C6 - the rate and extent of chemical change

• how can the rate of reaction can be measured?

  the rate at which a reactant is used up, or the rate at which a product is formed

  looking at how fast solid reactants are used up

  how quickly gas is produced or how quickly light is blocked
• how can the quantity of reactant r product be measured by?

  mass in grams or volume

  quantity if reactants in terms if moles and units for rate of reaction in mol/s
• how can the mean rate of reaction be formed?

  quantity of reactant used ÷ time

  or

  quantity of product formed ÷ time
• what are the three different methods used to measure the rate of reaction?

  • measuring the decreasing mass of a reaction mixture e.g. marble chips
  • increasing volume of gas given off
  • decreasing light passing through a solution e.g. disappearing x
• when do reactions take place?

  when particles collide with a certain amount of energy
• what does the rate of a reaction depend on?

  the frequency of successful collisions between particles and the energy with which particles collide
• what happens when particles collide with less energy than the activation energy?

  they will not react, they will just bounce off each other
• what are the factors that affect the rate of reaction?

  • increased temperature
  • increased concentration of dissolved reactants
  • increased pressure of gaseous reactants
  • increased surface area of solid reactants
  • use of a catalyst
• what is the effect of temperature on the rate of reaction?

  • at higher temperature particles have more energy which allows them to move faster and are more likely to collide with other particles
  • when the particles collide they do so with more energy and so the number of frequent successful collisions increases
• what is the effect of concentration on the rate of reaction?

  • at higher concentration there are more particles in the same amount of space
  • this means that the particles are more likely to collide and therefore more likely to react
• what is the effect of pressure on the rate of reaction?

  • as the pressure increases the space in which the gas particles are moving becomes smaller which means they are closer together
  • this will mean that more frequent successful collisions will increase and mean that particles are more likely to react
• what is the effect of surface area on rate of reaction?

  • there is an increased area fr the reactant particles to collide with
  • the smaller the pieces the larger the surface area meaning more collisions and a greater chance of reaction
• what is a catalyst?

  substances that change the rate of reaction without being used up in the reaction
• what do catalysts do in a reaction?

  the produce the same amount of product more quickly - never produce more product
• what are examples of catalysts?

  • nickel - in the production of margarine
  • iron - in the production of ammonia from nitrogen and hydrogen
  • platinum - in the catalytic converters of car exhaust, it catalyses the conversion of carbon monoxide and nitrogen oxide into the less polluting carbon dioxide and nitrogen
• why are catalysts so important for industry?

  • products can be made more quickly, saving time and money
  • they reduce the need for high temperatures, saving fuel and reducing pollution
• why do catalysts often come in the form of powders?

  as it provides the largest possible surface area for them to work
• when do reversible reactions take place?

  when the backwards reaction takes place relatively easily under certain conditions

  the products turn back into the reactants
• what can reversible reactions be?

  endothermic and exothermic
• how does a reaction reach equilibrium?

  when reversible reactions reach equilibrium the forward and backward reactions are still happening but at the same rate, so the concentrations of reactants and products do not change
• what can the balance point of equilibrium be affected by?

  temperature and the pressure
• what happens when you heat ammonium chloride?

  • a reversible reaction takes place
  • ammonium chloride breaks down on heating, it forms ammonium chloride and hydrogen gas which is an example of a decomposition reaction
• what happens if the equilibrium lies to the right?

  the concentration of products is greater than that of the reactants
• what happens if the equilibrium lies to the left?

  the concentration of reactants is greater than that of the products
• what does the position of equilibrium depend on?

  • temperature
  • pressure
  • concentration
• what is Le Chatelier's principle?

  if a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium shifts to counteract the change to reestablish an equilibrium.
• what happens if the temperature of a system at equilibrium is increased?

  • the relative amount of products at equilibrium increase for an endothermic reaction
  • the relative amount of products at equilibrium decreases for an exothermic reaction
• what happens if the temperature of a system at equilibrium id decreased?

  • the relative amount of products at equilibrium decreases for an endothermic reaction
  • the relative amount of products at equilibrium increases for an exothermic reaction
• what does an increase in pressure cause for equilibrium?

  the equilibrium position shifts towards that side with the smaller number of molecules as shown by the symbol equation for that reaction
• what does a decrease in pressure cause for equilibrium?

  the equilibrium position to shift towards the side with the larger number of molecules as shown by the symbol equation for that reaction
• how does concentration affect equilibrium?

  if the concentration of one of the reactants or products is changed, the system is no longer at equilibrium and the concentrations of all the substances will change until equilibrium is reached again
• what happens to equilibrium if the concentration is increased?

  more products will be formed until equilibirum is reached again
• what happens to equilibrium is concentration is decreased?

  more reactants will react until equilibrium is reached again
• what is the method to produce a cloudy precipitate?

  • measure 50cm of NaSO solution into a flask
  • measure 5cm of dilute HCl into a measuring cylinder
  • draw a dark cross on a piece of white paper and put it underneath the falsk
  • add the aid into the flask and immediately start the stop watch
  • solid sulfur is formed which precipitates in solution, turning cloudy
  • look down at the cross and stop timing when the cross can no longer be seen
  • repeat using different concentrations of sodium thiosulfate solution
• what is the conclusion for producing a cloudy precipitate?

  as there are more reactant particles in a given volume collisions occur more frequently, increasing the rate of reaction
• what is the method to produce hydrogen gas?

  • measure 50cm of 1.0 mol/dm HzSO4 solution into a flask
  • ass the magnesium ribbon to the flask and connect it to the gas collection equipment
  • start the stopwatch and record the volume of gas every 10 seconds
  • when the reaction is complete, repeat using 1.5 mol/dm sulfuric acid