C6

• Mean rate of reaction = amount of reactant used or product formed / time
• How to measure the amount of reactant used:

  If one of the products is gas measure the mass before and after the experiment along with the time it takes for the experiment to be completed

  The mass will decrease

  The units for the rate if reaction can be given in g/s

  OR

  The amount of reactant can be measured in moles

  As the reaction takes place the reactant will be used up so the remaining amount of reactant will decrease

  The concentration of the reactant is calculated by amount (mol) over volume (dm^3) measure is mol/dm^3
• Rate of reaction can be calculated by measuring the amount of gas formed:

  If one of the products is gas measure the total volume of gas produced in cubic centimetre with a gas syringe and the time it takes for the reaction to take place

  The unit for rate of reaction will be cm^3/s
• Measuring the time it takes for a reaction mixture to turn opaque (turbidity) or change colour:

  Time how long it take for the reaction to change colour

  Rate of reaction = 1 / time taken for solution to change colour
• Chemical reactions only occur when reacting particles collide with each other and have sufficient energy to do so
• The minimum amount of energy required to cause a reaction is called the activation energy
• There are 4 main factors which influence the rate of reaction

  Temperature

  Concentration

  Surface area

  Catalyst
• How does temperature affect rate of reaction?

  In a hot rection the mixture particles move quicker. Therefore there are more collisions and they have more energy so the collisions are more successful
• How does concentration affect rate of reaction?

  A higher concentration means the particles are more crowded meaning they collide more and the collisions are more successful

  Increasing the pressure of reacting gasses also increasing the frequency of collisions
• How does surface area affect rate of reaction?

  Smaller solid pieces have a larger surface area compared there volume

  More particles are exposed and available for collisions so there are more collisions and a faster reaction
• Graphs can be plotted to show the rate of reaction.

  There are three key aspects

  The steeper the line = faster the reaction

  When one of the reactant is used up the reaction stops and the line goes horizontal

  The same amount of product is formed form the same amount of reactant regardless of rate
• The rate of reaction at a particular time is given by graphs:

  Draw a tangent to the graph

  Find the gradient of the tangent

  The gradient is equal to the rate of reaction at that time
• A catalyst is a substance that increase the rat of a chemical reaction without being used up in the process
• Catalysts are not included in the chemical equation for the reaction
• A catalyst:

  Recued the amount of energy need for a successful Collison

  Makes Collison more successful

  Speeds up the rate of reaction

  provided a surface for the molecules to attach to which increase chances of them bumping into each other
• Enzymes act as catalyst in biological systems
• Some chemical reactions are reversible meaning they can go forwards and backwards
• Complete diagram

  A) Reversible

  B) Reactants

  C) products

  
• The direction of reversible reactions can be changed by changing the conditions
• If a reversible reaction is exothermic in one direction it is endothermic in the opposite direction

  Meaning the amount of energy given out during the exothermic reaction is the same amount taken in during the endothermic reaction
• In a closed system no reactants are added and no products are removed
• When a reversible reaction occurs in a closed system it reaches a point of equilibrium
• Le Chatelier's principle states that if a system in equilibrium is subjected to change in conditions then the system shift to resist the change
• (Le Chatelier's principle)In a exothermic reaction:

  if the temperature is raised the yield decrease

  If the temperature is lowered the yield increased
• (Le Chatelier's principle)In a endothermic reaction:

  if the temperature is raised the yield increases

  If the temperature is lowered the yield decreases
• (Le Chatelier's principle)In a reaction involving gases:

  An increase in pressure favours the reaction that produces the least number of gas molecules

  A decrease in pressure favours the reaction that produces the greater number of gas molecules
• (Le Chatelier's principle)If the concentration of a reactant or product is changed:

  The system is no longer in equilibrium

  The system adjust until it can reach equilibrium once more