energy changes (5)

• Law of Conservation of Energy


  Energy is conserved in chemical reactions. It is neither created nor destroyed.
• Exothermic Reaction


  A reaction in which energy is transferred from the reacting substances to their surroundings (increasing the temperature) is called an exothermic reaction.

  E.g. Neutralisation reactions.
• Endothermic Reaction


  A reaction in which energy is transferred to the reacting substances from their surrounding (decreasing the temperature) is called an endothermic reaction.

  E.g. Thermal Decomposition reactions.
• Uses of exothermic changes


  Exothermic changes can be used in hand warmers and self-heating cans.
• Uses of endothermic changes

  Endothermic changes can be used in instant cold packs for sports injuries.
• Activation Energy


  The minimum amount of energy required to start a chemical reaction.
• Energy Changes


  Bond breaking is endothermic, whereas bond making is exothermic.

  Energy changes are measured in kJ/mol.
• Reaction profile for exothermic reaction

  The products are at a lower energy level than the reactants.

  
• Reaction profile for endothermic reaction

  The products are at a higher energy level than the reactants.

  
• Bond energies


  The energy needed to break the bond between two atoms is called the bond energy for that bond. You can use bond energies to calculate overall energy change in a chemical reaction using bond energies. (ENERGY IN - ENERGY OUT).
• Chemical cells


  Cells contain chemicals that react to produce electricity. The voltage produced by a cell is dependent upon a number of factors, including the type of electrode and electrolyte.
• Cells and Batteries


  A simple cell can be made by connecting 2 different metals in contact with an electrolyte. A battery consists of 2 more cells giving greater voltage.
• Rechargeable and non-rechargeable cells

  Non-Rechargeable cells - reactions stop when one reactant gets used up (e.g. alkaline batteries)

  Rechargeable cells - reactions are reversed when an external current is supplied.
• Electrical cells


  Metals lose electrons and form positive electrons. When two metals are dipped in a wire and salt solution, the more reactive metal will donate its electrons to the less reactive metals.
• Increasing voltage in cells

  The greater the difference in reactivity, the higher the voltage produced by the cell.
• Fuel Cells


  Fuel cells are an alternative fuel source. It was an external source of fuel (e.g. Hydrogen) and oxygen. The hydrogen is oxidised within the fuel cell to produce p.d.
• The reaction


  The overall reaction is the oxidation of hydrogen to produce water. It can be used in combustion engines, rechargeable cells and fuel cells.
• Positives of hydrogen fuel cells

  It's Eco-friendly, there are 0% harmful emissions, there's a continuous supply and there's no combustion involved.
• Negatives of hydrogen fuel cells

  It is very expensive, can be dangerous under pressure, hydrogen can be polluting and there is no simple way to get hydrogen.
• Exothermic Reaction


  - one that transfers energy into the surroundings

  - causes temperature to rise

  - products have a lower energy content than reactants

  
• examples of exothermic reactions

  combustion, magnesium reacting with hydrochloric acid
• endothermic reaction


  - transfers energy from the surroundings to the reacting chemicals

  - takes in energy from the surroundings

  - reactions cause a fall in temperature

  - have a higher energy content than the reactants so energy is transferred from the surroundings

  
• examples of endothermic reactions

  dissolving, photosynthesis
• bond making is


  exothermic: gives off energy

  
• bond breaking is


  endothermic: requires energy
• describe the movement of the particles in an exothermic reaction

  the particles come towards each other and collide, forming bonds and giving off energy
• describe the movement of particles in an endothermic reaction

  the particles are heated up, and they break when taking energy from the surroundings
• what does a bond require in order to break

  energy
• in energy change calculations , will a exothermic reaction have a negative or positive result?

  negative: the reaction is transferring energy to the surroundings
• in energy change calculations, will a endothermic reaction have a negative or positive result?

  positive: the reaction is taking in energy from the surroundings
• Reactivity series from top to bottom

  Potassium

  Sodium

  Calcium

  Magnesium

  Aluminium

  (carbon)

  Zinc

  Iron

  Tin

  Lead

  (hydrogen)

  Copper

  Silver

  Gold

  Platinum
• What is a simple cell?

  two metal electrodes dipped into an electrolyte that produces electrical energy
• What is a battery?


  two or more simple cells connected
• What is an electrolyte?

  solution or molten compound that allows irons to move fully therefore transferring electrons
• How to increase voltage in electrolysis

  Greater difference in reactivity between the metals
• The greater the difference in reactivity between the metals used, the ___ the voltage produced

  higher
• Things that affect voltage in an experiment?

  type of solution, concentration of solution
• What is a non rechargeable battery?

  runs out when reactions have run out
• ex of non rechargeable battery?

  alkaline battery
• things you should keep the same in an experiment?

  concentration of sodium chloride, length of metal/electrodes