topic 7

• rates of reaction

  • the rate of a chemical reaction is the speed at which it takes place
• measuring rates of reactions

  • initial reaction needs to be measured at the start of the reaction , where the rate is fastest as none of the reactants are limiting
• collision theory

  • for a reaction to occur particles of the reactants must collide with each other
  • not all collisions leads to reaction
  • particles must collide with enough energy to break the bonds in the reactants - ACTIVATION ENERGY
  • the more energy the particles have - more chance of successful collision
  • rate of reaction will increase if there are more high energy collisions between particles
• effect of temperature - collision theory

  • higher the temperature of the reactants , faster the reaction
  • as temp increases , particles move faster
  • resulting in more frequent collisions
  • particles have more KE , having a greater chance of successful collisions
  • increasing rate of reaction
• effects of concentration - collision theory

  • higher the concentration of reactants in a solution , faster the reaction
  • more concentrated a solution , the more solute particles there are in a given volume
  • more likely reactant particles will collide with one another
  • increasing rate of reaction
• effects of surface area - collision theory

  • greater the surface area of a solid , the faster the rate of reaction
  • increasing the surface areas increase number of particles exposed on the surface ( that cancollide and react
  • more frequent collisions between reactant particles
  • increases rate of reaction
• the effects of pressure - collision theory

  • increasing the pressure of the gas increases the rate of reaction .
  • the reactant particles are squeezed closer together so collisions happen more often
• catalyst

  • substance that speeds up a reaction without being used in the reaction ( not a product or reactant )
  • catalysts reduce activation energy
• catalytic converters

  • Combustion in car engines produces toxic carbon monoxide and unburned hydrocarbons.
  • Catalytic converters reduce emissions by converting these pollutants into carbon dioxide and water.
  • Converters contain costly transition metals (platinum, rhodium, or palladium) that act as catalysts.
  • Their honeycomb structure increases surface area and decreases metal usage.
  • They work most efficiently at high temperatures and are less effective when the engine is cold.
• core practical - RoR - calcium carbonate + acid - Surface area

  • bubbling marble chips or powdered calcium carbonate in acid - in a conical flask
  • gas produces goes through delivery tube to measuring cylinder filled with water
  • amount of gas in measuring flask is volume of of gas produced
  • to calculate RoR - products made (g or cm3 ) / time taken ( s )
• sodium thiosulfate and acid - ror - concentration

  • measure known volume , and concentration of sodium thiosulfate and pour into conical flask which is placed on top of paper with cross
  • add known volume of acid into conical flask and time how long it takes for sulfur precipitate to be formed ( yellow ) , which covers the cross on paper
  • repeat with different concentrations
  • calculate rate using 1/time or 1000 / time
• exothermic

  • reaction gives out heat energy to surroundings
  • temperature of reaction mixture and surrounding increases
• exothermic reaction examples

  • all combustion reactions - e.g - combustions of methane
  • displacement reactions
  • neutralisation reactions
• endothermic

  • reactions take heat energy from surroundings
  • temperature of reaction mixture and surroundings decreases
• example of endothermic reaction

  • reaction of sodium hydrogen carbonate with hydrochloric acid ( test tube feels cold - heat energy is take from hand )
  • dissolving ammonium nitrate in water
  • photosynthesis ( take in heat energy from the sun )
• measuring temperature change

  • add reactant to polystyrene cup ( good insulator of heat )
  • record initial temperature
  • and second reactant and stir
  • add lid to reduce evaporation to escape
  • record final temperature and calculate difference
• energy changes

  • making and breaking bonds - bonds in reactants break to form new bonds in products
  • breaking bonds requires energy - endothermic process
  • making bonds releases energy - exothermic process
• energy changes endothermic and exothermic

  • exothermic - more heat energy is released making bonds ( in products ) than is required to break bonds ( in reactants ) - heat energy is released - hotter
  • endothermic - more heat energy is required breaking bonds ( in reactants ) than is released to make bonds ( in products ) - heat energy is absorbed - colder
• bond energy calculations

  • energy change = energy required to break bond in reactants - energy required to break bonds in product
  • if negative reaction is exothermic
  • if positive reaction is endothermic