C8

• Le Chatelier's principle


  A principle which states, "If a system is at equilibrium and a change is made to any of the conditions, then the system responds to counteract the change"
• Dynamic Equilibrium


  An equilibrium where the forward and backward reactions are happening at the same rate
• Activation Energy


  The minimum energy required for a chemical reaction to take place
• Collision Theory


  The theory that states for a chemical reaction to happen, particles must collide with sufficient energy
• Gradient


  The measurement of how steep a line is on a graph
• Frequency


  The amount of times something happens in one second
• Concentration


  The number of particles in a given volume
• Collision Theory


  1. Particles must collide with a certain amount of energy

  2. The minimum amount of energy needed is the activation energy

  3. The rate of a reaction depends on the frequency of collisions and the energy of collisions
• If the concentration of a solution is increased


  There are more particles in a given volume, therefore collisions are more frequent and the chemical reaction is faster
• Concentration is directly proportional to rate of reaction (if you double the concentration you double the rate)
• Reaction Profile


  A graph which show the energies of the reactants and products at different stages of the chemical reaction
• Catalyst


  A substance which speeds up a chemical reaction without being used up
• Catalysts are not included in a chemical equation as they are not used up in a chemical reaction
• Enzymes


  Biological catalysts that speed up chemical reactions in biological systems
• Enzymes have an optimum temperature where they work best, this is usually around 37
• Collision Theory - Temperature

  Increasing the temperature increases the frequency of collisions and the energy of collisions, making it more likely that collisions exceed the activation energy
• Rate of Reaction


  The speed at which a chemical reaction is happening
• Measuring Rate of Reaction


  1. Measuring the quantity of reactant used or product made in a certain length of time

  2. The quantity can be measured in volume (cm3) or mass (g)
• Measuring Rate of Reaction - Graphical Method

  1. Draw a tangent to the curve

  2. Measure the gradient of the tangent to find the rate at that point
• There are several experiments that can be used to measure the rate of a chemical reaction, such as measuring mass lost, volume of gas produced, or time taken for an X to disappear
• Calculating Mean Rate of Reaction

  Pick two y values and two x values on the graph

  Subtract the largest from the smallest for both axes

  Divide the y-axis value by the x-axis value
• Rate of chemical reaction

  The speed with which the reactants get turned into products
• Measuring the rate of reaction

  1. Measure how fast the reactants are being used up

  2. Measure how fast the products are being formed
• Rate of reaction = quantity of reactants used / time taken
• Rate of reaction = quantity of products formed / time taken
• Slow reaction

  • Rusting of iron (years or decades)
• Typical reaction

  • Reaction between magnesium and acid (gentle stream of hydrogen bubbles)
• Fast reaction

  • Explosions like fireworks (fraction of a second)
• The rate of reaction starts off really fast when there are lots of reactants, then slows down as the reaction progresses and the reactants get used up
• As time increases

  Mass of reactants remaining decreases
• As time increases

  Volume of products produced increases
• Units for rate of reaction

  • Grams/seconds
  • Centimetres cubed/seconds
  • Moles/minutes
  • Decimetres cubed/seconds
• The average rate of reaction is different from the rate at a particular time
• Disappearing cross reaction


  1. Use a measuring cylinder to put 10 cm³ of sodium thiosulfate solution into a conical flask

  2. Place the conical flask on a printed black cross

  3. Add 10 cm³ of hydrochloric acid to the conical flask

  4. Swirl the solution and start a stopwatch

  5. Look down through the top of the flask until the solution turns cloudy and the cross can no longer be seen

  6. Stop the clock

  7. Repeat the experiment using lower concentrations of sodium thiosulfate solution

  8. Calculate mean values for each concentration, excluding any anomalous results
• Measuring volume of gas produced

  1. Use a measuring cylinder to place 50 cm³ of hydrochloric acid into a conical flask

  2. Attach the conical flask to a bung and delivery tube

  3. Place the delivery tube into a container filled with water

  4. Place an upturned measuring cylinder filled with water over the delivery tube

  5. Add a 3 cm strip of magnesium to the hydrochloric acid and start a stopwatch

  6. Measure the volume of hydrogen gas in the measuring cylinder every 10 seconds until no more hydrogen is given off

  7. Repeat the experiment using different concentrations of hydrochloric acid
• The greater the concentration of a chemical in a reaction


  The faster the reaction takes place
• This finding is reproducible because it is shown by two different experiments
• Collision theory


  In order for particles to react they have to collide with each other with sufficient energy (activation energy)
• If particles collide with less energy than the activation energy, they will just bounce apart again
• Rate of reaction


  Depends on the amount of energy the particles have and the frequency of the collisions