Reactions

• Chemical reaction


  Evidence that a chemical reaction has taken place is that either heat is taken in or given out
• Exothermic reaction


  Heat energy is transferred from the chemicals to the surroundings, the temperature of the reaction rises (heat is given out), e.g. fireworks burning gas
• Exothermic reaction


  • Vinegar and baking powder
• Endothermic reaction


  Energy is taken in from the surroundings in order for the reaction to occur, the reaction shows a fall in temperature
• Endothermic reaction


  • Ammonium chloride and water
• During a chemical reaction old bonds are broken, and new bonds are formed
• Endothermic reaction


  Breaking bonds requires energy
• Exothermic reaction


  New bonds are formed, energy is released
• Bond energy


  Energy required to break different bonds, the units are kilojoules per mol (KJ/mol)
• Energy profile diagrams


  Shows energy stored in the reactants compared to the energy stored in the products, so they tell us if the reaction is exothermic or endothermic

  
• ΔH


  The change in energy, if ΔH is negative then it is an exothermic reaction, if ΔH is positive then it is an endothermic reaction
• Physical change


  A change that occurs where no new substance is formed
• Physical change


  • Melting chocolate
  • Folded piece of paper

  
• Chemical change


  A change where a new substance is formed
• Chemical change


  • Rusting
• Chemical change


  • Energy is usually released or absorbed
  • It is hard to reverse
• Chemical reactions do not always involve heat, photosynthesis is a chemical reaction but uses light instead of heat
• During a reaction


  1. Atoms rearrange to form a new substance

  2. There can be signs that the reaction has taken place
• Signs of reaction


  • Colour change
  • Change in temperature
  • Light is given out
  • Bubbles can be given off
• Law of Conservation of mass: a substance cannot be created or destroyed so during a physical or chemical change there is no change in overall mass
• Experiment


  1. 10g sugar, 100ml water

  2. Find mass independently

  3. Mix the two together

  4. Find mass
• Particle model diagrams


  Diagrams that show how particles rearrange themselves to form a new substance
• Rate of reaction


  How long does a reaction take to occur
• Rates of reaction


  • Nails rusting - slow
  • Explosion - fast
• Particle Theory


  • During a chemical reaction, particles collide with each other
  • For a reaction to occur the particles must collide with enough force to break the bonds in the molecules and make new products
• Increasing temperature


  Particles have more energy to move around quickly, resulting in more collisions so reactions get faster
• Effects of temperature on reaction rate

  • Milk goes off - put into fridge, lower temp, lower reaction collisions
  • Glow sticks - drop into hot water, will glow more intensely, reaction occurs faster at higher temps
• Increasing concentration


  More collisions in a certain time, increase in the rate of reaction
• Surface area


  The smaller the particle size of a solid, the greater the surface area on which the reaction can take place, so the faster the reaction

  
• Catalyst


  Makes a reaction happen faster without being part of the reaction
• Activation energy


  The minimum amount of energy that reactant particles must have in order for them to react
• Adding a catalyst


  Lowers the activation energy, making it easier for the particles to react

  
• Enzymes


  Biological catalysts
• Investigating surface area and reaction rates

  1. Break stick of chalk in half, make sure mass is equal

  2. Grind 1 stick of chalk into a powder

  3. Place 20ml of vinegar into each conical flask

  4. Add whole stick of chalk into 1 deflated balloon and attach to neck of conical flask

  5. Add powdered stick to the other balloon and attach to conical flask neck

  6. Lift up both balloons so contents drop into conical flask at the same time

  7. Observe how fast the balloons fill with carbon dioxide
• To make carbon dioxide

  1. Pour vinegar into a small conical flask - warm vinegar

  2. Place baking powder into the balloon

  3. Attach balloon to conical flask

  4. Lift balloon so baking powder falls into the vinegar

  5. Hold neck of balloon and let carbon dioxide fall into a beaker

  6. Pour contents of beaker over the lit night lights

  7. Repeat with ice cold vinegar
• To make Oxygen


  Catalyst: Manganese Dioxide AND Hydrogen peroxide
• Activation energy - light a match

  1. Sulphur on match uncreative

  2. Strike match adds heat - reaction occurs
• Heat - Vitamin C


  1. Boiling water crushed vitamin C tablet

  2. Cold water crushed vitamin C tablet

  3. Balloon - collect gas given off, test with night light
• Factors effecting Rate of Reaction

  • Surface Area - More surface area, faster reaction
  • Temperature - Greater temperature, faster reaction
  • Concentration - Greater concentration, faster reaction
  • Catalyst - Lowers activation energy