Energy Changes

• Exothermic and Endothermic

  • If the product of a reaction has more energy than the original reactants, then they must have taken in energy from the surroundings
  • If the products of the reaction store less energy then the excess energy was transferred to the surroundings during the reaction
• Exothermic

  • Transfers energy to the surroundings - shown by rise in temperature
  • Combustion, respiration, neutralisation, reactions of metals w/ acids
  • Everyday: Hand warmers, self-heating cans
• Endothermic

  • Takes in energy from the surroundings - shown by fall in temperature
  • Thermal decomposition, photosynthesis
  • Everyday: Sports injury packs
• Exothermic and Endothermic reactions

  1. Put 25cm³ of 0.25 mol/dm³ of hydrochloric acid and sodium hydroxide in separate beakers
  2. Place beakers in 25°C water bath until both are same temperature
  3. Add HCl followed by NaOH to a polystyrene cup with a lid
  4. Take temperature every 30 seconds, record highest temperature
  5. Repeat 1-4 using 0.5 mol/dm³ then 1 mol/dm³ of hydrochloric acid

  • Energy can be lost to the surroundings so decrease this by putting the polystyrene cup in a beaker of cotton wool for insulation and a lid on to stop evaporation
• Bond energies

  • Energy is supplied to break bonds, energy is released when new bonds are formed
  • Exothermic = energy released by forming bonds is greater than energy used to break
  • Endothermic = energy used to break bonds is greater than energy used to form
• Endothermic reaction:

  A) energy is absorbed

  
• Exothermic reaction:

  A) Energy is released

  
• Potential difference = how much potential energy or charge is between two specific points in an electrical circuit
• Electrolyte = liquid which can conduct electricity
• Electrode = the electrical conductor, which makes contact with the electrolyte so that a full circuit is achieved
• Cell = a device which stores chemical energy, then converts to electrical energy
• Batteries = two or more cells connected together in series to provide a greater voltage
• Simple cell = made by connecting two different metals in contact with an electrolyte

  • Amount of electricity produced depends on reactivity and metal used
• • Potassium - Please
  • Sodium - Stop
  • Lithium - Lying
  • Calcium - Ceiling
  • Magnesium - Me
  • Aluminium - A
  • Carbon - Cordless
  • Zinc - Zebra
  • Iron - Instead
  • Tin - Try
  • Lead - Learning
  • Hydrogen - How
  • Copper - Copper
  • Silver - Saves
  • Gold - Gold
• Non-rechargeable batteries

  • In non-rechargeable cells and batteries, the reactions stop when one of the reactants has been used up
  • Alkaline batteries are non-rechargeable
• Rechargeable batteries

  • The reaction is reversed when connected to an external electric current
• Fuel cells

  • An electrical cell that is supplied with a fuel and oxygen or air and uses energy from the reaction to produce electrical energy efficiently
• Hydrogen-oxygen fuel cells in vehicles

  • Fuel cell vehicles:
  • Don’t produce as many pollutants as other fuels
  • By-products = water and heat
  • Electric vehicles have more polluting batteries when disposed of as they’re made from highly toxic metal compounds
  • Electric vehicle batteries are rechargeable but there is a limit before it needs to be replaced
  • Fuel cells are cheaper than batteries and store more energy, so would need to be recharged less often
• Hydrogen-oxygen fuel cell redox reaction

  • At the anode: Hydrogen loses electrons to produce H⁺ ions (oxidation)
  • At the cathode: Oxygen gains electrons from the cathode and reacts with H⁺ ions (from the acidic electrolyte) to make water (reduction)