Home > Chemistry > Chemistry 1A Part Deux: Lecture 28 Notes

Chemistry 1A Part Deux: Lecture 28 Notes

Overview

Today is a 48 minute lecture on heat and work.

Details

Chemical energy can be converted into work and used to move objects.

2H + O2 = 2H2O   Space Shuttle

C + O2 = CO2         Trains

C8H18 + 12.5O2 = 8CO2 + 9H2O    Cars

Many exothermic reactions have a cost due to negative entropy.  The second law must be obeyed.  dG is a measure of the free energy, the energy available once the second law is obeyed.  Some heat goes into the entropy cost.

Converting Heat to Work

G = H – TS

dH < 0

This portion can be converted into work after the entropy cost is paid for.

dS < 0

This portion must be compensated for in order to pay for the second law.

dS >o

This portion can also be converted into work

Work maximum = dG = dGo + RT ln Q initial

Solids don’t have much entropy.

Converting Entropy to Work

Using a concentration gradient.  Use membranes.  Water moves across the membrane to equalize concentrations.  Entropy tries to balance concentration by moving either water or solute.

Heat is often KE, sometimes abbreviated as Q.

Mechanical work is potential energy, W = F x d.

You can convert work into heat.  Drop a weight, connect a rope to a pulley that spins.

W = Q

W = F d = mg dh = Q = m cp dT

Normally, we want heat into work.  When a gas is heated, it performs work on the surroundings.

W = -F ext dh = -P ext A dh = – P ext dV

Pressure volume work.  A change in volume, a change in temperature, heat transferred to the gas from the fire Q = n Cp dT.  Work done by the gas to push up the piston W = -P ext dV.

Cp (N2 gas) = 29 J/(mol K)

The efficiency of gas in a cylinder being heated and gas expanding is about 30%.  Much work went into moving the molecules faster.

More heat is transferred to increase the temperature of 1 mole of gas to the same final temperature at constant pressure.

dH = dE + P ext dV

= internal energy + work

Chemistry is done at constant pressure, not constant volume.

efficiency = 1 – T low/Thigh

The temperature of the exhaust gas is wasted heat.  Gas does work then cools.

Review

PV = nRT

Electronegativity: NOF Cl Kr

K = products/reactants

 

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