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Chem31 Chapt. 9, pt. I: Energy, Enthalpy & Ideal Gases by Mind Map: Chem31 Chapt. 9, pt. I: Energy,
Enthalpy & Ideal Gases
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Chem31 Chapt. 9, pt. I: Energy, Enthalpy & Ideal Gases

Energy in Daily Life

Chemical Sources

Fossil fuel



Mineral deposits

Nuclear fission & fusion

Non-chemical Sources



Motion (waves, wind)

Definitions & Laws of Energy

Energy: capacity to do work or produce heat

Law of Conservation of Energy

Can be converted from one form to another

Can be neither created or destroyed

Classifications of Energy

Potential Energy

Kinetic Energy E1

Interconversion Between Kinetic & Potential Energy E2

Heat & Work

Difference between heat & temperature

Temperature: property reflecting the random motions of particles within a substance

Heat: transfer of energy between objects: not conserved, not a substance

Work: Force acting over a distance

Two ways to transfer nrg: heat & work

Proportion of heat & work depends on process

Chemical Energy

Combustion of methane: energy as a product E3

Universe is divided into two parts

System: part of system on which we want to focus attention (e.g. reactants & products)

Surroundings: everything else

Heat and chemical reactions

Exothermic reaction: evolves heat: nrg flows out of system into surroundings (e.g. combustion of methane) F1

Endothermic reaction: absorbs heat: energy flows from surroundings into the system (e.g. formation nitric oxide from nitrogen & oxygen) E4, F2

Relative energy of reactants & products

Exothermic reaction: Bond in products stronger than reactants: energy is a product

Endothermic reaction: Bonds of products weaker than reactants: energy is a reactant

Thermodynamics: study of energy and its interconversions

1st Law of Thermo: The energy of the universe is constant

Conservation of energy

The energy lost by a system is equal to the energy gained by the surroundings

The internal energy (E) of a system is the sum of the kinetic and potential energy of all of its particles

When a change in the internal energy of a system occurs there is a flow of heat and/or work done E5

SIGN CONVENTIONS: from system's point of view F3

q positive when heat flows into the system from the surroundings (endothermic)

q negative when heat flows out of the system to the surroundings (exothermic)

w positive when work is done on the system by the surroundings nrg from surroundings into system

w negative when work is done by the system on the surroundings (nrg from system into surroundings

Pressure-volume work (PV) of a gas in a cylinder F4

Expansion: work done by a gas on the surroundings (w<0)

Compression: work done on a gas by the surroundings (w>0)

Pressure: Force/Area (external pressure)

Work= Force x Distance E6



Definition: H= E +PV

E: internal energy of system

P: pressure of system

V: Volume of system

All state functions, so H is also a s.f.

Process at constant pressure where only PV work allowed (w=-PΔV)





At constant P, Δ(PV)=PΔV, so ΔH=ΔE+PΔV

ΔH=q: at constant pressure a process with only PV work, change in enthalpy is equal to the flow of energy as heat

For a chemical reaction, the enthalpy change, is ΔH=Hproducts-Hreactants

Thermodynamics of Ideal Gases T2

Ideal Gas Law: PV=nRT

Kinetic energy of an ideal gas: KE=3/2RT

The only way to change the kinetic energy of an ideal gas is to change its temperature

Heat required to raise the temperature of an ideal gas = 3/2RΔT

molar heat capacity: nrg required to raise the temperature of a substance by 1 K

Heating an ideal gas at constant volume

ΔV=0, therefore no PV work


Heating an ideal gas at constant pressure

Volume changes, work is done

Energy required="heat"=energy to change translational energy+energy to do work

work=PΔV=nRΔT=RT per mole=R per mole per 1 K

heat = Cp = 3/2R + R= 5/2 R or Cv+R

Heating a polyatomic gas T1

Cv is higher than for monatomic gas because of rotational and vibrational motion

Cp still= Cv+ R

State Functions

Property that depends on the present state of system, not on its past or future

Change in a state function in going from one state to another is independent of the pathway it takes

Energy is a state function, heat and work are not