[Music] today we're going to be discussing enthalpy changes the property that allows modern air conditioning or Refrigeration to be possible enthalpy H is a way to talk about the energy in a system especially when things are heating up or cooling down think of it as the total heat content of something when a chemical reaction or physical process happens the change in enthalpy Delta H tells if the reaction absorbs heat endothermic or releases heat exothermic it's like checking whether a process makes things hotter or cooler I find it easiest to remember it being H by emphasizing the th in enthalpy technically enthalpy is a measure of the total energy of a thermodynamic System including its internal energy plus the product of its pressure and volume it's defined by the equation H = e+ PV where H is enthalpy e is the internal energy of the system p is the pressure and V is the volume of the system when a process occurs at constant pressure such as in the room you're sitting in the change in enthalpy Delta H is equal to the heat exchanged between the system and its surroundings this is why enthalpy is often Associated as heat when we measure enthalpy changes we are looking at the difference in enthalpy between reactants and products during a chemical reaction or physical change heat q and temperature T are related but different concepts heat is a form of energy transfer between a system and its surroundings due to a temperature difference and is measured in Jewels J day while temperature measures the average kinetic energy of the particles in the substance in the air conditioning example the air conditioner absorbs heat from the inside of your room making the air cooler temperature change as a result this heat is then transferred to the refrigerant a special fluid that changes from a liquid to a gas as it absorbs heat the gaseous refrigerant carries the heat outside where it releases the heat into the outdoor air once this is complete it's condensed back into a liquid and the cycle repeats continually removing heat from your room and keeping it cool in air conditioning the change in enthalpy Delta H represents the heat energy involved in the process of moving heat from inside a space to the outside while temperature is an indicator of how hot or cold of substance is and is measured in degrees Fahrenheit F Celsius C or kelvin K since the temperature of a substance is directly proportional to the average kinetic energy of its particles as the kinetic energy of the particles increases so does the temperature for example when you heat water on a stove you are adding heat energy to it this added energy increases the kinetic energy of the water MO ules which in turn raises the temperature of the water in chemical reactions and physical changes energy is either absorbed from or released to the surroundings and are classified as either exothermic or endothermic this energy transfer can be observed as a change in temperature exothermic processes release energy to the surroundings usually in the form of heat or light the heat causes the temperature of the surroundings to increase and the light can illuminate the surroundings a great chemical reaction example of this that you can see and feel is the use of space heaters often seen at outdoor events or restaurants these space heaters use the combustion of propane C3 h8 to release heat and light flame to the surroundings the reaction is c3h8 + 502 going to 3 CO2 plus 4 H2O plus heat and light when you are near a propane flame heat is transferred directly to your skin the molecules in your skin gain kinetic energy from the heat which increases the temperature of your skin endothermic processes absorb energy from the surroundings causing the temperature of the surroundings to decrease a great physical change example of this that you can feel is ice melting ice is water in a solid state which means that the molecules are arranged in a structured rigid lattice when ice melts it transitions from a solid to a liquid this process requires energy to break the bonds holding the water molecules in the solid structure this energy can come from the air place your hand above a stack of ice you can feel the air is colder or from your skin grab an ice cube quickly the cold you feel is heat being transferred from your skin to the ice the reaction is heat plus H2O solid going to H2O liquid most combustion reactions are exothermic because they release energy and the reactants form more stable products with lower potential energy however the combustion of nitrogen N2 is endothermic looking closer at the bonding structure of the nitrogen molecule we can see why this is the case nitrogen molecules have a triple bond the strongest out of the three calent bonds between the two nitrogen atoms breaking this triple bond requires an enormous amount of energy more than the products N2 plus O2 going to 2 N of the combustion reaction therefore since since the net energy is required and not released it's endothermic the relative stability of reactants and products plays a crucial role in determining whether a chemical reaction is endothermic or exothermic this can be understood through the concepts of chemical potential energy and enthalpy potential energy is a form of energy that's stored in an object or system due to its position Arrangement or state you can think of it as the energy something potentially has books on a shelf have potential energy due to their position and gravity but because that shelf is holding them Against Gravity it's just potential this is the same for chemicals which we call chemical potential energy and is the energy stored within the chemical bonds of molecules and atoms in a chemical reaction reactants are converted into products each substance involved has a certain amount of stored chemical potential energy in its bonds more stable compounds have lower potential energy while less stable compounds have higher potential energy consequently chemical reactions tend to move toward a state of Greater stability by forming products that are more stable than the reactants chemist visualize these potential energy changes between re re ANS and products of endothermic and exothermic reactions through energy profiles also known as reaction coordinate diagrams they also illustrate the activation energy the minimum energy required for the reaction to start and the energy released or absorbed for the overall reaction in an exothermic reaction the products are more stable than the reactants this means that the products have lower potential energy compared to the reactants for example with the combustion of propane the products CO2 plus H2O are more stable and have lower energy than the reactants c3h8 and n O2 since the products are more stable lower energy the excess energy is released to the surroundings often in the form of heat and light the change in enthalpy is negative Delta h less than zero since Delta is a symbol denoting final minus initial looking at the reaction coordinate we can arbitrarily give the products any number lower than the reactants and Delta H will always be less than zero indicating that energy is released in an endothermic reaction the reactants are more stable than the products this means that the products have higher potential energy compared to the reactants for example with ice melting the products H2O liquid are less stable and have higher energy than the reactants heat plus H2O solid since the products are less stable higher energy energy must be absorbed from the surroundings to drive the reaction with endothermic reactions the change in enthalpy is always positive Delta H is greater than zero indicating that energy is absorbed as discussed previously when a reaction occurs at constant pressure the change in enthalpy is equal to the heat exchanged between the system and its surroundings this is referred to as the standard enthalpy change for a reaction Delta H knot and the units are KJ per mole and can be determined from the change in temperature of a pure substance these temperature changes indicate whether a reaction is exothermic or endothermic and can quantify the energy changes involved this is often done using a calorimeter which measures the initial and final temperatures of the water that a reaction is conducted in from the changing temper temperature you can calculate the heat Q absorbed or released using the formula q = m * C * delta T where m is the mass of the water C is the specific heat capacity of the water 4.18 KJ per kilogram per Kelvin and delta T is the change in temperature we've discussed all these terms before other than specific heat capacity specific heat capacity is the amount of heat required to raise the temperature of 1 gram of a substance by 1° C or kelvin once you have q you can use the equation Delta h = Q / n where n equals the number of moles of the substance not water involved in the reaction to calculate the enthalpy change Delta H let's run through an example with the neutralization of the acid base reaction of vinegar by baking soda vinegar is approximately 5% by volume acetic acid ch3 Co a weak acid and baking soda is sodium bicarbonate nah3 a weak base the reaction between the two is ch3 co plus nah3 going to ch3 Co na plus H2O plus CO2 you set up the reaction in a styrofoam cup with a stirer lid thermometer and 100 ml of vinegar you record the initial temperature 22° C and add 10 G of baking soda replace the lid and start the stirring the final temperature is 24° c a change of 2° C or 2 Kelvin now we measure Q with the formula q = m * C * delta T the mass of the water m equals 95 G because 5% acetic acid means 5 MLL 95 M left that's water we're disregarding the water being formed by the reaction for ease of calculations and the density of water is 1 G per Mill so Q = 95 G of water * 4.18 K per kg per Kelvin * 2 Kelvin before we multiply across we must first convert the grams of water to kilogram to keep the units consistent which we can do by dividing by a th000 and we get qal 0.794 2 K now we need to determine n the number of moles of the substance not water involv D in the reaction to do this we divide the mass of acetic acid by the molar mass of it 60.6 g per Mo but we just have the Volume 5 millit to convert volume to mass we use the density which is approximately 1.49 G per Mill for acetic acid so 5 m * 1.49 G per Mill equal 5245 G of acetic acid 5245 G divided 6.06 G per mole equal 0.873 mol of acetic acid plugging these values into Delta h = q / n we get 0.794 2 k / 0.873 moles which equals 9.94 KJ per mole notice that is negative Delta H is less than zero which means that it's an exothermic reaction which coincides with what we just saw the release of energy if you were to compare this to what is expected from theoretical values you'd likely see a higher value theoretically this is due to various factors including heat loss to the surroundings thereby not being absorbed by the calorimeter contents Additionally the calorimeter itself our Styrofoam cup absorbs some heat which is not accounted for in complete neutralization of the substance being tested an efficient heat transfer from the reaction to the water evaporation and condensation during the experiment and finally inaccuracies in measuring temperature and mass can cause the discrepancies between theoretical and experimental values [Music]