WCLN - Enthalpy, Entropy, and Spontaneity Explained

Enthalpy, Entropy, and Spontaneity Explained http://www.BCLearningNetwork.com. 0:01this video will help you understand 0:10changes in enthalpy and entropy and how 0:12these can be used to predict whether a 0:14reaction will be spontaneous or not 0:17let's start by reviewing enthalpy change 0:19enthalpy change or delta h is the amount 0:23of heat released or absorbed in a 0:25reaction carried out at constant 0:26pressure most of the reactions done in 0:29chemistry take place at constant 0:31pressure will start by looking at an 0:33endothermic reaction in an endothermic 0:36reaction enthalpy change or Delta H is 0:39positive an example could be the 0:42reaction a plus B give see and Delta H 0:45written beside the equation with the 0:47value of positive 45 kilojoules another 0:51way of identifying and endothermic 0:52reaction is the reaction is written with 0:55the heat term on the left side an 0:58example could be a plus B plus 45 1:00kilojoules give see this tells us that 1:03for every mole of they reacted 45 1:06kilojoules of heat is absorbed from the 1:08surroundings the potential energy 1:11diagram for an endothermic reaction 1:13looks like this the energy difference 1:16between the reactants and the products 1:19is Delta H will use this example in 1:22which Delta H is positive 45 kilojoules 1:25notice when Delta H is positive the 1:28energy of the products is higher than 1:30that of the reactants now we'll look at 1:33an exothermic reaction and an exothermic 1:35reaction Delta H is negative an example 1:39could be the reaction X plus y gives 1:41that and Delta H is negative 36 1:44kilojoules another way to identify an 1:47exothermic reaction is when the heat 1:49term is written on the right side 1:51an example would be X plus y gives that 1:55plus 36 kilojoules because the thirty 1:58six kilojoules is on the right side of 2:00the equation 2:01it means that when one mole of X reacts 2:03thirty six kilojoules of heat is 2:06released to the surroundings 2:07therefore the reaction is exothermic 2:11the potential energy diagram for an 2:12exothermic reaction looks like this 2:15notice the products have lower energy 2:18than the reactants the energy difference 2:20between the reactants and products is 2:23Delta H in this example it's negative 36 2:27colleges think of a coconut in a tree if 2:31it's released there's a natural tendency 2:33for it to fall downward 2:37it falls downward in order to reach a 2:40state of minimum gravitational potential 2:42energy in a gravitational field there's 2:45a natural tendency for objects to reach 2:47a state of minimum potential energy now 2:50consider a chemical system there's a 2:52natural tendency for chemical systems to 2:55reach estate of minimum enthalpy we 2:58could also Express this by stating that 3:00there's a natural tendency for the 3:01enthalpy of a chemical system to 3:03decrease just like there's a natural 3:05tendency for objects to fall downward in 3:08a gravitational field yet another way to 3:11state the same thing would be say that 3:13equilibrium tends to favor a state of 3:16minimum Mantha p 3:17let's focus on an Ambo thermic reaction 3:20the potential energy diagram Fernando 3:23thermic reaction looks like this Delta H 3:26is positive and the products have more 3:28potential energy than the reactants the 3:31potential energy of a chemical system is 3:34closely related to its anthony PP if we 3:37change the title of the axis from 3:39potential energy to enthalpy the graph 3:41will have the same shape the reactants 3:44are lower in enthalpy than the products 3:46so we can say that the reactants have 3:49minimum methyl-p and we can generalize 3:52and state that in any endothermic 3:54reaction the reactants have minimum 3:57methyl-p also we recently learned that 4:00equilibrium tends to favor estate of 4:03minimum antha p so if just enthalpy is 4:06considered in an endothermic reaction 4:09equilibrium tends to favor the reactants 4:11now let's consider an exothermic 4:14reaction the potential energy or 4:17enthalpy diagram for an exothermic 4:19reaction looks like this notice the 4:21products have lower enthalpy than the 4:23reactants and Delta H is negative notice 4:27that in this case the products have 4:29minimum antha p so we can state that in 4:32an exothermic reaction the products have 4:35minimum antha p and since equilibrium 4:38tends to favor estate of minimum antha p 4:41when only enthalpies considered 4:44equilibrium tends to favor products as 4:47an example consider the following 4:48reaction h2 plus 4:50have to give us to hf and Delta H equals 4:53negative 537 kilojoules the question 4:57asks if the tendency toward minimum 4:59enthalpy favorite the reactants or the 5:01products because the delta h is shown 5:04and its sign is negative it means the