Concept Map: Collision Theory, Rate of Reaction
Rate of Reaction
Rate takes the reciprocal of time ,so is expressed per unit time or in SI units per second.
Rate = 1/time = s-1
As the reaction proceeds, reactants are converted into products , and so the concentration of reactant decreases as the concentration of product increases.
Rate of reaction = increase in product conc./time taken
Rate of reaction = dec in reactant conc/time taken
As rate is change in concentration per time, its units are mol dm-3s-1
Collision Theory
Graphical Representations
Calculating Rate of Reaction
Gas Collection
Mass Loss Method
Colorimeter
Titration
conductivity meter
Non-continous methods
Clock Reactions
Activation Energy
Maxwell-Boltzmann Distribution Curve
Factors affecting rate of reaction
Temperature
Concentration
Particle Size
Pressure
Catalyst
For a reaction to occur between two species --> the two species much collide
not necessary that just because two species collide there will be a reaction
convenient method if one product is gas--collecting gas and measuring the change in volume at regular time intervals enables a graph to be plotted of volume against time
The rate of reaction of a metal with dilute acid to release hydrogen gas can be followed in this way.
Eg: Mg(s) + 2HCl(aq) --> MgCl2 (aq) + H2 (g)
If the reaction is giving off a gas, the corresponding decrease in mass can be measured by standing the reaction mixture directly on a balance.
The method allows for continuous readings , so a graph can be plotted directly of mass against time,
For example : CaCO3 (s) + 2HCl(aq) --> CaCl2 (aq) + CO2(g) + H2O (l)
Colorimeter may be used if one of the constituents is colored. It follows the change in intensity of color.
A colorimeter or spectrophotometer works by passing light of a selected wavelength through the solutions being studied and measures the intensity of light transmitted by the reaction components.
As the concentration of the coloured compound increases, it absorbs proportionally more light , so less is transmitted.
A photocell generates an electric current according to the amount of light transmitted and is recorded on a meter or is connected to a computer.
The method allows for continuous readings to be made , so a graph of absorbance against time can be plotted directly.
for a collision to be successful the species must collide with the correct orientation, and have sufficient energy to break pre-existing bonds in the reactant species
In some reactions it may be possible to measure the concentration of one of the reactants or products by titrating it against a known standard.
The process of titration takes time ,during which the reaction mixture in the sample will continue to react .
The quenching which occurs as a result of change in the chemical nature of a substance is known as chemical quenching. It can be due to change in pH, presence of oxygen , halide and heavy metals.
The reaction between H2O2 and acidified KI liberates I2, which can be titrated against sodium thiosulphate to determine its concentration. Sodium carbonate is used to quench the reaction by neutralising the added acid.
A conductivity meter is used if there is change in conductivity i.e. the no. of ions present is changing. A pH meter follows the change in H+ ions.
The total electrical conductivity of a solution depends on the total concentration of its ions and on their charges.
Conductivity can be measured with conductivity meter which involves immersing inert electrodes in the solution.
The apparatus can be calibrated using solutions of known concentration so that reading can be converted into the concentrations of the ions present.
minimum amount of energy required for collision to occur
measuring the loss in mass due to carbon dioxide escaping from the flask
measuring the change in concentration of the hydrochloric acid
Sometimes it is difficult to record the continuous change in the rate of reaction, it may be more convenient to measure the time it takes for a reaction to reach a certain chosen point or endpoint (observable entity).
The time taken to reach this point for the same reaction under different conditions can then be compared and used to measure different rates of reaction.
measuring volume of carbon dioxide evolved over time
increasing temperature increases the rate of reaction.
it increases the average kinetic energy of the particles
the frequency of collisions between reactant particles increases.
there is an increase in the proportion of particles that have energy equal to, or greater than, the activation energy.
increasing the concentration increases the rate of reaction
as particle size decreases, the rate of reaction increases
as particle size decreases, the surface area per unit volume increases, which results in an increase in the frequency of collisions between reactant particles.
for reactions that involve gases, increasing the pressure increases the rate of reaction
at higher pressure, the gas is compressed which has the effect of increasing its concentration
a catalyst increases the rate of a chemical reaction by providing an alternative reaction pathway with lower activation energy (Ea)
In the catalysed reaction, a greater proportion of particles have energy greater than the activation energy, therefore the rate of reaction increases
this increases the frequency of successful collisions between reactant particles, increasing rate of reaction
catalyst does not undergo permanent change in a chemical reaction