6.2- rates of reaction: observing colour change

• what is the aim

  To investigate the effect of changing concentration in the reaction between sodium thiosulfate and hydrochloric acid
• what apparatus is needed?

  • 40 g dm-3 sodium thiosulfate solution
  • 1.0 mol dm-3 dilute hydrochloric acid
  • Conical flask (100 cm3)
  • Black cross on paper
  • White paper or white tile
  • Stopwatch or timer
• what is the method?

  • Measure 50 cm3 of sodium thiosulfate solution into a flask
  • Measure 5 cm3 of dilute hydrochloric acid into a measuring cylinder
  • Draw a cross on a piece of paper and put it underneath the flask
  • Add the acid into the flask and immediately start the stopwatch
  • Look down at the cross from above and stop the stopwatch when the cross can no longer be seen
  • Repeat using different concentrations of sodium thiosulfate solution (mix different volumes of sodium thiosulfate solution with water to dilute it)
• what are the results?

  • With an increase in the concentration of a solution, the rate of reaction will increase
  • This is because there will be more reactant particles in a given volume, allowing more frequent and successful collisions, increasing the rate of reaction
• whats the balanced equation for this?

  $Na 2 ​ S 2 ​ O 3 ​ (aq)+$$2HCl(aq)→S(s)+$$SO 2 ​ (g)+$$2NaCl(aq)+$$H 2 ​ O(l)$
• Sodium thiosulfate solution

  Clear and colorless
• Sodium thiosulfate solution reacting with dilute hydrochloric acid

  1. Initially clear and colorless

  2. Reaction begins

  3. Sulfur (S) forms as yellow precipitate

  4. Precipitate clouds the solution, making it opaque and changing color to yellowish

  5. More sulfur precipitates out, entire solution turns opaque and yellow
• The formation of the yellow precipitate indicates that the reaction is progressing and sulfur is being produced
• what safety precautions should you take in this investigation?

  wear gloves to protect skin from acids

  well ventilated area
• what is the best practical method to determine the rate of this reaction and why?

  "disappearing cross" method because its simple, minimal equipment required
• how do you change the temperature of the solutions?

  • waterbath
  • heating or cooling plate
• what happens to the time taken for the reaction to occur as the temperature increases?

  Overall, increasing temperature generally accelerates chemical reactions by increasing the kinetic energy of the molecules and promoting more frequent and energetic collisions between reactant molecules. As a result, the time taken for the reaction to occur decreases with increasing temperature.