An investigation into the effect of concentration on the rate of chemicl reaction between sodium thiosulphate and hydrochloric acid.

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Mona Ibrahim

Chemistry Coursework

Aim

The aim of this investigation is to find out and observe how concentration affects the rate of reaction. I am going to investigate the changes caused to the solution of hydrochloric acid and sodium thiosulphate when the concentration is changed.

Background information

The rate or speed of a chemical reaction is a measure of how fast the reaction takes place. The rate of reaction gives the measure of how the reaction is proceeding.

How is the rate of reaction found?

To find the rate of reaction, it is necessary to measure either:

  • The amount of a reactant used per unit of time (or)
  • The amount of product produced per unit of time.

If one products of the reaction is a gas, it can be collected and its volume measured as it is evolved by means of a gas syringe. The rate of evolution of gas can then be used as a measure of the reaction.

Example- during the experiment between Magnesium and dilute hydrochloric acid, the products are magnesium chloride and hydrogen. The gas can be collected and its  volume measured at intervals using the apparatus below.

Collision theory of reaction

Reaction takes when particles collide.

Most collision does not produce a reaction because colliding particles need a minimum energy to react when they collide.

E.g.:  2H2 (g) + O2 (g) ==> 2H2O

When H2 and O2 molecule collide, they must have energy to break the H-H and the O=O bonds in H2 and O2 before they can react to form water. The collisions must be enough of energy to overcome the energy barrier; called the activation energy.

The more frequently particles collide, the faster is the reaction.

How rate of reaction is measured

  1. Measure how long it takes for a reaction to be completed. E.g.: the rate of reaction of magnesium with hydrochloric acid can be measured by timing how long it takes a piece of magnesium ribbon to dissolve in hydrochloric acid.

The shorter the time, the faster the reaction

  1. Measure the amount of reaction product produced in a given. E.g. the rate of reaction in a marble chips(calcium carbonate) with hydrochloric acid can be measured by recording the volume of CO2 gas produced at a regular time intervals

            The steeper the graph, the faster the reaction

  1. Measure the amount of reactant used in a given time.

The steeper the graph, the faster the reaction.

There are five factors that affect the rate of reactions:

  • Temperature

Increasing the temperature raises the kinetic energy of the reactant molecule. This results in:

  • Harder collisions so that a greater proportion of collisions are successful in causing reaction (particles collide with more energy).
  • More frequent collisions leading to an increase in the frequency of successful collisions.

The rate of reaction increases with rise in temperature. Heat energy increases the kinetic movement of the molecules, hence increases the chances of collision between the molecules.

  • Pressure

Increasing the pressure in a reaction involving gases is equivalent to increasing the concentration. At high pressure, the molecules of gases are closer together, hence increases the chances of collision between the molecules.

  • Concentration

The rate of reaction is directly proportional to the concentration of the reacting substances.                                                                                                                Increasing the number of particles per unit volume (concentration) will cause more frequent collisions and hence increase the rate of the reaction.  It can also change the concentration of a gas mixture by compressing it into a smaller volume. A gas at high pressure is more concentrated than at low pressure.

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  • Catalyst

A catalyst is a substance which affects the rate of reaction but is chemically unchanged at the end of the reaction.  Higher temperature and greater surface area make them more effective. It is possible that the catalysts speed up reactions by facilitating a different reaction with lower activation energy than the uncatalysed reactions. This results in a higher proportion of collisions being successful, leading to a higher reaction rate.                                                           ...

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