Energy Changes

• exothermic reaction


  A reaction which transfers energy to the surroundings usually by heating, it is shown by rise in temperature
• Examples of exothermic reactions


  Combustion,neutralisation,oxidation reactions also for everyday uses like hand warmers
• endothermic reaction


  A reaction where energy is taken in from the surroundings, it is usually shown by fall in temperature
• Examples of endothermic reactions

  Reaction between citric acid and sodium hydrogencarbonate and thermal decomposition also everyday uses likes sports injury packs
• How to measure energy released by a chemical reaction?

  By taking the temperatures of the reagents and mixing them in a polystyrene cup and measuring the temperature of the solution at the end of the reaction
• Factors that affect energy measurement

  Amount of energy lost to surroundings
• How to test the effect of acid concentration on energy released in a neutralisation reaction between HCL and NaOH?

  First put 25cm^3 of 0.25mol/dm ^3 of hydrochloric acid and sodium hydroxide in separate beakers. Place the beakers in a water bath set to 25 degrees until they are both the same temperature. Add the HCL followed by the NaOH to a polystyrene cup with a lid with cotton wool on the outside to prevent energy transfer to the surroundings. Take the temperature of the mixture every 30 seconds and record the highest temperature. Repeat steps 1-4 using 0.5mol/dm^3 and then 1mol/dm^3 of HCL
• reaction profile


  Diagrams that show the relative energies of the reactants and products in a reaction and how the energy changes over the course of the reaction
• exothermic reaction profile


  Products are at a lower energy than the reactants
• activation energy


  Minimum amount of energy the reactants need to collide with each other and react. The greater it is the more energy needed to start the reaction
• endothermic reaction profile


  Products are at a higher energy than the reactants
• What process is bond breaking?

  endothermic process
• What process is bond forming?


  exothermic process
• Overall energy change


  Sum of the energies needed to break bonds in the reactants minus the energy released when the new bonds are formed
• Overall energy change formula

  energy required to break bonds - energy released by forming bonds
• electrochemical cell


  A basic system made up of two different electrodes in contact with an electrolyte
• What do chemical reactions between the electrodes and electrolyte set up?

  a charge difference between the electrodes
• What affects the voltage of a cell?

  1) The type of electrodes used

  2) bigger difference in reactivity of electrodes

  3) electrolyte used
• Is the voltage of cells in batteries bigger or smaller?

  Bigger
• What happens in non rechargeable batteries?

  The chemical reactions that happen at the electrodes are irreversible. One of the reactants gets used up so the reaction can't happen and therefore don't produce anymore charge.
• rechargeable cell/battery


  Reaction can be reversed by connecting it to an external electric current
• Fuel cells


  An electrical cell that's supplied with a fuel and oxygen and uses energy from the reaction between them to produce electrical energy efficiently
• What happens to the fuel when it enters the cell?

  It becomes oxidised and sets up a potential difference within the cell
• Hydrogen-oxygen fuel cells


  Electrolyte is often an acid and electrodes are often porous carbon with a catalyst. Hydrogen goes into the anode compartment and oxygen goes into cathode compartment. At negative electrode (anode) hydrogen loses electrons to produce H+ ions (oxidation). H+ ions in the electrolyte move to the cathode. At the positive electrode (cathode) oxygen gains electrons and reacts with H+ ions to make water (reduction). Electrons flow through external circuit from anode to cathode (electrical current).
• Advantages of hydrogen oxygen fuel cells

  -no pollutants

  -no charging limit

  -less expensive than batteries

  -store more energy
• Disadvantages of hydrogen oxygen fuel cells

  Hydrogen is agasand takes uptoo much space.

  It is veryexplosiveandunsafe.

  Oftenhydrogen is madefromhydrocarbonsorelectrolysis of water.
• Hydrogen oxygen fuel cell equation

  2H2(g) + O2(g) -> 2H2O(l)