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23 terms

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

Energetics, Kinetics, and Equilibrium
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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