Thermochemistry, Thermochemistry, Hess’s Law and Hess’s Law and Driving ForcesDriving ForcesReview p. 531-549-Review p. 531-549-

Define: temperature, heat, Define: temperature, heat, specific heat, calorie and joule, specific heat, calorie and joule, exothermic, endothermic, enthalpy, exothermic, endothermic, enthalpy, Hess’s law, entropy and free energyHess’s law, entropy and free energy

Some review terms for this Some review terms for this unit:unit: TemperatureTemperature

HeatHeat

Specific heat Specific heat capacitycapacity

A measure of the A measure of the average kineticaverage kinetic energyenergy of random motion of the of random motion of the particles in a sample of matterparticles in a sample of matter

The The transfer of energytransfer of energy from one from one system at high temperature to system at high temperature to another system at low another system at low temperaturestemperatures

The amount of heat energy The amount of heat energy required to increase the required to increase the temperature of one gram of a temperature of one gram of a substance by one degree Celsiussubstance by one degree Celsius

Distinguishing endo and Distinguishing endo and exo-exo- Exothermic rx.:Exothermic rx.:

Release energyRelease energy Has a –Has a –ΔΔH valueH value Has a smaller value Has a smaller value

with a decreasing with a decreasing slope on an enthalpy slope on an enthalpy diagramdiagram

Has + heat as a Has + heat as a productproduct in a rx. in a rx.

Endothermic rx.:Endothermic rx.: Absorbs energyAbsorbs energy Has a +Has a +H valueH value Has a larger Has a larger

enthalpy value enthalpy value after the rx.after the rx.

Has + heat as a Has + heat as a reactant reactant

Enthalpy:Enthalpy: The amount of energy absorbed by a system as The amount of energy absorbed by a system as

heat during a process at constant pressureheat during a process at constant pressure H = HH = Hproductsproducts – H – Hreactantsreactants

Thermochemical equation:Thermochemical equation: HH2(g)2(g) + ½ O + ½ O2(g)2(g) H H22OO(g)(g) + 241.8kJ + 241.8kJ * notice that heat is a product = exothermic* notice that heat is a product = exothermic Most enthalpy of formation reactions are Most enthalpy of formation reactions are

exothermic. Compounds produced with a large exothermic. Compounds produced with a large negative enthalpy are stablenegative enthalpy are stable

Elements in their standard states are defined as Elements in their standard states are defined as H = 0H = 0

Compounds produced with a high positive enthalpy Compounds produced with a high positive enthalpy change are relatively unstable. They can react change are relatively unstable. They can react violently.violently.

Hess’s Law: Hess’s Law: the overall enthalpy the overall enthalpy change in a reaction is equal to the sum change in a reaction is equal to the sum of enthalpy changes for the individual of enthalpy changes for the individual steps in the process.steps in the process.

Use Hess’s law to calculate the enthalpy of reaction Use Hess’s law to calculate the enthalpy of reaction for the combustion of nitrogen monoxide gas, NO, to for the combustion of nitrogen monoxide gas, NO, to form nitrogen dioxide gas, NOform nitrogen dioxide gas, NO22, as given in the , as given in the following thermochemical equation:following thermochemical equation:

NONO(g)(g) + ½ O + ½ O2(g)2(g) NO NO2(g)2(g)

NONO(g)(g) ½ N ½ N2(g)2(g) = ½ O = ½ O2(g)2(g) H= -90.29 kJH= -90.29 kJ

½ N½ N2(g)2(g) + O + O2(g)2(g) NO NO22 H= +33.2 H= +33.2 H = -57.1H = -57.1

Driving Forces:Driving Forces: The change in energy is one of two factors that allow The change in energy is one of two factors that allow

chemists to predict whether a reaction will occur chemists to predict whether a reaction will occur spontaneously and to explain how it occurs. The spontaneously and to explain how it occurs. The randomness of the particles or the disorder termed randomness of the particles or the disorder termed entropyentropy is the second.is the second.

Entropy is an actual measure of the degree of Entropy is an actual measure of the degree of randomness in a system, randomness in a system, S S

There is a tendency in nature towards more entropy or There is a tendency in nature towards more entropy or randomnessrandomness

So, processes in nature are driven:So, processes in nature are driven: toward least enthalpy and toward least enthalpy and toward largest entropy.toward largest entropy.

Free energyFree energy This combination enthalpy-entropy function is called free This combination enthalpy-entropy function is called free

energy, G, of a system.energy, G, of a system. G = G = H - TH - TSS The H and S can be positive or negative, so there are The H and S can be positive or negative, so there are

four possible combinations of terms:four possible combinations of terms: 1. 1. -H-H (exo) and (exo) and +S+S (more random) (more random) = = always always – G – G

(spontaneous)(spontaneous) 2.2. -H -H (exo) and (exo) and –S–S (less random) = (less random) = - G at low temperature- G at low temperature 3. 3. +H+H (endo) and (endo) and +S+S (more random) = (more random) = - G at high pressure- G at high pressure 4. 4. +H+H (endo) and (endo) and –S–S (less random) = (less random) = never negative or never negative or

spontaneousspontaneous