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Physical factors affecting rate of reaction.

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  1. 1. Reaction Rates (Chemical Kinetics)Key ConceptsThe rate of a chemical reaction is the speed with which reactants are converted to products.Collision Theory is used to explain why chemical reactions occur at different rates.Collision Theory states that in order for a reaction to proceed, the reactant particles must collide.The more collisions there are per unit of time, the faster the reaction will be.In order for a reaction to proceed, the reactant particles must:collide with sufficient energy to break any bonds in the reactant particles.The activation energy is the minimum amount of energy the colliding reactant particles must have inorder for products to form.be in an orientation favourable for breaking those bonds. THE EFFECT OF TEMPERATURE ON REACTION RATES Usually, an increase in temperature is accompanied by an increase in the reaction rate. Temperature is a measure of the kinetic energy of a system, so higher temperature implies higher average kinetic energy of molecules and more collisions per unit time. A general rule of for most (not all) chemical reactions is that the rate at which the reaction proceeds will approximately double for each 10°C increase in temperature. Once the temperature reaches a certain point, some of the chemical species may be altered (e.g., denaturing of proteins) and the chemical reaction will slow or stop. With the exception of some precipitation reactions involving ionic compounds in solution, just about all chemical reactions take place at a faster rate at higher temperatures. The question is why? Temperature (in Kelvin degrees) is proportional to the kinetic energy of the particles in a substance. For example, if the Kelvin temperature of a substance is doubled, then the average kinetic energy of the particles in that substance is doubled.
  2. 2. At higher temperatures, particles collide more frequently and with greater intensity.ExamplesSome reactions are virtually instantaneous - for example, a precipitation reactioninvolving the coming together of ions in solution to make an insoluble solid, or thereaction between hydrogen ions from an acid and hydroxide ions from an alkali insolution. So heating one of these wont make any noticeable difference to the rate ofthe reaction.Almost any other reaction you care to name will happen faster if you heat it - either inthe lab, or in industry.The explanationIncreasing the collision frequencyParticles can only react when they collide. If you heat a substance, the particles movefaster and so collide more frequently. That will speed up the rate of reaction. Collisionsonly result in a reaction if the particles collide with enough energy to get the reactionstarted. This minimum energy required is called the activation energy for the reaction.It turns out that the frequency of two-particle collisions in gases is proportional to thesquare root of the kelvin temperature.The key importance of activation energyIn a sample of substances, at a given temperature, the particles will not all possess thesame amount of energy as each other.A few will have a relatively small amount ofenergy.A few particles will have a relatively large amount of energy.Most particles willhave an amount of energy somewhere in between.The distribution of energies at a
  3. 3. given temperature can be shown on a graph called boltzman distribution.You can mark the position of activation energy on a Maxwell-Boltzmann distribution toget a diagram like this:Only those particles represented by the area to the right of the activation energy willreact when they collide. The great majority dont have enough energy, and will simplybounce apart.To speed up the reaction, you need to increase the number of the very energeticparticles - those with energies equal to or greater than the activation energy. Increasingthe temperature has exactly that effect - it changes the shape of the graph.In the next diagram, the graph labelled T is at the original temperature. The graphlabelled T+t is at a higher temperature.If you now mark the position of the activation energy, you can see that although thecurve hasnt moved very much overall, there has been such a large increase in the
  4. 4. number of the very energetic particles that many more now collide with enough energy to react. Remember that the area under a curve gives a count of the number of particles. On the last diagram, the area under the higher temperature curve to the right of the activation energy looks to have at least doubled - therefore at least doubling the rate of the reaction. Summary Increasing the temperature increases reaction rates because of the large increase in the number of high energy collisions. It is only these collisions (possessing at least the activation energy for the reaction) which result in a reactionIncreasing the temperature of a reaction increases the kinetic energy of the particles which increasesthe number of collisions so the reaction rate increases.Increasing the kinetic energy of reactant particles also means more of the reactant particles will havethe minimum amount of energy required to form products (ie, activation energy) which leads to moresuccessful collisions and therefore increases the reaction rate.Increasing the temperature will increase the reaction rates of both endothermic and exothermicreactions, it will also, by Le Chetaliers Principle, affect the equilibrium position.pHpH is a measure of the concentration of hydrogens ions (= H+) (= protons) in a solution.Numerically it is the negative logarithm of that concentration expressed in moles per liter (M).
  5. 5. Pure water spontaneously dissociates into ions, forming a 10-7 M solution of H+ (and OH-). The negativeof this logarithm is 7, so the pH of pure water is 7.Solutions with a higher concentration of H+ than occurs in pure water have pH values below 7 and areacidic.Solutions containing molecules or ions that reduce the concentration of H+ below that of pure waterhave pH values above 7 and are basic or alkaline.The Effect of pH on Enzyme ActivitypH is a measure of the concentration of hydrogen ions in a solution.The higher the hydrogen ion concentration, the lower the pH. Mostenzymes function efficiently over a narrow pH range. A change in pHabove or below this range reduces the rate of enzyme reactionconsiderably.Changes in pH lead to the breaking of the ionic bonds that hold thetertiary structure of the enzyme in place. The enzyme begins to loseits functional shape, particularly the shape of the active site, suchthat the substrate will no longer fit into it, the enzyme is said tobe denatured. Also changes in pH affect the charges on the amino acidswithin the active site such that the enzyme will not be able to forman enzyme-substrate complex.The pH at which an enzyme catalyses a reaction at the maximum rate iscalled the optimum pH. This can vary considerably from pH 2 for pepsinto pH 9 for pancreatic lipaseparticle sizeSmaller reactant particles provide a greater surface area which increases the chances for particlecollisions so the reaction rate increases.
  6. 6. presence of a catalystA catalyst lowers the activation energy for the reaction so more reactant particles will have theminimum amount of energy required to form products so the reaction rate increases.Catalysts (e.g., enzymes) lower the activation energy of a chemical reaction and increase the rate of achemical reaction without being consumed in the process. Catalysts work by increasing the frequency ofcollisions between reactants, altering the orientation of reactants so that more collisions are effective,reducing intramolecular bonding within reactant molecules, or donating electron density to thereactants. The presence of a catalyst helps a reaction to proceed more quickly to equilibrium. Asidefrom catalysts, other chemical species can affect a reaction. The quantity of hydrogen ions (the pH ofaqueous solutions) can alter a reaction rate. Other chemical species may compete for a reactant or alterorientation, bonding, electron density, etc., thereby decreasing the rate of a reaction.intensity of light affects some reactionsSome reactions occur very slowly in the dark but much more quickly in light.eg, methane reacts very slowly with chlorine in the dark, but the rate of reaction is much faster in thepresence of ultraviolet light.Light provides the activation for a reaction to occur. Radiation of properfrequency and sufficient energy must be absorbed to activate the molecules.Energy unit of radiation is photon and is equal to Quantum of energy. Photochemical reactions areindependent of temperature. After a molecule has absorbed quantum of radiant energy,it will raise theirkinetic energy and temperature of system increases. Example of photosynthesis.
  7. 7. Ionic StrengthIonic strength is a characteristic of an electrolyte solution (a liquid with positive and negativelycharged ions dissolved in it). It is typically expressed as the average electrostatic interactionsamong an electrolytes ions. An electrolytes ionic strength is half of the total obtained bymultiplying the molality (the amount of substance per unit mass of solvent) of each ion by itsvalence squared.The ionic strength of a solution is a measure of the concentration of ions in that solution. Ioniccompounds, when dissolved in water, dissociate into ions. The total electrolyte concentration insolution will affect important properties such as the dissociation or the solubility of differentsalts. One of the main characteristics of a solution with dissolved ions is the ionic strength.Quantifying ionic strengthThe ionic strength, I, of a solution is a function of the concentration of all ions present in thatsolution.where ci is the molar concentration of ion i (mol·dm-3), zi is the charge number of that ion, andthe sum is taken over all ions in the solution. For a 1:1 electrolyte such as sodium chloride, theionic strength is equal to the concentration, but for MgSO4 the ionic strength is four times higher.Generally multivalent ions contribute strongly to the ionic strength.Ionic strength is closely related to the concentration of electrolytes and indicates howeffectively the charge on a particular ion is shielded or stabilized by other ions (the so-calledionic atmosphere) in an electrolyte. The main difference between ionic strength and electrolyteconcentration is that the former is higher if some of the ions are more highly charged. Forinstance, a solution of fully dissociated (broken down) magnesium sulfate (Mg+2 SO4-2) has 4times higher ionic strength than a solution of sodium chloride (Na +Cl -) of the sameconcentration. Another difference between the two is that ionic strength reflects theconcentration of free ions, and not just of how much salt was added to a solution. Sometimes a
  8. 8. salt may be dissolved but the respective ions still bound together pairwise, resembling unchargedmolecules in solution. In this case the ionic strength is much lower than the salt concentration.Importance It is also important for the theory of double layer and related electrokinetic phenomena in colloids and other heterogeneous systems.. Increasing the concentration or valence of the counterions compresses the double layer and increases the electrical potential gradient. Media of high ionic strength are used in stability constant determination in order to minimize changes, during a titration, in the activity quotient of solutes at lower concentrations.Ionic strength is an important factor in biochemical reactions like metabolism to respiration.Ionic strength is a key factor in these reactions because it affects the rates at which ions reactwith each other and, thus, the extent to which the reaction occurs. Enzymes, protein moleculesthat catalyze and regulate reactions important to life, can also be extremely sensitive to ionicstrength and may become insoluble or inactive if the organisms ionic strength is too high or toolow, much in the same way that they are extremely sensitive to temperature.Take for example, the case of a person running. When he or she begins to perspire they will losemoisture as well electrolytes, or ions. This loss of electrolytes is a practical example of how ionicstrength works. If the runner does not replace those lost electrolytes, he or she will becomethirsty, sluggish, and overheated. Ionic strength is one of the basic characteristics of an organisms chemical makeup thatdetermines whether that organism can exist in a state of homeostatis, or internal stability. Inhigher animals, the kidneys regulate the bodys ionic strength by maintaining electrolyte andwater balances.Acetylcholine, a positively charged ion that organisms release at the ends of certain neurons.Acetylcholines jobs are to serve as a bridge between neurons, passing along nerve impulses fromone to the next, and to start muscular contractions. Ionic strength determines the rate at whichacetylcholine reacts with other chemicals in an organism, so if the ionic strength of theorganisms electrolytes was too high, the acetylcholine would react at a rate too slow or too fast,or may bind too strongly or too weakly to its receptor for the organism to function normally.Ionic strength is a also a useful parameter in the laboratory. If a researcher knows the ionicstrength of an electrolyte, it can tell him or her a great deal about the dynamics of specificchemical reactions.