Energetics, Kinetics, and Equilibrium (Topics 5-7)

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Energetics, Kinetics, and Equilibrium

endothermic

the breaking of bonds in a chemical reaction which requires energy (on the reactants side)
energy absorbed from surroundings is stored up as potential energy in the system (+)

exothermic

the formation of bonds in a chemical reqction which releases energy (on the products side)
potential energy in the system is released as heat to the surroundings (-)

average bond enthalpy

the energy needed to break one mole of covalent bonds between atoms in the gas state

specific heat capacity

amount of heat needed to raise the temperature of 1 gram of a substance by 1 degree Celcius

Hess's Law

when several reactions can be added to give one overall reaction, the enthalpy change for the overall reaction is the sum of the enthalby change for the individual reactions
- reverse a reaction, reverse the enthalpy sign
- if you need to multiply a reaction, multiply it's enthalpy with the same number

kinetics

study of reaction rates

rate of reaction

change in concentration over time (unit is mol dm-3 sec-1)

collision theory

- particles must have sufficient energy (activation energy - Ea)
- particles must collide in the correct orientation
more effective collisions = faster reaction

activation energy

the minimum amount of energy needed to bring about a reaction

catalyst

chemical that speeds up a reaction without being consumed
(lowers the activation energy)

homogeneous catalyst

catalyst is in the same phase of matter as reactants

heterogeneous catalyst

catalyst is in a different phase than reactants

Haber Process

3H2 + N2 --> (Fe) 2NH3
iron is the catalyst

Contact Process

SO3 + H2O --> (V2O5) H2SO4
vanadium is the catalyst

Decomposition of Hydrogen Peroxide

2H2O2 --> (MnO2) 2H2O + O2
manganese oxide (i.e. manganese dioxide) is the catalyst

reaction mechanism

the series of steps by which a reaction occurs

rate determining steps (RDS)

the slowest step in the mechanism; the overall reaction cannot go any faster than this step

chemical equilibrium

the rate of the forward reaction equals the rate of the reverse reaction in a closed system (concentrations are constant but not equal)

equilibrium constant

indicates the "extent" of the reaction
- large Keq means there are more products than reactants
- very large Keq means the reaction goes to completion
- small Keq means there are more reactants than products
- affected by temperature

Le Chatelier's Principle

when a stress is applied to a reaction at equilibrium, the reaction shifts left or right to relieve the stress and establish a new equilibrium

enthalpy

the energy contained in chemical bonds that can be converted into heat (i.e. H)

first electron affinity

the enthalpy change when 1 mole of gaseous atoms each gain 1 electrons

standard enthalphy change of reaction

the enthalphy change when molar quantities of reactants in their normal states react to form products in their normal states under standard conditions of temperature and pressure

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