaction and is stored in product
• BENDO MEXO

  • Breaking bonds = endothermic
  • Making bonds = exothermic
• Simple chemical cells are a source of electrical energy
• Electrolyte = a solution containing ions that carries a charge
• Electrochemical cell = a basic system made of 2 different electrodes in contact with an electrolyte
• Electricity in an electrochemical cell

  • More reactive metals form positive ions quicker
  • Will give away electrons
  • Electricity generated as free electrons can flow
• A cell will only produce electricity is metals with different reactivities are used

  • Different combinations of metals cause different voltages - BIGGER DIFFERENCE IN REACTIVITY = BIGGER VOLTAGE OF CELL
• REDOX reactions

  • Reduction = gain of electrons
  • Oxidation = loss of electrons
• OILRIG

  • Oxidation
  • Is
  • Loss
  • Reduction
  • Is
  • Gain
• REDOX

  • At positive electrode a metal is reduced - metals gain electrons and go from an ion to an atom
  • At negative electrode a metal is oxidised - metals lose electrons and go from an atom to an ion
• Batteries = 2 or more cells connected together in series
• Total voltage of a battery = sum of cells - bigger overall voltage
• Types of battery

  • Rechargeable eg phone battery
  • Non-rechargeable eg alkaline
• Rechargeable batteries

  • Reaction is reversible
  • Battery can be charged using an external energy supply - reversed the reaction so more reactants are present
• Non-rechargeable batteries

  • All reactants get used up (during use)
  • Irreversible reaction
  • Once all reactants are used up, no more reaction occurs and no voltage is produced
• Fuel cell = a device that generates energy by a chemical reaction
• Fuel cells

  1. Hydrogen goes to anode and oxygen goes to cathode
  2. Hydrogen loses electrons to form H+ ions at the anode - OXIDATION
  3. Sets up a potential difference (voltage) across electrodes
  4. Electrons move - produces voltage
  5. H+ ions in electrolyte move to the cathode
  6. Oxygen gains electrons and reacts with H+ ions from electrolyte and forms water - REDUCTION
  7. Electrons flow through an external circuit from anode to cathode (electric circuit)
• Fuel cells half equations at the anode - oxidation of hydrogen

  $H2 —> 2H^+$$+$$2e^-$
• Fuel cells half equations at the cathode - reduction of oxygen

  $O2 +$$4e^- +$$4H^+$$—> 2H2O$
• What is often the electrolyte in fuel cells?

  Potassium hydroxide
• What are often the electrodes in fuel cells?

  Porous carbon with a catalyst
• Fuel cells vs batteries - fuel cells

  • + Generate electricity with minimal pollution
  • + Store more energy than batteries
  • + No batteries to dispose of - better for environment
  • — Catalyst is toxic
  • — Hydrogen is made by burning fossil fuels
  • — Material to produce cells is expensive
  • — High pressure tanks are needed to store gases - dangerous
  • — Hydrogen is expensive to produce and store
• Fuel cells vs batteries - batteries

  • + Can be recharged - limits use of resources
  • + Portable
  • + Doesn’t produce pollutants
  • + Alkaline cells are cheaper to make
  • — Batteries are toxic to dispose of
  • — Limit to how often you can recharge before replacing
  • — Rechargeable batteries are more expensive than fuel cells
  • — Batteries store less energy so need replacing often