The Disappearing Cross

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Chemistry

The Disappearing Cross

In this experiment, we shall be adding sodium thiosulphate to hydrochloric acid together and  placing a drawn cross underneath and  seeing how long the rate of reaction lasts until you cannot see this cross. Before the reaction starts, the liquids are both clear. When added they turn cloudy and milky with a yellow tinge to it, due to the sulphur released.

Na2S203 (aq) + 2HCL   2Nacl (aq) + H20 (l) + SO2 (g) + S(s)

Sodium thiosulphate + hydrochloric acid  sodium chloride + water +                                                sulphur dioxide + Sulphur

 The aim of this experiment is to see how each factor affects the rate of reaction.

The factors we are concentrating on are:

  • Concentration
  • Surface area/Particle size
  • Temperature
  • Catalyst

Collision Theory: In order to understand rates of reactions, we use this model

  1. All chemicals are made from particles (atoms/Molecules)
  2. The particles have energy and can move
  3.  Chemical reactions only happen when particles collide
  4. Some collisions are not successful. Thy must have enough energy to react

Particle size.

6 surfaces therefore a slow reaction  because of a smaller surface area

Small chips therefore a faster reaction because of a larger surface area

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The collision theory model explains why smaller particles have a faster reaction

Temperature

Lower energy therefore particles move slowly so a slow reaction

More energy therefore particles move faster so more collisions, so faster reaction

The collision theory explains that increasing temperature makes reactions faster.

Concentration

Concentration measures how many moles per 1000cm3

KEY

1mole=6 x 1023 particles

1 Litre= 1dm1000cm3

1dm x 1dm x 1dm= 1cm3

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