Irreversible reactions Most chemical reactions are considered irreversible
Irreversible reactions Most chemical reactions are considered irreversible – the products that are made cannot readily be changed back into their reactants. For example, when wood burns it is impossible to turn it back into unburnt wood again! Similarly, when magnesium reacts with hydrochloric acid to form magnesium chloride and hydrogen, it is not easy to reverse the reaction and obtain the magnesium.
CONCENTRATION CHANGE IN A REACTION As the rate of reaction is dependant on the concentration of reactants. . . the forward reaction starts off fast but slows as the reactants get less concentrated FASTEST AT THE START THE STEEPER THE GRADIENT, THE FASTER THE REACTION SLOWS DOWN AS REACTANTS ARE USED UP CONVERSION TO PRODUCTS
What are reversible reactions? Reversible reactions are reactions that can proceed in both directions at the same time A + (reactants) B C + D (products) For example, during a reversible reaction reactants A and B react to make products C and D. However, products C and D can also undergo the reverse reaction, and react together to form reactants A and B.
Reversible and irreversible reactions What kind of reactions are reversible and irreversible?
Reversible biochemical reactions Many biochemical reactions (those that take place inside organisms) are reversible. For example, in the lungs, oxygen binds to haemoglobin (Hb) in red blood cells to create oxyhaemoglobin. When the red blood cells are transported to tissues, the oxyhaemoglobin dissociates back to haemoglobin and oxygen. Hb + 4 O 2 Hb. 4 O 2 There also some very important industrial reactions, like the Haber process, that are reversible.
Heating copper sulfate
Heating ammonium chloride An ammonium salt can be made by reacting ammonia with an acid. Some of the salt will decompose back into the reactants when heated. ammonia + hydrogen chloride NH 3 (g) + HCl (g) NH 4 Cl decomposes back into NH 3 and HCl gases when heated ammonium chloride NH 4 Cl (s) NH 4 Cl reforms in the cooler part of the test tube
EQUILIBRIUM REACTIONS Initially, there is no backward reaction but, as products form, it speeds up and provided the temperature remains constant there will come a time when the backward and forward reactions are equal and opposite; the reaction has reached equilibrium. FASTEST AT THE START NO BACKWARD REACTION FORWARD REACTION SLOWS DOWN AS REACTANTS ARE USED UP In an equilibrium reaction, not all the reactants end up as products; there is not a 100% conversion. BUT IT DOESN’T MEAN THE REACTION IS STUCK IN THE MIDDLE BACKWARD REACTION RATE STARTS TO INCREASE AT EQUILIBRIUM THE Rate of BACKWARD Reaction equals rate of FORWARD Reaction
Reversible Reactions proceed in both directions at the same time Indicates Reversible Reaction
DYNAMIC EQUILIBRIUM • The equilibrium achieved is dynamic and is called a dynamic equilibrium The steady state dynamism could be compared with the state of water within a reservoir. In a reservoir when the water is filled simultaneously and discharged as well and if the rate of water inflow is found to be equal to the rate of the water outflow, the reservoir water quantity will remain unaffected. Rate of water Inflow = Rate of Water Outflow (The level of water is constant) This is exactly similar to what we observe in the quantities of reactants and products obtained when they are in a chemical equilibrium state.
DYNAMIC EQUILIBRIUM • a reversible chemical reaction is a dynamic process • everything may appear stationary but the reactions are proceeding in both directions • the position of equilibrium can be varied by changing certain conditions Trying to get up a “down” escalator gives an excellent idea of a nonchemical situation involving dynamic equilibrium. Summary When a chemical equilibrium is established. . . • Both reactants and products are present at all times • the reaction is dynamic – the forward reaction and background reactions are still occurring but the concentrations of reactants and products remain constant
THE EQUILIBRIUM LAW Simply states “If the concentrations of all the substances present at equilibrium are raised to the power of the number of moles that appear in the equation, the product of the concentrations of the products divided by the product of the concentrations of the reactants is a constant, provided the temperature remains constant”
THE EQUILIBRIUM CONSTANT Kc
THE EQUILIBRIUM CONSTANT Kc for an equilibrium reaction of the form. . . a. A + b. B then (at constant temperature) c. C + d. D [C]c. [D]d = a constant, (Kc) [A]a. [B]b VALUE OF Kc AFFECTED by NOT AFFECTED by a change of temperature a change in concentration of reactants or products a change of pressure adding a catalyst
Reversible or irreversible?
True or false?
LE CHATELIER’S PRINCIPLE “When a change is applied to a system in dynamic equilibrium, the system reacts in such a way as to oppose the effect of the change. ”
Le Chatelier’s Principle and THE POSITION OF EQUILIBRIUM CONCENTRATION The equilibrium constant is not affected by a change in concentration at constant temperature. To maintain the constant, the composition of the equilibrium mixture changes. If you increase the concentration of a substance, the value of Kc will theoretically be affected. As it must remain constant at a particular temperature, the concentrations of the other species change to keep the constant the same.
Le Chatelier’s Principle and THE POSITION OF EQUILIBRIUM
Le Chatelier’s Principle and THE POSITION OF EQUILIBRIUM CONCENTRATION example Decreasing [H 2 O] - will make the bottom value smaller so would Kc increase - some CH 3 CH 2 OH reacts with CH 3 COOH to replace the H 2 O - more CH 3 COOC 2 H 5 is also produced - this reduces the value of the bottom line and increases the top - eventually the value of the constant will be restored
Le Chatelier’s Principle and THE POSITION OF EQUILIBRIUM SUMMARY REACTANTS PRODUCTS THE EFFECT OF CHANGING THE CONCENTRATION ON THE POSITION OF EQUILIBRIUM INCREASE CONCENTRATION OF A REACTANT EQUILIBRIUM MOVES TO THE RIGHT DECREASE CONCENTRATION OF A REACTANT EQUILIBRIUM MOVES TO THE LEFT INCREASE CONCENTRATION OF A PRODUCT EQUILIBRIUM MOVES TO THE LEFT DECREASE CONCENTRATION OF A PRODUCT EQUILIBRIUM MOVES TO THE RIGHT Predict the effect of increasing the concentration of O 2 on the equilibrium position EQUILIBRIUM MOVES TO RHS Predict the effect of decreasing the concentration of SO 3 on the equilibrium position EQUILIBRIUM MOVES TO RHS
Concentration and equilibrium Changing the concentration of a substance affects the equilibrium of reversible reactions involving solutions. increasing the concentration of substance A decreasing the concentration of substance A = = equilibrium shifts to decrease the amount of substance A equilibrium shifts to increase the amount of substance A
Opposing changes in concentration (1) Bismuth chloride reacts with water to produce a white precipitate of bismuth oxychloride and hydrochloric acid. bismuth chloride + Bi. Cl 3 (aq) + water bismuth oxychloride + hydrochloric acid H 2 O (l) Bi. OCl (s) + 2 HCl (aq) What will happen if more H 2 O is added? l The equilibrium will shift to decrease the amount of water, i. e. to the right. l More Bi. OCl and HCl will be produced. If H 2 O is removed, more Bi. Cl 3 and H 2 O will be produced.
Opposing changes in concentration (2) Chlorine gas reacts with iodine chloride to produce iodine trichloride. chlorine + Cl 2 (g) + pale green iodine chloride ICl (l) brown iodine trichloride ICl 3 (s) yellow What effect will adding more Cl 2 have on the colour of the mixture? It will become more yellow. What effect will removing Cl 2 have on the colour of the mixture? It will become more brown.
Le Chatelier’s Principle and THE POSITION OF EQUILIBRIUM PRESSURE When studying the effect of a change in pressure, we consider the number of gaseous molecules only. The more particles you have in a given volume, the greater the pressure they exert. If you apply a greater pressure they will become more crowded (i. e. they are under a greater stress). However, if the system can change, as in a reversible reaction, it will move to the side with fewer gaseous molecules - it is less crowded. No change occurs when equal numbers of gaseous molecules appear on both sides. THE EFFECT OF PRESSURE ON THE POSITION OF EQUILIBRIUM INCREASE PRESSURE MOVES TO THE SIDE WITH FEWER GASEOUS MOLECULES DECREASE PRESSURE MOVES TO THE SIDE WITH MORE GASEOUS MOLECULES
Le Chatelier’s Principle and THE POSITION OF EQUILIBRIUM THE EFFECT OF PRESSURE ON THE POSITION OF EQUILIBRIUM INCREASE PRESSURE MOVES TO THE SIDE WITH FEWER GASEOUS MOLECULES DECREASE PRESSURE MOVES TO THE SIDE WITH MORE GASEOUS MOLECULES For a reversible reaction there is no change in the position of equilibrium if pressure is increased or decreased when there are equal numbers of gaseous molecules on both sides of the reaction. Predict the effect of an increase of pressure on the equilibrium position of. . MOVES TO RHS : - fewer gaseous molecules NO CHANGE: - equal numbers on both sides
Pressure and Chemical Equilibrium Changing the pressure only has an effect on the equilibrium of reversible reactions when the number of moles of gaseous reactants differ from the number of moles of gaseous products. If the pressure is increased: l equilibrium shifts to decrease the pressure l equilibrium shifts in the direction of fewest molecules If the pressure is decreased: l equilibrium shifts to increase the pressure l equilibrium shifts in the direction of most molecules
Opposing changes in pressure Nitrogen dioxide is in constant equilibrium with dinitrogen tetroxide. Two molecules of nitrogen dioxide react to form one molecule of dinitrogen tetroxide. nitrogen dioxide dinitrogen tetroxide 2 NO 2 (g) N 2 O 4 (g) What will happen if the pressure is increased? l The equilibrium will shift to reduce the number of molecules, i. e. to the right (only 1 molecule). l More N 2 O 4 will be produced. If the pressure is decreased, more NO 2 will be produced.
Le Chatelier’s Principle and THE POSITION OF EQUILIBRIUM TEMPERATURE Temperature is the only factor that can change the value of the equilibrium constan Altering the temperature affects the rate of both backward and forward reactions Temperature alters the rates to different extents The equilibrium thus moves producing a new equilibrium constant. The direction of movement depends on the sign of the enthalpy change.
Le Chatelier’s Principle and THE POSITION OF EQUILIBRIUM TEMPERATURE Temperature is the only factor that can change the value of the equilibrium constan Altering the temperature affects the rate of both backward and forward reactions Temperature alters the rates to different extents The equilibrium thus moves producing a new equilibrium constant. The direction of movement depends on the sign of the enthalpy change. REACTION TYPE DH INCREASE TEMP DECREASE TEMP EXOTHERMIC - TO THE LEFT TO THE RIGHT ENDOTHERMIC + TO THE RIGHT TO THE LEFT
Le Chatelier’s Principle and THE POSITION OF EQUILIBRIUM REACTION TYPE DH INCREASE TEMP DECREASE TEMP EXOTHERMIC - TO THE LEFT TO THE RIGHT ENDOTHERMIC + TO THE RIGHT TO THE LEFT Predict the effect of a temperature increase on the equilibrium position of. . . DH = + ive Equilibrium position moves to the RHS DH = - ive Equilibrium position moves to the LHS
Exothermic and endothermic reactions All reactions are exothermic (give out heat) in one direction and endothermic (take in heat) in the other. If the temperature is increased: l equilibrium shifts to decrease the temperature l equilibrium shifts in the endothermic direction If the temperature is decreased: l equilibrium shifts to increase the temperature l equilibrium shifts in the exothermic direction
Opposing changes in temperature at Equilibrium Nitrogen dioxide is in constant equilibrium with dinitrogen tetroxide. The forward reaction is exothermic and the backwards reaction is endothermic. nitrogen dioxide dinitrogen tetroxide 2 NO 2 (g) N 2 O 4 (g) What will happen if the temperature is increased? l The equilibrium will shift to decrease the temperature, i. e. to the left (endothermic). l More NO 2 will be produced. If the temperature is decreased, more N 2 O 4 will be produced.
Le Chatelier’s Principle and THE POSITION OF EQUILIBRIUM CATALYSTS An increase in temperature can be used to speed up chemical reactions but it can have an undesired effect on the yield of the product when the reaction is reversible and exothermic. In this case the equilibrium position is reached quicker but with a reduced yield of N 2 O 4 because the increased temperature moves the equilibrium to the left. In many industrial processes a compromise temperature is used (Haber and Contact Processes). To reduce the problem one must look for a way of increasing the rate of a reaction without decreasing the yield i. e. with a catalyst.
Le Chatelier’s Principle and THE POSITION OF EQUILIBRIUM CATALYSTS An increase in temperature can be used to speed up chemical reactions but it can have an undesired effect on the yield of product when the reaction is reversible and exothermic. In this case you get to the equilibrium position quicker but with a reduced yield because the increased temperature moves the equilibrium to the left and decreases the yield of the products. Adding a catalyst DOES NOT AFFECT THE POSITION OF EQUILIBRIUM. However, it does increase the rate of attainment of equilibrium. This is especially important in reversible, exothermic industrial reactions such as the Haber or Contact Processes where economic factors are paramount.
Le Chatelier’s Principle Whenever a change is made to a reversible reaction in dynamic equilibrium, the equilibrium will shift to try and oppose the change. Condition Effect Temperature Increasing the temperature shifts the equilibrium in the direction that takes in heat. Concentration Increasing the concentration of a substance shifts the equilibrium in the direction that produces less of that substance. Pressure Increasing the pressure shifts the equilibrium in the direction that produces less gas.
Dynamic equilibrium and change
Le Chatelier’s Principle Investigate Le Chatelier’s Principle at the following website http: //www. learnerstv. com/animation. php? ani=120&ca t=chemistry
Haber Process
Changing the yield of ammonia
HABER PROCESS N 2(g) + 3 H 2(g) 2 NH 3(g) : DH = - 92 k. J mol-1 Equilibrium theory favours low temperature as there is a higher yield at lower temperature high pressure as it favours a decrease in number of gaseous molecules
HABER PROCESS N 2(g) + 3 H 2(g) 2 NH 3(g) : DH = - 92 k. J mol-1 Equilibrium theory favours Low Temperature exothermic reaction - higher yield at lower temperature High Pressure Higher yield as it favours a decrease in number of gaseous molecules Whereas Kinetic theory favours high temperature high pressure catalyst greater average energy + more frequent collisions for gaseous molecules lower activation energy
HABER PROCESS Compromise conditions Which is better? A low yield in a shorter time or a high yield over a longer period. The conditions used are a compromise with the catalyst enabling the rate to be increased, even at a lower temperature.
HABER PROCESS
Temperature, pressure and yield
Manufacture of Sulfuric acid H 2 SO 4 Contact Process Stage
Contact Process Stage
Contact Process
Contact Process SUMMARY REACTANTS PRODUCTS Predict the effect of increasing the concentration of O 2 on the equilibrium position EQUILIBRIUM MOVES TO RHS Predict the effect of decreasing the concentration of SO 3 on the equilibrium position EQUILIBRIUM MOVES TO LHS
Contact Process SUMMARY REACTANTS PRODUCTS Predict the effect of a temperature increase on the equilibrium position of. . . DH = - ive Equilibrium position moves to the LHS Predict the effect of a temperature decrease on the equilibrium position of. . DH = - ive Equilibrium position moves to the RHS
Contact Process SUMMARY REACTANTS PRODUCTS Predict the effect of an increase of pressure on the equilibrium position of. . MOVES TO RHS : - Less gaseous molecules Predict the effect of a decrease of pressure on the equilibrium position of. . MOVES TO LHS : - More gaseous molecules
CONTACT PROCESS Compromise conditions Which is better? A low yield in a shorter time or a high yield over a longer period. The conditions used are a compromise with the catalyst enabling the rate to be increased, even at a lower temperature. Room Pressure is used as it is economically the best compromise
CONTACT PROCESS : DH = Actual Conditions used in Contact Process Pressure 1 atm Temperature 450°C Catalyst Vanadium(V) Oxide
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