Energy

• Exothermic reaction


  A reaction which gives out energy into the surroundings via light or heat. Reactants have more energy than products. Activation energy is needed to make new bonds.
• types of Exothermic reactions

  • Respiration
  • Neutralisation
  • Combustion
  • Oxidation
• Real life examples of exothermic reactions

  • Hand warmers - (disposable)Oxidation of iron oxide( reusable) Uses crystallisation of salt solutions, and boiling th solution redissolves the crystals, so it can be activated once more
  • Self heating cans - Calcium oxide reacts with melting aluminium, generating heat
• This diagram shows 3 things: Energy in reactants and products, Activation Energy, Energy overall change
• Endothermic reaction


  A reaction which takes in energy from the surroundings via light or heat. Energy in the products is larger than reactants, meaning that bonds are Broken.
• Endothermic reactions


  • Thermal decomposition
  • Photosynthesis
  • Reaction of sodium bicarbonate (baking soda) and citric acid
  • Sports injury packs - Ammonium nitrate and water mix in the pack, very fast, making it useful for situations where ice is not available or applicable rate +water
• More energy in products because energy is taken in from surroundings
• Conservation of energy/ 1st law of thermodynamics = Energy can never be created or destroyed, only transferred between 1 store and another.
• Successful chemical reaction


  • Particles to collide - There need to be collisions between reacting particles, with sufficient energy
  • Activation energy - These particles must collide with enough energy. (The minimum energy needed to start a reaction)
• Catalysts


  Decrease the activation energy to speed up the reaction, by finding alternate pathways increasing the likelihood of successful collisions
• Bond energy


  The energy required to break chemical bonds
• Energy must be supplied to break bonds (BB)


  Energy is released when the products' bonds are formed (BF)
• If we subtract the bond energy of the products from the reactants, we can see if it's exo or endothermic (exothermic would be positive number, endo would be negative)
• Exothermic reactions


  • Energy released by making bonds > energy required to break bonds (Bond breaking - bond forming energy) BB-BF = NEGATIVE
  • Overall energy change < 0 (NEGATIVE)
• Endothermic reactions


  • Energy released by making bonds< Energy required to break bonds BB-BF = POSITIVE
  • Overall energy change> 0 POSITIVE
• Chemical bonds have energy in their bonds - It's measured in kj/mol
• Bond making - Releases heat energy into the surroundings - Exothermic process
• Bond breaking - Requires heat energy from surroundings - Endothermic process
• Chemical cells


  • Batteries and cells contain chemicals than can react, which produces electricity
  • We can make a simple cell by connecting 2 different electrodes(conductors) and putting them in a liquid electrolyte
  • The difference in reactivity produces a voltage. (think of reactivity series)
  • The greater the difference , the greater the voltage. (usually magnesium to zinc)
  • A voltage is produced until all the reactants are used up
  • Changing the electrolytes changes reactions which happen at the electrodes, resulting in the output voltage changing
• Batteries


  • More than one cell. They are all individually joined together in series.
  • The overall voltage of a battery is determined by adding up all of the voltages of each cell inside the battery
  • When at least one of the reacting chemicals in a battery run out, the battery cannot produce electricity anymore, meaning that it cannot be used anymore
• Voltage of a cell


  • Depends on the electrolyte (temperature, type, and concentration)
  • Depends on the electrodes (types, difference in reactivity of metal electrodes)
• Rechargeable batteries


  • Reversible reactions
  • Connection to an electrical current (charger)allows reactions to be reversed at the electrodes. This allows electricity to be produced as long as there is access to this electrical current (charger)
  • Electrons which have moved from negative to positive electrodes in a battery are moved back to the negative side. Though charging again and again means that the overall capacity of the battery decreases over time (cycles), and the electrolyte hardens.
• Non-rechargeable batteries


  • Non-reversible reactions
  • In non-rechargeable cells and batteries (e.g alkaline batteries), irreversible reactions take place at the electrode, meaning that ions cannot be moved back to the negative terminal when one of the reacting chemicals runs out
  • Hydroxide ions are usually present, which separates them from rechargeable ions
• Fuel cells


  • Produce electrical energy using a reaction between an external source ( usually hydrogen) and oxygen
  • The fuel is added to the cell, and then there is a constant supply of oxygen
  • The fuel is oxidised (losing electrons) and this creates a voltage
  • Advantages: Less pollution than generators, Compact and lightweight, Reliable - Without moving parts, High efficiency
• Hydrogen fuel cells


  • Hydrogen is supplied to the cell, reacting with oxygen
  • This oxidises hydrogen, producing a waste product of water
  • The anode and cathode are reversed. This used to be PANIC (positive anode negative cathode) , but now it is the other way round (Positive cathode negative anode)
  • At the anode: Hydrogen (H2) Is pumped in the anode, and it loses electrons to form Hydrogen ions (H+)
  • At the cathode: Oxygen is pumped in at the cathode, reacting with the H+ ions from the anode (travelling across the Proton exchange membrane) to form water
• The overall equation (excluding what happens at the electrodes): 2H2 + O2 → 2H2O
• The direction of electron flow and the positive/negative designations of the electrodes are based on conventional current direction (from positive to negative outside the cell), not the actual flow of electrons (which is from negative to positive).
• Exothermic reaction


  A reaction which gives out energy into the surroundings via light or heat. Reactants have more energy than products. Activation energy is needed to make new bonds.
• Exothermic reactions


  • Respiration
  • Combustion
  • Neutralisation
  • Oxidation
• Real life examples of exothermic reactions

  • Hand warmers - Oxidation of iron oxide (disposable)
  • Hand warmers - Uses crystallisation of salt solutions, and boiling the solution redissolves the crystals, so it can be activated once more (reusable)
  • Self heating cans - Calcium oxide reacts with melting aluminium, generating heat
• This diagram shows 3 things: Energy in reactants and products, Activation Energy, Energy overall change
• Endothermic reaction


  A reaction which takes in energy from the surroundings via light or heat. Energy in the products is larger than reactants, meaning that bonds are broken.
• Endothermic reactions


  • Thermal decomposition
  • Photosynthesis
  • Reaction of sodium bicarbonate (baking soda) and citric acid
  • Sports injury packs - Ammonium nitrate and water mix in the pack, very fast, making it useful for situations where ice is not available or applicable
• More energy in products because energy is taken in from surroundings
• Conservation of energy/ 1st law of thermodynamics = Energy can never be created or destroyed, only transferred between 1 store and another.
• Successful chemical reaction


  • Particles to collide - There need to be collisions between reacting particles, with sufficient energy
  • Activation energy - These particles must collide with enough energy. (The minimum energy needed to start a reaction)
• Catalysts


  Decrease the activation energy to speed up the reaction, increasing the likelihood of successful collisions
• All chemical reactions have a bond energy which is the energy required to break chemical bonds.
• Energy must be supplied to break bonds (BB)

  Endothermic
• When the products' bonds are formed

  Energy is released (BF) Exothermic