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KU CHEM 130 - Chapter 6: Thermochemistry

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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*1066.3 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 WorkHeat- 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 be no NET transfer of heat.- Heat capacity is the amount of heat require to change temperature by 1 degree Celsius. 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*atm6.5 Measuring Energy: Constant-Volume Calorimetry- If reaction is carried out at constant volume, the heat at constant volume is the change in energy.- Measure heat in chemical reaction using calorimetry by observing temperature change in surroundings.- 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 constant pressure.- Conceptually and numerically energy and enthalpy are very similar.-ΔH = ΔE + PΔV- If chemical reaction result in large volume changes, energy and enthalpy will have slightly different values.- If chemical reaction does not cause large change in volume, energy and enthalpy value are very similar.-Endothermic positive ΔH while exothermic negative ΔH.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.- Endothermic when bonds break and and atom has higher potential energy absorb heat.Stoichiometry Involving ΔH: thermochemical Equations- Enthalpy is an extensive property that depends on amount of material undergoing reaction.- Amount of heat depends on amount of reactant.-Specify ΔH for balanced chemical equation.-Magnitude of ΔH is for stoichiometric amounts of reacts and products.6.8 Relationships Involving ΔH-If chemical equation is multiplied by some fact, ΔH is also multiplied by the same factor.-If a chemical equation is reversed, ΔH changes signs (e.x positive to negative).-If a chemical equation is a sum of a series of steps, ΔH is the sum of the ΔH 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 ΔH in relation to the chemical equation to get the overall reaction’s

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