Chapter 6 Thermochemistry Thermochemistry is the relationship between energy and chemistry Exothermic reactions emit heat and endothermic reactions requires heat Temperature is proportional to heat 6 2 The Nature of Energy Definition Energy is the capacity to do work and work is force acting through a distance Heat and work are ways energy is exchanged Kinetic energy is energy associated with motion of object Thermal energy is energy associated with temperature of object Type of kinetic energy because it is the motion of atoms Potential energy is energy associated with position and composition Chemical energy is energy stored within the bonds of chemical substances Nuclear energy is energy stored in the neutrons and protons of atom Electrical energy is the energy associated with the flow of electrons Law of conservation of energy is energy cannot be created or destroyed Energy can change forms and be transferred System is the object under investigation and surroundings is everything the system can exchange energy with System lose energy the surrounding gains the lost energy Units of Energy Kinetic Energy 0 5 mass kg velocity m s 2 Potential Energy mass kg gravity 9 8 m s2 height m Energy is measured in joules J kg m2 s2 Calorie cal is amount of energy needed to raise 1g of water by 1 degree Celsius 1 cal 0 001 Cal 4 184 J 1 kWh 3 60 106 6 3 The First Law of Thermodynamics Thermodynamics is the study of energy and its conversions Total energy of the universe is constant There can be no energy output without energy input Internal Energy It is the sum of kinetic and potential energy of a system Only depend on the the state of the system state function does not consider how the system arrived at state Can t determine value of energy Can only determine change in energy Change in state function is difference between final and initial value Enthalpy of system Enthalpy of surroundings Energy products reactants Energy q heat w work Heat and work are not state function because it depends on surroundings 6 4 Quantifying Heat and Work Heat Heat is the exchange of thermal energy Temperature is the measure of thermal energy Thermal energy flow from high temperature to low temperature Energy transfer stops when thermal equilibrium is reached with same temperature There will Heat capacity is the amount of heat require to change temperature by 1 degree Celsius be no NET transfer of heat Extensive property Specific heat capacity is amount of heat needed to raise temperature of 1 g of substance by 1 degree Celsius Intensive property Same concept for molar heat capacity Heat J Mass g Specific heat capacity J g C Temperature change C mC T system mC T surroundings Work Pressure Volume Work associated with force caused by volume change against an external pressure Work Pressure atm Change in volume L Negative because work is done on the surroundings not the system 101 3 J 1 L atm 6 5 Measuring Energy Constant Volume Calorimetry If reaction is carried out at constant volume the heat at constant volume is the change in Measure heat in chemical reaction using calorimetry by observing temperature change in Heat of calorimeter Heat capacity of calorimeter J C Temperature change C Heat of calorimeter Heat of reaction 6 6 Measuring Enthalpy Constant Pressure Energy is the total energy of both heat and work but enthalpy is only the heat exchanged under Conceptually and numerically energy and enthalpy are very similar H E P V If chemical reaction result in large volume changes energy and enthalpy will have slightly If chemical reaction does not cause large change in volume energy and enthalpy value are very Endothermic positive H while exothermic negative H Exothermic and Endothermic Processes Molecular View Thermal energy of system is not the thermal energy emitted in exothermic reaction Thermal energy of system is the kinetic energy of atoms and molecules of system Heat emitted or absorbed during reaction is from potential chemical energy Exothermic when bonds form with lower potential energy emit heat energy surroundings constant pressure different values similar Endothermic when bonds break and and atom has higher potential energy absorb heat Stoichiometry Involving H thermochemical Equations Enthalpy is an extensive property that depends on amount of material undergoing reaction Amount of heat depends on amount of reactant Specify H for balanced chemical equation Magnitude of H is for stoichiometric amounts of reacts and products 6 8 Relationships Involving H If chemical equation is multiplied by some fact H is also multiplied by the same factor If a chemical equation is reversed H changes signs e x positive to negative If a chemical equation is a sum of a series of steps H is the sum of the H of all steps Hess s Law is that enthalpy is only dependent of initial and final values not the pathway it takes It is a state function Combining reactions to find an overall reaction manipulate chemical equation in a way that reactant and products will cancel out to the final overall equation Remember to manipulate the H in relation to the chemical equation to get the overall reaction s H