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Chemical Equilibrium

Chemical Equilibrium
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Dr.SamuelHunt,United Arab Emirates,Teacher
Published Date:21-07-2017
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Chemical Equilibrium www.ThesisScientist.comEquilibrium vs. Disequilibrium • When systems are at equilibrium with their surroundings, their conditions are the same as the surroundings and they stay that way. • When systems are in disequilibrium with their surroundings, their conditions are not the same as the surroundings. • Systems that are in disequilibrium tend to change until they reach equilibrium with their surroundings. • Living things are in controlled disequilibrium with their environment—they are not at the same conditions as the environment and do not tend to change toward those conditions. www.ThesisScientist.comReaction Rates • Some chemical reactions proceed rapidly.  Like the precipitation reactions studied in Chapter 7 where the products form practically the instant the two solutions are mixed. • Other reactions proceed slowly.  Like the decomposition of dye molecules of a sofa placed in front of a window. • The rate of a reaction is measured in the amount of reactant that changes into product in a given period of time.  Generally moles of reactant used per second. Like miles per hour. • Chemists study ways of controlling reaction rates. www.ThesisScientist.comCollision Theory • In order for a reaction to take place, the reacting molecules must collide with each other. • Once molecules collide they may react together or they may not, depending on two factors: 1. Whether the collision has enough energy to ―start to break the bonds holding reactant molecules together." 2. Whether the reacting molecules collide in the proper orientation for new bonds to form. www.ThesisScientist.comEffective Collisions • Collisions in which these two conditions are met (and therefore the reaction occurs) are called effective collisions. • The higher the frequency of effective collisions, the faster the reaction rate. • There is a minimum energy needed for a collision to be effective. We call this the activation energy. The lower the activation energy, the faster the reaction will be. www.ThesisScientist.comEffective Collisions: Kinetic Energy Factor For a collision to lead to overcoming the energy barrier, the reacting molecules must have sufficient kinetic energy so that when they collide, it can form the activated complex. www.ThesisScientist.comEffective Collisions: Orientation Effect www.ThesisScientist.comReaction Energy Diagram www.ThesisScientist.comFactors Effecting Reaction Rate: Reactant Concentration • The higher the concentration of reactant molecules, this increases the frequency of reactant molecule collisions the faster the reaction will generally go. • Since reactants are consumed as the reaction proceeds, the speed of a reaction generally slows over time. www.ThesisScientist.comEffect of Concentration on Rate Low concentrations of reactant molecules leads to fewer effective collisions, therefore a slower reaction rate. High concentrations of reactant molecules lead to more effective collisions, therefore a faster reaction rate. www.ThesisScientist.comFactors Effecting Reaction Rate: Temperature • Increasing the temperature increases the energy so that their collisions can overcome the activation energy. • And, increasing the temperature also increases the frequency of collisions. • Both these mean that increasing temperature increases the reaction rate. www.ThesisScientist.comEffect of Temperature on Rate Low temperatures lead to fewer molecules with enough energy to overcome the activation energy, and less frequent reactant collisions, therefore a slower reaction rate High temperatures lead to more molecules with enough energy to overcome the activation energy, and more frequent reactant collisions, therefore, a faster reaction rate.Chemical Equilibrium • When a reaction reaches equilibrium, the production of both reactants and products is constant. • The forward and reverse reactions still continue. • Because they go at the same rate, the amounts of materials does not appear to change. www.ThesisScientist.comEquilibrium As the reaction proceeds, the forward reaction Initially, we only have reactant molecules in the slows down as the reactants get used up. At the mixture. The reaction can only proceed in the same time, the reverse reaction speeds up as forward direction, making products. product concentration increases. Once equilibrium is established, the concentrations of the reactants and products in Eventually, the forward and reverse rates are equal. the final mixture do not change, (unless At this time equilibrium is established. conditions are changed). Equilibrium  Equal • The rates of the forward and reverse reactions are equal at equilibrium. • But that does not mean the concentrations of reactants and products are equal. www.ThesisScientist.comEquilibrium, Continued Initially, only the forward B As Onc ecthe ae use e fquil or the wa ibrium rre d ar ce tan a c is ti t co e on pr stnc abli eoc ntra shed, eeti ds on Eventually, the reaction proceeds reaction takes place. de it th make c ere for ase wa s produc s, rd the f and re or ts wa aver nd use rd se rere as re ca ticon ti ac ons tants. slows. in the reverse direction as fast as As proc the ee pr d a oduc t the ts same accumulate rate, so t , the he it proceeds in the forward direction. re conce verse ntra reati cons tion spe of all eds mater up. ials At this time equilibrium is established. stay constant. Rate forward Rate reverse Time Rate• What occurs in a reaction at equilibrium. • When the number of people moving up is the same as the number of people moving down, the number of people on each floor remains constant, and the two populations are in equilibrium. • Equilibrium occurs when the forward and reverse reactions are the same. mAn Analogy: Population Changes When Narnians feel However, as time passes, overcrowded, some will emigration will occur in both emigrate to Middle Earth. directions at the same rate, leading to populations in Narnia and Middle Earth that are constant, though not necessarily equal. www.ThesisScientist.comEquilibrium Constant • Even though the concentrations of reactants and products are not equal at equilibrium, there is a relationship between them. • For the reaction H (g) + I (g)  2HI(g) at 2 2 equilibrium, the ratio of the concentrations raised to the power of their coefficients is constant. 2 HI K eq HI 2 2 www.ThesisScientist.comEquilibrium Constant, Continued • For the general equation aA + bB  cC + dD, the relationship is given below: The lowercase letters represent the coefficients of the balanced chemical equation. Always products over reactants. • The constant is called the equilibrium constant, K . eq c d CD K eq a b AB