Energy changes

• exothermic reaction

  chemical reaction in which energy is primarily given off in the form of heat, increases the temperature of surroundings
• endothermic reaction

  a reaction in which energy is absorbed, decreases the temperature of surroundings
• energy level diagram for exothermic reaction

  
• energy level diagram for endothermic reaction

  
• activation energy

  the minimum amount of energy required to start a chemical reaction
• activation energy in exothermic reaction

  
• activation energy in endothermic reaction

  
• bond energy

  energy required to break chemical bonds, the same as the amount of energy released when the same bond is formed
• what is the unit of bond energy

  kJ/mol
• electrolyte

  solution containing ions
• chemical cell
• which of these reactions are exothermic?

  combustion, neutralisation, oxidation
• What are the reactants for the endothermic reaction used in sports injury packs?

  ammonium nitrate and water
• which of these reactions are endothermic?

  citric acid + baking soda, thermal decomposition, sports injury packs
• What do we call the minimum amount of energy that reacting particles must have in order for a reaction to occur?

  activation energy
• what does the x axis show in a reaction profile diagram

  progress of reaction
• what does the y axis show in a reaction profile diagram

  energy

  
• catalyst

  increase reaction rates by lowering the activation energy, which increases the likelihood of successful collisions.
• bond energy in exothermic reactions

  energy released by making bonds > energy required to break bonds
• bond energy in endothermic reactions

  energy released by making bonds < energy required to break bonds
• simple cell

  connecting 2 different electrodes and putting them in a liquid electrolyte
• In a chemical cell, what does the difference in the reactivity of the metals produce?

  voltage
• electrode

  A metal strip that conducts electricity
• how can voltage output of a cell be altered?

  changing the type of electrode and/ or electrolyte
• battery

  a collection of more than one cell joined in series (end-to-end)
• non-rechargeable battery

  a cell that is designed to be used only once since the reactions involved at the electrodes are irreversible
• rechargeable battery

  connection to an electric current reverses the reactions that happen at the electrodes
• fuel cell

  produce electrical energy using a reaction between an external fuel source (often hydrogen) and oxygen
• advantages of fuel cells

  no moving parts, compact and lightweight, less pollution that traditional methods, high efficiency
• what reaction takes place in fuel cell energy production?

  the reaction is between an external fuel source and oxygen. once added to the cell, the fuel is oxidised, and this causes a voltage to be established
• what does the oxidation of hydrogen generate in hydrogen fuel cells?

  a voltage, waste product is water
• hydrogen fuel cell - anode half equation

  2H2 → 4H+ + 4e-
• hydrogen fuel cell - cathode half equation

  O2 + 4H+ + 4e- → 2H2O
• hydrogen fuel cell - overall equation

  2H2 + O2 → 2H2O
• advantages of hydrogen fuel cells

  longer lifetime, less toxicity, greater capacity
• disadvantages of batteries

  must be replaced after a number of recharges, made from toxic metal compounds
• A molecule of ethane and a molecule of iodine react. What needs to happen for the reaction to occur?

  The ethane and iodine molecules must collide.

  They must collide with more energy than the activation energy.
• Bond Energy and Exothermic Reactions

  In exothermic reactions, the energy released by making new bonds is greater than the energy required to break existing bonds, resulting in a release of energy in the form of heat
• Bond Energy

  The energy required to break a chemical bond between two atoms
• Energy Balance

  The overall energy balance between the energy required to break bonds and the energy released by making new bonds in a chemical reaction