Thermochemistry Part 2

Thermochemisty Study of chemical reactions in relation to energy The capacity to do work or the transfer of heat Work The energy used in moving an object against a force.

Thermochemisty Ek = ½ mv2 Kinetic energy Ek = kinetic energy m= mass The energy of motion Depends on mass and velocity Ek = kinetic energy m= mass v= velocity Chemistry is interested in kinetic energy of molecules Ek = ½ mv2

Thermochemisty Eel = kQ1 Q2 d Potential energy Electrostatic energy The energy by virtue of its position in relation to other things, stored energy Electrostatic energy Eel = electrostatic energy k= constant of proportionality (8.99 x109 J-m/C2) C in k is coulomb, unit of electrical charge d= distance (in meters) Q1 and Q2 are the charges of the electron (1.60 x10-19 C) Eel = kQ1 Q2 d

Units of Energy joule (J): SI unit of energy 1J = 1 kg-m2/ s2 kJ: kilojoules calorie: non SI unit of energy 1 cal = 4.184 J (exact definition) Calorie (capital C) in nutrition is: 1 Cal = 1000 cal = 1 kcal (exact definition)

System vs. Surroundings What is under observation or study Surroundings Everything else

Different Systems Open System Closed System Isolated System Matter and energy can be exchanged between system and surroundings Closed System ONLY energy can be exchanged between system and surroundings Isolated System NEITHER matter or energy is exchanged between system and surroundings

Work w= F x d Remember work is energy of a force moving an object Work Product of a force over a distance w: work F: force, influence exerted on an object (push or pull) d: distance w= F x d

1st Law of Thermodynamics Energy can be neither created nor destroyed; it is conserved. Involves energy, system and surroundings

Internal energy ΔE = Efinal - Einitial Sum of all of the potential energy and kinetic energy of a system. Looking at the energy in a system after some change to the system. ΔE = Efinal - Einitial Δ read as delta, always means: final - initial

ΔE = Efinal - Einitial If ΔE is + If ΔE is - Efinal > Einitial Endothermic If ΔE is - Efinal < Einitial Exothermic

Energy Diagram Exothermic or Endothermic? Why?

ΔE = q + w ΔE = q + w ΔE q w Change in internal energy of a system Heat added or released from a system w work done on or by a system ΔE = q + w

You Try If you have a reaction that releases 873 J of heat and is able to move a piston up by doing 283 J of work against the surroundings, what would the change in energy be? What is the sign of the energy? Would it be gaining or losing energy?

You Try ΔE = q + w ΔE = -873 J + -283 J q = -873 J (why is it negative?) w = -283 J (why is it negative?) ΔE = -873 J + -283 J ΔE = -1156 J Sign is negative Losing energy

Energy as a State Function a property of a system that is determined by specifying the system’s condition or state (in terms of temperature, pressure, etc) Value of a state function depends on the present state of the system, not the path it took to get there. ΔE = Efinal - Einitial

Wrap your ear around this! E is a state function q and w are not state functions Yet Explain? ΔE = q + w p. 168-169

Enthalpy H = E + PV Abbreviated with an H Enthalpy From enthalpein (Greek: to warm) H = E + PV

Pressure-Volume Work w = -PΔV ΔV is + ΔV is - Work involved in the expansion or compression of gases under a constant pressure ΔV is + Vfinal > Vinitial Work is done on the surroundings ΔV is - Vfinal < Vinitial Work is done in the system w = -PΔV

Change in Enthalpy (at a constant pressure) ΔH = Δ(E + PV) ΔH = ΔE + PΔV (@ a constant pressure)

Let’s Derive… ΔH = ΔE + PΔV constant pressure ΔE = q + w w = -PΔV Remember: Now substitute: ΔH = ΔE + PΔV constant pressure ΔE = q + w w = -PΔV so… -w = PΔV ΔH = (qp + w) - w ΔH = qp (qp denotes constant pressure)

ΔH = qp ΔH > 0 endothermic ΔH < 0 exothermic More useful than internal energy since most change in volume in reactions is small H is a state function, even though q is not. Equation is misleading in that it is true because pressure volume work is involved and pressure is constant. ΔH > 0 endothermic ΔH < 0 exothermic

Enthalpy Enthalpy is an extensive property, depends upon how much we have Enthalpy change is equal in magnitude, but opposite in sign to the change in H for the reverse reaction. Enthalpy for a reaction, depends on states of reactants and products.

Enthalpy Enthalpy is an extensive property, depends upon how much. Explanation: If there is more reactants, when they react, there will be more energy released if it is exothermic

Enthalpy Enthalpy change is equal in magnitude, but opposite in sign to the change in H for the reverse reaction. Explanation: Follows the First Law of Thermodynamics

Enthalpy Enthalpy for a reaction, depends on states of reactants and products. Explanation: If the substance is a solid, it would take in energy to change to a liquid. The change in state inherently involves a change in enthalpy.

Enthalpies of a Reaction For a chemical reaction (rxn): Could be called heat of a reaction ΔH = Hfinal - Hinitial ΔH = Hproducts - Hreactants ΔHrxn = Hproducts - Hreactants

Practice The decomposition of slaked lime, Ca(OH)2 (s), into lime, CaO (s), and water at a constant pressure requires the addition of 109 kJ of heat per mole of Ca(OH)2. Write a balanced thermochemical equation for the reaction Draw an enthalpy diagram for this reaction How much heat is needed to decompose 148 grams of slaked lime?