have you ever noticed that some reactions and processes give off heat for example you might neutralize an acid with a base and notice your reaction vessel gets quite warm to the touch or a more obvious example is burning hydrocarbons like methane which gives off so much heat it can be used to heat a home on the other hand some reactions and processes require energy to occur an example of this is photo synthesis which uses the energy from sunlight to produce glucose we call reactions and processes that give off energy to the surroundings in the form of heat exothermic and those that absorb energy from the surroundings in the form of heat endothermic so why is it useful to know this well first of all if a reaction is very exothermic it gives off a lot of energy and that's useful to know because we can use that energy as we just mentioned very exothermic reactions like burning hydrocarbons can be used to heat our homes when it's cold power our vehicles or generate electricity it's also really helpful to know which reactions are endothermic as those reactions might not happen unless we add some energy now we can quantify the change in energy when heat is given off or absorbed in in terms of a quantity called enthalpy which is given the letter H like all forms of energy we cannot directly measure the energy a system has so instead we'll always discuss changes in enthalpy which is indicated by Delta H the Delta here means change in so how does the enthalpy change during an endothermic process and an exothermic process while well as said before an exothermic process is one that gives off energy in the form of heat this means that the energy of the products will be lower than the energy of the reactants since heat is lost the change in enthalpy is equal to the final enthalpy minus the initial enthalpy or in this case the product enthalpy minus the reactant enthalpy since that product enthalpy is lower the enthalpy change for an exothermic reaction is always negative endothermic reactions absorb energy in the form of heat so the enthalpy of the products is higher than the enthalpy of the reactants and this means that the change in enthalpy will be positive well this is all good to know but why does this happen why do some reactions give off energy and not others well all chemical reactions involve the making and breaking of bonds Bonds in the reactant molecules must be broken which requires energy in other words this is an endothermic process we call this quantity of energy that is needed to break a bond the bond energy then the new Bonds in the product molecules have to be formed which releases energy making it an exothermic process this energy that's released will be opposite the bond energy so the total amount of energy that's absorbed or released during a reaction is the combination of these two factors which we can express using those Bond energies represented in an equation this means that the enthalpy change of a reaction is the sum of the enthalpy changes for all the reactant bonds broken minus the sum of the enthalpy changes of all the product bonds formed the this is the same as the sum of the bond energies of the reactants minus the sum of the bond energies of the products if the amount of energy required to break the bonds in the reactants is smaller than the amount of energy released when the product bonds are formed the change in enthalpy will be negative and the reaction will be exothermic that extra energy from the bonds being formed will be released to the surroundings in the form of heat warming the surroundings and causing the temperature to increase however if the amount of energy required to break the bonds in the reactants is greater than the amount of energy released when the product bonds are formed the change in enthalpy will be positive and the reaction will be endothermic that energy difference will be absorbed from the surroundings causing the temperature to decrease it's worth mentioning here that while certain processes like changes in state don't involve the making or breaking of chemical bonds they do involve making and breaking intermolecular forces instead so the same ideas still apply here now here's another question why are some species lower in energy than others that is why are these reactant molecules higher in energy than these product molecules in this exothermic reaction well when a state is lower in energy it's more stable energetically we released energy to get from that higher energy state so now we're in a lower energy more stable State now as we know chemical species generally want to become more stable this leads us to an interesting point it deviates a bit from our discussion of enthalpy but it's an important one to make say we have a hydrocarbon like methane that's out in the open so it's exposed to some oxygen we know that the combustion of methane which forms carbon dioxide and water is very exothermic so the energy of methane and oxygen is higher than the energy of carbon dioxide and water in other words carbon dioxide and water are much more stable as we said chemical species want to become more stable so why doesn't methane spontaneously combust in oxygen we all know that it doesn't we actually need to provide some energy to get this reaction going why is that well as we said earlier we need to break Bonds in the reactant molecules to get the reaction going and this requires some energy the energy that is required to start the reaction is called the activation energy essentially those reactant molecules need to hit each other with enough energy for the reaction to begin in the case of the combustion of methane our reactants just don't have enough energy at room temperature to overcome the activation energy and break those Bonds in the reactants so even though a reaction is exothermic it may have a high activation energy which means the reaction won't happen unless we put in some energy and the change in enthalpy just doesn't tell us about that now we've gone through quite a bit of theory so let's talk about how we can put this to practical use how do we go about measuring all of this well as we know by now these reactions involve changes in heat which will change the temperature of the surroundings there's a relationship between a change in temperature and and the amount of heat that is released or absorbed and this is qal MC delta T here Q is the heat that is released or absorbed m is the mass C is the specific heat capacity which is unique to each substance and delta T is the change in temperature we can accurately measure the temperature change during a reaction or process which can then be used to calculate the heat by performing something called a calorimetry experiment in a calorimetry experiment we measure the temperature of a reaction or process by performing the reaction in an insulated container so that the heat doesn't escape the reaction vessel say we want to know the change in enthalpy for an exothermic aquous reaction we could perform that reaction in our insulated container and then measure the temperature change during the course of the reaction the reactants will release heat which will be absorbed by the water the reactants are dissolved in this heat exchange will cause the temperature of the water to increase which will measure now that we know the change in temperature during the reaction we can use our equation to calculate the heat the water absorbed using the mass of water in the solution and the specific heat capacity of water which happens to be 4.18 J per G per Kelvin of course we didn't really want the heat the water absorbed we wanted the heat the reaction gave off but these two quantities are equal in magnitude just opposite in sign that's because we're performing this experiment in an insulated container so whatever heat the reaction gives off will be absorbed by the water and if we know the heat the reaction gave off we we can calculate the molar change in enthalpy as the change in enthalpy is equal to the heat divided by the amount in moles of one of our reactants or products let's end with an example applying the equation Q = MC delta T the question is as follows in a calorimeter a reaction in a solution of 100 G of water raises the temperature from 20 to 30° C what is the heat given off in Jewels Q is the heat given off this is of course what we're solving for and we want this in the units of jewels m is the mass and we're given this in the question as it says that the reaction happens in a solution of 100 G the solution is made up of water as the solvent so we know to use the specific heat capacity for water which is 4.18 Je per G per Kelvin finally delta T is the temperature change which is the difference between 20 and 30° 10° importantly a difference in 10° Celsius is the same as a difference in 10° Kelvin since the absolute Celsius and Kelvin scales are the same just shifted by different amounts if we plug those values into our equation we get 100 * 4.18 * 10 notice how the units of G and Kelvin cancel out from these terms so we're left with just Jewels which is the unit we want the final answer is 4,180 jewles so now we've covered quite a lot of material in the topic of energetics we now know about EXO and endothermic reactions about enthalpy changes and how to measure and calculate them of course there's more to learn in this topic but that's the subject of another video