Chemistry controlling the rate

• Chemical reactions occur at different speeds (or rates) and can be affected by variables such as particle size, concentration, temperature, and the use of a catalyst.
• Increasing the concentration of the reactants results in more successful collisions, so the reaction rate increases.
• Particle size is a variable that can affect the speed of a chemical reaction, with smaller particles leading to a faster rate of reaction.
• Concentration is another variable that can affect the speed of a chemical reaction, with higher concentrations leading to a faster rate of reaction.
• Temperature is a variable that can affect the speed of a chemical reaction, with higher temperatures leading to a faster rate of reaction.
• Using a catalyst can speed up a chemical reaction without being used up in the reaction.
• Reaction rates in the Chemical industry must be controlled, with too slow a rate leading to an uneconomical process and too fast a rate risking an explosion.
• The time taken for a reaction to be complete can be used to calculate the relative rate of reaction, which can be found by calculating the reciprocal of the time taken.
• As the concentration increases, the rate of reaction increases.
• Molecules with enough energy to react increase in number at low temperatures and decrease at high temperatures.
• Chemical reactions involve a change in energy, usually a loss or gain of heat energy, and the overall enthalpy change for a reaction is represented by ΔH.
• Units for ΔH are kJmol-1 (kilojoules per mole).
• Potential energy diagrams can be used to calculate the ΔH and the activation energy for a reaction.
• Exothermic reactions give out energy, meaning that the products have less energy than the reactants, resulting in a drop in the potential energy and ΔH is negative.
• Endothermic reactions take in energy, meaning that the products have more energy than the reactants, resulting in an increase in the potential energy and ΔH is positive.
• One definition of activation energy (E A ) is the minimum kinetic energy reacting particles require for a successful collision to break the bonds in the reactants.
• Activation energy can be seen as an ‘energy barrier’ to a chemical reaction taking place, with the greater the activation energy, the slower the reaction rate.
• The activated complex is an unstable arrangement of atoms formed at the maximum of the potential energy barrier, during a reaction.
• In a reaction, a catalyst provides an alternative reaction pathway with a lower activation energy, represented as follows: For reversible reactions, the catalyst lowers the E A for both the forward and the reverse reaction by the same amount.
• When a catalyst is used, the E A is lowered which means that more particles have kinetic energy equal to or greater than E A.
• Energy is given out as new bonds are formed and the atoms are rearranged into the products.
• Successful collision: A collision in which the reactants collide and form products.
• Unsuccessful collision: A collision in which the reactants collide but do not form products.
• Activated complex: A complex formed during a reaction.
• Observations: The magnesium powder reacted faster than the ribbon due to the larger surface area of the powder.
• Decreasing the particle size increases the surface area of the magnesium so there is a greater area over which collisions can occur, leading to a faster rate of reaction.
• More collisions per second means a faster rate of reaction.
• The more concentrated acid produced hydrogen gas more quickly due to the increased number of particles present, resulting in more collisions per second.
• Increasing the concentration means there are more particles present which results in more collisions per second, leading to a faster rate of reaction.
• Pressure is similar to concentration but for gases, and can be increased by adding more gas to the same volume of container or reducing the volume of the container.
• Increasing the pressure means there are more particles per volume present which results in more collisions per second, leading to a faster rate of reaction.
• Temperature is a measure of the average kinetic energy of all of the particles in a substance, and a higher temperature results in the molecules having a higher kinetic energy, leading to more successful collisions and an increase in the rate of reaction.
• Most particles have energy near the average kinetic, a few particles have energy well above the average kinetic energy, and a few particles have energy well below the average kinetic energy.
• A higher temperature results in the molecules having a higher kinetic energy, leading to more successful collisions and an increase in the rate of reaction.
• When dilute hydrochloric acid is added to a solution of sodium thiosulfate in a beaker, solid sulfur forms in the solution.
• The effect of concentration on the rate of reaction can be studied by varying the sodium thiosulfate concentration and timing how long it takes for enough sulfur to be formed, to obscure a cross drawn on a piece of card placed below the beaker.
• The collision theory explains the effect of particle size, concentration, pressure, temperature and a catalyst on the rate of a reaction.
• The collision theory states that substances can only react if: the particles collide, the particles collide with energy in excess of the activation energy and the particles collide with the correct collision geometry.
• The greater the number of successful collisions, the faster the rate of reaction will be.
• When reacting particles collide, they must collide with a minimum amount of kinetic energy in order for the collision to be successful.