reaction kinetics

• The rate of a reaction is the speed at which a chemical reaction takes place and has units mol dm s.
• The rate of a reaction can be calculated by: Worked example: Calculating the rate of reaction.
• The collision theory states that for a chemical reaction to take place the particles need to collide with each other in the correct orientation and with enough energy.
• The minimum energy that colliding particles must have for a collision to be successful and a reaction to take place is called the activation energy (E).
• An ineffective collision is when particles collide in the wrong orientation or when they don’t have enough energy and bounce off each other without causing a chemical reaction.
• The collision frequency is the number of collisions per unit time.
• When more collisions per unit time take place, the number of particles with energy greater than the E increases, causing an increase in the rate of reaction.
• The graph shows that the rate is directly proportional to the concentration of cyclopropane.
• The calculated rates for both methods are very similar, with a rate of 0.0009 mol dm s.
• The calculated rates can be summarised in a table to show how the rate of reaction changes with changing concentration of the reactants or products.
• The rate of reaction can be calculated by taking short time intervals.
• The data from the table can be used to plot a rate-concentration graph.
• The graph shows a directly proportional correlation between the concentration of cyclopropane and the rate of reaction.
• If you double the concentration of cyclopropane, the rate of reaction will double too.
• The rate of reaction can be calculated at different concentrations of reactant or product at particular time points by drawing tangents on the graph.
• To calculate the rate of reaction, you can either use the increase in concentration of products or the decrease in concentration of reactants.
• A catalyst is a substance that increases the rate of reaction without taking part in the chemical reaction by providing the particles an alternative mechanism with a lower activation energy.
• A catalyst increases the rate of a reaction by providing an alternative pathway which has a lower activation energy.
• The diagram shows a higher concentration of particles in (b) which means that there are more particles present in the same volume than (a) so the chances and frequency of collisions between reacting particles is increased causing an increased rate of reaction.
• The average rate over the first 5 minutes for propene is:
• The isomerisation of cyclopropane to propene will be taken as an example:
• The more concentrated a solution is, the greater the number of particles in a given volume of solvent.
• An increase in pressure therefore increases the rate of reaction.
• An increase in pressure in reactions that involve gases has the same effect as an increased concentration of solutions.
• An increase in concentration causes an increased collision frequency and therefore an increased rate of reaction.
• Line a shows the average rate over the first 5 minutes whereas line b shows the actual initial rate found by drawing a tangent at the start of the curve.
• At the start of the reaction, the concentration-time curve looks almost linear, meaning that the rate at this point can be found by treating the curve as a linear line and using:
• During a reaction, the reactants are used up and changed into the products, meaning that as the reaction proceeds, the concentration of the reactants is decreasing and the concentration of the products is increasing.
• The concentrations of reactant (cyclopropane) and product (propene) over time can be experimentally obtained.
• When the pressure is increased, the molecules have less space in which they can move, meaning that the number of effective collisions increases due to an increased collision frequency.
• A concentration-time graph for the concentration of propene as well as cyclopropane can be obtained from the above results.
• The rate of the reaction during the reaction can be calculated from a concentration-time graph.
• The graph shows that the concentration of propene increases with time.
• When taking the measurements, the temperature should be kept the same at all times as a change in temperature will change the rate of reaction.
• Heterogeneous means that the catalyst is in a different phase to the reactants, for example, the reactants are gases but the catalyst used is a solid.
• Boltzmann distribution curve.
• The catalyst speeds up a reaction that would normally be slow due to the high activation energy.
• Catalysts can be divided into two types: Homogeneous catalysts and Heterogeneous catalysts.
• A catalyst increases the rate of a reaction by providing the reactants with an alternative reaction pathway which is lower in activation energy than the uncatalysed reaction.
• Catalysts provide the reactants another pathway which has a lower activation energy, resulting in a greater proportion of molecules in the reaction mixture having sufficient energy for an effective collision.