ENERGY CHANGES J

• Shows how energy changes over course of a reaction

  
• Exothermic reaction is one which transfers energy to the surrounding , usually by heating. This Is shown by a rise in temperature. 
• Examples of exothermic reactions 

  • burning fuels ( combustion )
  • Neutralisation reactions
  • Oxidation reactions 
• Everyday uses of exothermic reactions : hand warmers , self heating cans 
• Endothermic reactions is one which takes in energy from the surroundings. This is shown by a fall in temperature. 
• Endothermic reactions are less common.
• Examples of endothermic reactions include :

  • reaction between citric acid and sodium hydrogencarbonate 
  • Thermal decomposition eg heating calcium carbonate causes it to decompose into calcium oxide and carbon dioxide.
• Everyday uses of endothermic reactions : sports injury packs 
• Energy transfer can be measured 
• Testing the effect of acid concentration on the energy released in neutralisation reaction between hydrochloric acid and sodium hydroxide. 

  1. Put 25cm of 0.25mol/dm3 of hydrochloric acid and sodium hydroxide In separate beakers 
  2. Place the beakers in a water bath set to 25 degrees until they are both at the same temperature 
  3. Add the hydrochloric acid followed by sodium hydroxide to a polystyrene cup with a lid. 
  4. Take the temperature of the mixture every 30 seconds and record the highest temperature.
  5. Repeat steps 1-4 using 0.5 mol/dm and then 1 mol/dm of hydrochloric acid 
• Energy must always be supplied to break bonds 
• During a chemical reaction old bonds are broken and new bonds are formed 
• Bond breaking - endothermic - energy must be supplied
• Bond making - exothermic - energy is released 
• In exothermic reactions energy released by forming bonds is greater than energy used to break them
• In endothermic reactions the energy used to break bonds is greater the the energy released by forming them. 
• Bond breaking - bond making = overall energy change 
• Chemical reactions in cells produce electricity 
• -Electrodes must be able to conduct electricity so they are usually metals 
• -Electrolyte is liquid that contains ions which react with the electrodes 
• -The chemical reactions between the electrodes and the electrolyte set up a charge difference between the electrodes 
• -If the electrodes are then connected by a wire the charge is able to flow and electricity is produced 
• -A voltmeter can also be connected to the circuit to measure the voltage of the cell 
• Factors that effect the voltage of a cell 

  • the metals used for electrodes. The greater the difference in reactivity of the metals the greater the voltage. 
  • Type and concentration of electrolyte used 
  • Conditions eg temperature 
• A battery is formed by connecting two or more cells together in series 
• Rechargeable cells and batteries can be recharges because the chemical reactions are reversed when an external electrical current is supplied. These are the batteries used in laptops and mobile phones.
• In Non rechargeable cells and batteries the chemical reactions stop when one of the reactant has been used up. These are the batteries usually used in smoke alarms and tv remotes. Alkaline batteries are non rechargeable. 
• Fuel cells use fuel and oxygen to produce electrical energy 
• A fuel cell is an electrical cell that’s supplied with a fuel and oxygen and uses energy from reaction between them to produce electrical energy efficiently. 
• When fuel enters cells it become oxidised and sets up potential difference. 
• Hydrogen oxygen fuel cells can be used in vehicles 
• Fuels cells V batteries 

  • fuel cells don’t produce pollutants ( no greenhouse gases ) the only by products are water and heat 
  • Batteries are more polluting to dispose of 
  • Batteries are rechargeable but there us a limit of how many times you can re charge before a replacement Is needed
  • Batteries are more expensive to make than fuel cells 
  • Batteries store less energy so would need to be recharged more often.
• Disadvantages of hydrogen oxygen fuel cells 

  • it requires energy to produce hydrogen 
  • Hydrogen gas requires a large space to store 
  • Hydrogen is highly flammable so dangerous to store 
• Hydrogen oxygen fuel cells involve a redox reaction 
• HOFC:Electrolyte is often acid and electrodes are often carbon with a catalyst 
• HOFC: Hydrogen goes to anode and oxygen goes to cathode 
• HOFC: At negative electrode ( anode ) hydrogen loses electrons to produce hydrogen + ions . Oxidation.
• HOFC: At positive electrode ( cathode ) oxygen gains electrons from cathode and reacts with hydrogen + to make water . Reduction. 
• HOFC: Electrons flow through an external circuit from the anode to the cathode , this is the electric current 
• Overall reaction is hydrogen + water ——> water