when a piece of paper burns it is difficult to imagine this chemical reaction running in the reverse direction to combine the Smoke and Ashes and reform the cellulous molecules of which the paper is made we are generally familiar with reactions that are spontaneous that tend to move in the forward direction from reactants to products but we know from our study of kinetics that chemical reactions involve molecular collisions at the start of a reaction there are only reactant molecules with products form from successful collisions of these reactant molecules as the reaction proceeds and more product molecules are formed product collisions can occur if these collisions are thermodynamically favorable some of the product collisions can result in the stable formation of reactants meaning the reverse reaction occurs which we show with the double-sided Arrow the frequency and effectiveness of the forward and reverse reactions are reflected in the reaction rates equilibrium implies balance a chemical system reaches equilibrium when the rate of the forward reaction balances or is equal to the rate of the reverse reaction in this video we'll define equilibrium further examine the equilibrium expression and learn to use the equilibrium constant K to understand how much a chemical reaction proceeds in the forward or reverse Direction it is important to understand that equilibrium is a dynamic condition atoms and molecules are always moving and therefore they're always colliding a great way to understand the concept of equilibrium is to examine a simple physical system in which a liquid say methanol acts in a closed container at a constant temperature the methanol molecules are in the liquid phase as methanol is added and the container is closed however as the molecules move some of them will achieve enough energy to escape the surface of the liquid and move into the gas phase above the liquid as more methanol molecules vaporize the vapor pressure increases and the number of collisions above the surface of the liquid will increase as well some of the gas molecules will collide with liquid molecules and losing energy will move back into the liquid phase as this process proceeds and if the system remains closed the rate at which the liquid methan molecules vaporize to the gas phase will equal the rate at which gaseous methanol molecules condense back into the liquid phase the vaporization and condensation process never stops but the number of molecules in the liquid and gas phases will remain constant as we continue and begin to look at chemical reactions remember that this dynamic equilibrium state does not mean that the number of reactant and product species are equal but but rather it is the rates of the forward and reverse reactions that are the same we can see the general concept of equilibrium and its characteristics by graphing changes in the concentration of reactants and products over the course of a chemical reaction in this example just as with many chemical reactions the reaction will begin with only reactants present in our system as the reaction proceeds product molecules are formed increasing in number on our GR graph while reactant molecules are consumed decreasing in number in a product favored reaction we see the number of products surpasses the number of reactants in a reactant favored reaction we see that although the number of reactants decreases there are always more reactants than products in our system at some point in either situation the forward and reverse reaction rates become equal which we see when the concentration of reactants and products becomes constant this acts as an indicator that equilibrium has been established note that as the two graphs indicate at equilibrium there could be a higher concentration of products or there could be a higher concentration of reactants but once equilibrium is established indicated by the dashed vertical line concentrations of both reactants and products remain relatively constant in order to model chemical systems in equilibrium we'll need to understand the equilibrium expression which can be derived from a balanced chemical equation for example given the chemical equation 2 N plus cl2 makes 2 nocl the expression for the equilibrium constant K takes the concentration of no^ 2 divided by the concentration of no^ 2times the concentration of cl2 in general terms the expression for the equlibrium constant can be written for any balanced chemical equation in the following way where lowercase a b c and d are the balancing coefficients and uppercase a b c and d are the formulas for the reactants and products respectively the amount or concentration of reactant and product is expressed in marity which we represent using square brackets this equilibrium expression allows us to mathematically predict the reactant or product concentrations at equilibrium or the value for a particular equilibrium constant given the equilibrium concentrations of the reactants and products any one value of an equilibrium constant for a given reaction is only valid at one particular temperature I.E the value of K is true at one constant temperature let's look at a few equilibrium expressions and their corresponding reactions these will allow us to identify some important rules first recall that the square brackets tell us the concentration of each substance and marity which is measured in moles per decim cubed next you'll notice that water is omitted in the second and third Expressions that's because water is a pure liquid concentration is something that is only measured in the context of a mixture or of a gas pure solids and liquids have fixed concentrations meaning their concentrations don't change so it doesn't make sense to describe the concentration of a substance that is pure no matter how you choose to think of this the rule is that pure solids and pure liquids are always omitted from the equilibrium expression only the aquous and gas states are included thus calcium carbonate in the third reaction is also omitted in the expression along with water the numerical value of K for a given reaction at a given temperature tells us the extent to which the reaction proceeds from reactants to products the values for the equilibrium constant K are always unitless so in a very general sense K reflects the ratio of product concentrations to reactant concentrations therefore if the value of K is larger than one this indicates there are more product shown in the numerator than reactant shown in the denominator if K is less than one the denominator must be larger than the numerator meaning there are more reactants than products if K is equal to one then there are about equal concentrations of reactants and products at equilibrium let's see extremes of this visually with particle representations of exaggerated K values when K is small meaning less than 10 -3rd we see a lot more reactants and products at equilibrium a reactant favored reaction will hardly proceed with very few products actually made when K is large shown as being greater than 10 3 we have a reaction that has nearly gone to completion or the full production of products a good example of this would be a combustion reaction where nearly all of the reactants are consumed intermediate K values are somewhere between these two extreme conditions once the equilibrium constant value is known for a particular reaction at a particular temperature the value of K can be manipulated as the original chemical equation is varied for example at 25° C the production of hydrogen chloride from hydrogen gas and chlorine gas has an equilibrium constant or k value of 2.40 * 10 33rd since K is much greater than one we say this reaction lies further to the right at equilibrium meaning there are far more products than reactants a reaction that is this product favored is often shown with the single right pointing Arrow if the reaction is reversed we can see that the equilibrium expression is flipped as a result this means the value of K for the Reversed reaction will be the reciprocal of the original value note that K is far less than one meaning that the equilibrium for the reverse reaction lies far to the left with far more reactants than products in fact there are many instances where we might manipulate K some of which are shown here for example you may see situations where you square the value of K after doubling the value of an equation's coefficients or if you're adding two reactions together the K value of the subsequent equation would be the product of the K values of the equations you were combining in any case the relationship between the reaction equation and the equilibrium constant expression will always provide the path for determining the effect on the value of K itself in summary equilibrium is a dynamic state in which the rates of the forward and reverse reactions are equal we can see equilibrium using graphs that measure changes in concentration over time where at equilibrium the concentration of reactants and products remains relatively constant we measure equilibrium using the equilibrium expression in the equilibrium constant K these allow us to mathematically predict the concentrations of reactants and products at equilibrium as knowing the values of the equilibrium constant k at a particular temperature informs us about the extent of a chemical reaction we could manipulate the equilibrium constant to find its value for alternative equations or change its value by altering the temperature of a reaction system system equilibrium is more than just a concept in chemistry it mirrors the intricate balance that we find in our everyday lives just as in chemical equilibrium with a forward and reverse reactions balance life's equilibrium finds steadiness and adjusting to stress and change