Rate (Speed) Of Reaction 3 - Theory.pdf

The equation for the reaction between sodium thiosulfate and hydrochloric acid is Na2S2O3(aq) + 2HCl(aq) → 2NaCl(aq) + S(s) + SO2(g) + H2O(l). The speed of this reaction was investigated using an experiment where a beaker containing sodium thiosulfate was placed on a black cross, and hydrochloric acid was added. The time it took for the solution to become cloudy and the cross to no longer be visible was recorded. The experiment was repeated with different volumes of sodium thiosulfate and water.

The total volume of the solution was kept the same in all experiments to ensure that the concentration of the reactants was the only variable being changed. This allows for a fair comparison of the reaction rates.

The table of results shows that as the volume of sodium thiosulfate decreases, the time it takes for the reaction to occur increases. This is because there are fewer reactant particles available to collide and react.

The speed of the reaction varies from experiment 1 to 4 because the concentration of the reactants changes. In experiment 1, there is a higher concentration of sodium thiosulfate, resulting in a faster reaction rate. As the volume of sodium thiosulfate decreases, the concentration decreases, and the reaction rate slows down.

The idea of collisions between reacting particles is used to explain changes in the speed of reactions. When the temperature is increased from 20°C to 42°C, the particles have more kinetic energy, resulting in more frequent and effective collisions, and a faster reaction rate.

In a separate experiment, the rate of the reaction between iron and aqueous bromine was investigated using an apparatus with a piece of iron attached to a stirrer. The mass of iron was plotted against time, and the graph shows a decreasing mass of iron over time. The shape of the graph can be explained by the fact that the reaction rate decreases as the surface area of the iron decreases.

If a similar piece of iron with a much rougher surface had been used, the graph would show a faster reaction rate, as there would be more reactant particles available to collide and react.

To find out if the rate of this reaction depended on the speed of stirring, the experiment could be repeated with different stirring speeds, and the reaction rate could be measured and compared.

The redox reaction between iron and bromine can be represented by two possible equations. The change which is oxidation is the loss of electrons by the iron, resulting in the formation of Fe2+ or Fe3+ ions. The reductant in the first equation is bromine, as it gains electrons and is reduced.

To test the solution to find out which ion, Fe2+ or Fe3+, is present, a chemical test such as the addition of a specific precipitating agent could be used. The solution could be treated with a reagent that selectively precipitates one of the ions, allowing for identification of the ion present.