reaction rates (colour change)

• What is seen when sodium thiosulfate solution reacts with dilute hydrochloric acid?

  Solution turns clear and the cross appears
• What safety precautions should you take in this investigation?

  • Wear safety goggles to protect eyes from sodium thiosulfate and acid as irritants
  • Keep room well ventilated from fumes form sodium thiosulfate
• What is the best practical method to determine the rate of this reaction and why?

  • Place conical flask onto a drawn cross.
  • Time how long it takes for the cross to appear.
  • Can use light sensor and lamp to determine the solution going clear as this would be more accurate.
• How do you change the temperature of the solutions?

  Stand the conical flask in a water bath
• What happens to the time taken for the reaction to occur as the temperature increases?

  Time gets shorter / faster reaction
• How do you explain this change in terms of the energy of the particles and collisions?

  The higher temperature provides more kinetic energy to the particles so they move around faster and collide more frequently and react
• What is the best practical method to determine the rate of this reaction and why?

  Use a gas syringe to measure the volume of gas over a period of time.
• What are two different methods of collecting and measuring the volume of gas produced?

  Use a gas syringe to collect volume of gas over a known time or an upturned measuring cyclinder in a water bowl and measure volume of water displaced
• What is the biggest error in this experiment?

  There is gas lost in the time it takes to add the reactants and fix on the bung and delivery tube
• How could you reduce this error?

  Add the reactants through a side are or suspend the marble chips in a muslin bag from the bung and drop it in by loosening the bung slightly.
• How do you decrease the size of the marble chips?

  Use a pestle and mortar to break them up
• What specific safety precaution should you take when decreasing the size of marble chips?

  Wear safety glasses to protect eyes from shards of rock
• How does this affect the surface area of the marble chips?

  The surface area will increase
• What effect does this have on the rate of reaction?

  The rate of reaction increases
• How can you explain this effect from graphs of volume of gas plotted against time for two different sizes of marble chips?

  Calculate the reaction rate in cm3/min at a set time and compare. The smaller chips will have a steeper curve as the rate will be faster.
• How can you calculate the rate of reaction from these graphs?

  The average rate = volume change / time interval. On the graph find the point on the curve at e.g. 100 seconds, and draw the tangent line to the curve point. Choose two points on the tangent line where it is easy to read the time and volume, calculate the change in volume that occurs for that time interval. Calculate the rate by dividing the change in volume by the change in time. Repeat for the second graph and compare.
• What needs to be kept the same when you repeat the first experiment but use different size marble chips?

  Volume/concentration/type of acid, mass of marble chips, delivery tube length/width that attached to the gas syringe, temperature.
• How could you decrease the concentration of the hydrochloric acid?

  Add water and work out percentage strength
• What effect will decreasing the concentration of hydrochloric acid have on the rate of reaction?

  The rate of reaction will be slower
• How do you explain this effect in terms of particles and collisions?

  Lower concentration means fewer particles to collide with the marble chips and a slower reaction.