Topic 7

• How do you investigate the effect of surface area on rate of reaction?

  • Place marble chips in dilute HCl
  • Measure the volume of CO2 gas produced using a gas syringe attached to the flask. Take readings at regular intervals
  • Plot a graph of results
  • Repeat with same volume and concentration of acid, same mass of marble chips but scrunch them up for less surface area
  • Repeat with chalk
• Why do finer particles and more mass increase rate of reaction

  • The steeper the gradient of the graph is, the faster the rate of reaction
  • When line is flat no more gas is produced
  • Finer particles give larger surface area, so the reaction finishes sooner and the rate of reaction is greater
  • Using more mass will increase the surface area which will increase the amount of gas produced and the reaction will be faster
• How can you investigate concentration affecting rate of reaction?

  • use same method as investigating surface area
  • same mass and surface area of marble chips, same volume of acid but different concentration of acid
  • A higher concentration gives a faster reaction, more particles react and produce gas
• How do you investigate temperature affecting rate of reaction involving precipitation?

  • Measure out fixed volumes of the substances using a measuring cylinder
  • Gently heat them in a water bath to the desired temperature
  • Mix them in a conical flask and place them over paper with a black mark
  • Time how long it takes for the black mark to disappear due to the colour of the precipitate
  • Repeat at different temperatures
• What are the variables when investigating effect of temperature on rate of reaction?

  Control variables:

  • depth and volume of liquids
  • concentration of acids

  Dependent variable:

  • time it takes for cross to disappear (rate of reaction for precipitate to form)

  Independent variable:

  • the temperature of the solutions
• At lower temperatures, it took longer for the cross to disappear

  At higher temperatures, it took less time for the cross to disappear

  Therefore increasing the temperature increases the rate of reaction
• How can you use change in mass to measure reaction rate?

  • when gas is produced, the solution loses mass
  • The quicker the balance drops, the faster the rate of reaction
  • if the balance stops changing, no more gas is produced
  • but dangerous because gas released into the room, so use safety precautions e.g fume cupboard
• What does the rate of a chemical reaction depend on?

  • collision frequency of the particles
  • the more successful collisions, the faster the reaction is
  • the energy transferred during a collision
  • particles need to collide with at least the activation energy to have successful collisions
• What is activation energy?

  • The minimum energy particles need to react when they collide
• what is the equation for rate of reaction?

  rate of reaction = amount of reactant used or amount of product formed / time
• If you increase the number of collisions or the energy when they collide, the reaction happens more quickly
• How does temperature increase rate of reaction?

  • particles move faster
  • they will have more collisions
  • particles have more energy as they are moving faster
  • reactions only happen if particles have enough energy
  • therefore increasing temperature increases number of successful collisions because more particles will collide with enough energy to react
• How does increasing concentration/pressure increase rate?

  • more concentration = more reactant particles
  • more reactant particles in same volume = more successful collisions = rate of reaction increases

  
  

  • more pressure = more gas particles in one area
  • frequency of collisions between particles will increase = rate of reaction increases
• How does smaller solid particles/increased surface area increase rate?

  • solid reactant broken into small pieces
  • increases surface area : volume ratio (more solid exposed to cover its volume)
  • other reactant particles can react with smaller solid particles, frequency of collisions increases
  • rate of reaction is faster
• What is a catalyst?

  • a substance that speeds up the rate of a reaction
  • does not change products of reaction
  • it isn't used up - its mass will stay the same at the end
  • only need small amount to catalyse large amounts of reactants
• How does adding a catalyst to a reaction increase the rate of reaction?

  • they lower the activation energy of the reaction
  • by providing an alternative reaction pathway that has lower activation energy
  • more of the particles have the minimum energy they need to react when they collide
  • therefore more successful collisions and higher rate of reaction
• What is an exothermic reaction?

  A reaction that gives out energy to the surroundings
• What is an endothermic reaction?

  A reaction that takes in energy from the surroundings
• The breaking of bonds is endothermic and the making of bonds is exothermic
• Why would the overall heat energy change for a reaction be exothermic?

  • more energy is required to make the bonds in the products
  • than breaking the bonds in the reactants
• Why would the overall heat energy change for a reaction be endothermic?

  • more energy is required to break the bonds in the reactants
  • than to make the bonds in the products
• How can you tell if a reaction is overall exothermic?

  The heat energy change will be negative
• How can you tell if a reaction is endothermic or exothermic from its reaction profile?

  • An endothermic reaction will have the products higher than the reactants as they have more energy (taken in)
  • An exothermic reaction will have products lower than the reactants because they have less energy (release)
  • difference in height between them shows how much energy is taken in or given out
• How do you calculate overall energy change?

  energy required to break bonds - energy required to make bonds
• Describe heat changes for some reactions

  • Salts dissolving in water is either exothermic or endothermic
  • Neutralisation reaction is exothermic
  • Displacement is an exothermic or endothermic reaction
  • Precipitation is an exothermic reaction
  • if these reactions take place in a solution, you can carry them out in a polystyrene cup with a lid
  • measure the temperature change using a thermometer