PREVNEXTSUBMITMenuResourcesGlossaryNotesClear and return to MenuSearch ResultsFilterSlide TextNotesSearch in:Check to includeSearch...TermsDefinitiondefinitionresourcessearchpauseplayreplaysubmitnextpreviousvolumein the last learning activity we learned that over time, reversible chemical reactions tend to reach a point of dynamic equilibrium in which the rates of the forward and reverse reactions are the same. continue in general, a system wants to be at the equilibrium point, and if that equilibrium is disturbed, the system will react in such a way so that it can regain a state of equilibrium. continue the way a system reacts when its equilibrium is disturbed can be explained by a concept that is known as le chatelier's principle which we will now explore in this learning activity. next slide le chatelier's principle le chatelier's principle states that if a stress is applied to a system at equilibrium, the equilibrium will shift in such a way to oppose the stress. when we say stress in this case we mean a change in the conditions of the system, such as a change in pressure, temperature, concentration etc. one way to imagine le chatelier's principle is to imagine you are walking up an escaltor that is moving down. if you are walking up at the same rate that the escalator is moving down, you will remain in one place. in other words, you are at a state of equilibrium. (rate walking up = rate of escalator down) continue equilibrium point if, however the escalator starts moving down at a faster rate (i.e. the system is stressed), you will start to move down the escalator. (rate up < rate down) continue to counteract the stress, you must increase the speed that you are walking up. once you have increased your speed to the same speed of the escalator, you will once again remain in place, having established a new equilibrium at a different location. (rate up = rate down) new equilibrium point if, however the escalator starts moving down at a faster rate (i.e. the system is stressed), you will start to move down the escalator. (rate up < rate down) next slide le chatelier's principle: change in concentration for a system at equilibrium, when the concentration of the products or reactants is increased, the equilibrium will shift to reduce that concentration increase. when we say that the equilibrium shifts, we mean that the reaction will proceed in one direction until a new equilibrium is established. continue for example, in the following reaction that is in a state of equilibrium: 3h2(g) + n2(g) <--> 2nh3(g) continue if a stress is introduced to the system, in the form of an increase in concentration of n2, how do you think the reaction will respond to reestablish equilibrium? continue 3h2(g) + n2(g) <--> 2nh3(g) when the concentration of one of the reactants (n2) is increased, le chatelier's principle tells us that the system will respond to reduce that stress. in this case that means reducing the concentration of n2. the reaction will shift to the right, producing more nh3 and reducing the concentration of n2. the reaction will proceed in this way until a new equilibrium is established. continue 3h2(g) + n2(g) <--> 2nh3(g) when the concentration of one of the reactants (n2) is increased, le chatelier's principle tells us that the system will respond to reduce that stress. in this case that means reducing the concentration of n2. the reaction will shift to the right, producing more nh3 and reducing the concentration of n2. the reaction will proceed in this way until a new equilibrium is established. next slide le chatelier's principle: concentration let's take a closer look at the diagram to dissect what happens. continue 2. the system is stressed by the addition of more n2 which causes its concentration to increase. 2. continue let's take a closer look at the diagram to dissect what happens. 1. the original reaction proceeds until it reaches equilibrium 2. the system is stressed by the addition of more n2 which causes its concentration to increase 3. the system adjusts to the stress to reduce the concentration of n2 by shifting the reaction towards the right. 3. continue let's take a closer look at the diagram to dissect what happens. 1. the original reaction proceeds until it reaches equilibrium 2. the system is stressed by the addition of more n2 which causes its concentration to increase 3. the system adjusts to the stress to reduce the concentration of n2 by shifting the reaction towards the right. 4. the reaction proceeds until a new equilibrium is reached 4. next slide let's take a closer look at the diagram to dissect what happens. 1. the original reaction proceeds until it reaches equilibrium 1. let's take a closer look at the diagram to dissect what happens. 1. the original reaction proceeds until it reaches equilibrium continue le chatelier's principle: pressure now we will examine how le chatelier's principle would be applied to a change in pressure. the pressure of a system is related to the volume of the system as well as the number of molecules within the system. click the buttons below to see how the number of molecules and volume of a system affects pressure normal pressure higher pressure lower pressure smaller volume, same number of molecules same volume larger number of molecules higher pressure same number of molecules, larger volume same volume, fewer molecules lower pressure le chatelier's principle: pressure now, lets see how our reaction from earlier will respond to a stress to the system (an increase in pressure). recall our equation: 3h2(g) + n2(g) <--> 2nh3(g) notice that for every 4 moles of reactant molecules (3h2 + 1n2) we have 2 moles of product produced. so, lets consider our system after equilibrium has been reached (note for these examples, each molecule represents 1 mole): continue 3h2(g) + n2(g) <--> 2nh3(g) 18 moles total continue the volume is decreased, causing an increase in pressure. how do you think the system will respond? continue 18 moles total 18 moles total the reaction shifts to the right, producing more nh3 molecules, this reduces the pressure by lowering the total amount of molecules in the system. 3h2(g) + n2(g) --> 2nh3(g) 14 moles total le chatelier's principle: temperature what happens to a system at equilibrium if heat is added or removed from the system? to predict how changes in temperature affect a reaction we need to consider the change in enthalpy. in an exothermic reaction energy is released and is considered one of the products; whereas in an endothermic reaction, energy is considered one of the reactants. if we consider our same sample reaction as a thermochemical equation: 3h2(g) + n2(g) --> 2nh3(g) continue 3h2(g) + n2(g) --> 2nh3(g) δh = -92.58 or 3h2(g) + n2(g) --> 2nh3(g) + energy we can see it is an exothermic reaction. if the temperature of this system is raised, this is the same as adding more energy to it. continue so what do you think will happen if heat is added to the system? le chatelier's principle tells us the system will react to counteract the stress. the reaction will shift to the left if heat is added because adding heat is the same as adding more product. therefore the reverse reaction will act to reduce the increase in product. conversely, if you were to remove energy by cooling the system, the reaction would shift to the right to try to replace the heat that was removed. 3h2(g) + n2(g) <-- 2nh3(g) next slide so what do you think will happen if heat is added to the system? le chatelier's principle tells us the system will react to counteract the stress. lets summarize what we just learned in this lesson. when a system at equilibrium is stressed in some way, the system acts to regain a state of equilibirum. this is known as le chatelier's principle. continue for example, if the concentration of a reactant is all of a sudden increased, the system will act to reduce the concentration of that reactant by shifting to the right, or forward direction. over time, the concentration of that reactant will be lowered and a new equilibrium point will be reached. continue if the pressure of a system is increased, the system will adjust by shifting the reaction in the direction that lowers the overall number of particles within until a new equilibrium is reached. continue if the temperature of a system is increased, energy is being added to the system therefore, the system will shift in the direction consumes energy, whereas if the temperature is decreased, the system will shift in the direction that produces energy. click here to return to the beginning of this lesson