Rate and Extent of Chemical Change

• How can you measure rate of reaction

  by measuring the quantity of a reactant used or the quantity of product formed over time
• equation to determine mean rate of reaction using a reactant

  Mean rate of reaction = quantity of reactant used / time taken
• equation to determine mean rate of reaction using a product

  Mean rate of reaction = quantity of product formed / time taken
• how do you measure the amount of reactant or product in a reaction

  mass in grams or volume in cm3
• what are the units for rate of reaction

  g/s using mass, cm3/s using volume
• what are the units for rate using moles (HT only)

  mol/s
• How can you measure rate of reaction from a graph at a specific time (HT only)

  Draw a tangent and determine the gradient

  
• when drawing a tangent, how should the ruler be positioned on the graph (HT only)

  placed on the curve at the point where you want to know the slope. The space between the ruler and the curve should be equal on both sides of the point
• How do you calculate mean rate of reaction from a graph

  change in y (product or reactant)/change in x (time)
• what does the steepness of the slope on a rate graph tell you

  how fast the reaction is, steeper the slope, faster the reaction
• how can you tell from a rate graph that the reaction has finished

  the graph becomes flat
• What factors can affect the rate of a chemical reaction

  concentration, pressure, surface area, temperature and catalyst
• what is collision theory

  explain how factors affect rates of reaction
• what is activation energy

  the minimum amount of energy that particles must have to react
• what must particles have when they collide

  sufficient amount of energy
• what does increasing temperature in terms of collisions

  increases the frequency of collisions and the collisions become more energetic, increasing rate of reaction, get more successful collisions
• what does increasing concentration, pressure and surface area in terms of collisions

  increases the frequency of collisions, increasing rate of reaction
• how can you increase surface area of a solid

  break up the solid into smaller pieces/powder up the solid
• what is a catalyst

  a substance which increases rate of chemical reactions but are not used up during the reaction, different reactions need different catalysts
• what is an enzyme

  a biological catalyst
• how do catalysts increase the rate of reaction

  provide a different pathway for the reaction that has a lower activation energy
• what is a reversible reaction

  the products of the reaction can react to produce the original reactants
• why does the amount of products and reactant remain the same at equilibrium

  the rate of the forward reaction is equal to the rate of backward reaction so the concentrations of products and reactant remain constant
• how can you change the direction of a reversible reaction

  by changing the conditions
• If a reversible reaction is exothermic in one direction, what will it be in the opposite direction

  endothermic
• what is the amount of energy like in both directions in a reversible reaction

  the same
• what is equilibrium

  when forward and reverse reactions occur at exactly the same rate

  (when in a closed system so reactants and products cant escape from the apparatus)
• what happens if the conditions are changed in an equilibrium (HT only)

  the system responds to counteract the change
• what is le chateliers principle (HT only)

  a way to predict the effect of changing conditions on a system at equilibrium
• what happens if the concentration of one of the reactants or products is changed (HT only)

  the system is no longer at equilibrium and the concentrations of all of the substances will change until equilibrium is reached again
• what happens if the concentration of reactants are increased (HT only)

  more products will be made until equilibrium is reached again
• what happens if the concentration of products are decreased (HT only)

  more reactants will react until equilibrium is reached again
• what happens if the temperature is increased for an endothermic reaction (HT only)

  the amount of products will increase
• what happens if the temperature is decreased for an endothermic reaction (HT only)

  the amount of products will decrease
• what happens if the temperature is increased for an exothermic reaction (HT only)

  the amount of products will decrease
• what happens if the temperature is decreased for an exothermic reaction (HT only)

  the amount of products will increase
• what happens if pressure is increased in an equilibrium (HT only)

  equilibrium will shift towards the side with the smaller number of molecules
• what happens if pressure is decreased in an equilibrium (HT only)

  equilibrium will shift towards the side with the larger number of molecules
• On the image above, which of the lines represents the activation energy of the reaction?

  B

  
• The two graphs above show the same chemical reaction, but in the presence and absence of a catalyst. Which one shows the reaction occurring in the presence of a catalyst?

  Graph 1