Topic 5: Energetics Flashcards
energy
- measure of the ability to do work
- conserved in chemical reactions
enthaply
- measure of the amount of energy contained in a substance
- stored in chemical bonds and intermolecular forces as potential energy
- when substances react, the different in the enthalpy between the reactants and products results in a heat change
heat
a mode of energy transfer which occurs as a result of temperature difference
- increases the average kinetic energy of molecules
- its the TOTAL kinetic energy of a system
endothermic reactions
- heat absorbed by system
- breaking bonds; separation of particles which are held together by a force of attraction
- enthalpy is positive
- reactants more stable than products
exothermic reactions
- heat give out by system
- making bonds; bringing together of particles which haven an attrative force between them
- enthalpy is negative
- products more stable than reactants
e. g. combustion and neutralization reactions
standard conditions for enthalpy changes
- a pressure of 100 kPa
- concentrations of 1 mol dm^-3 for all solutions
- all substances in their standard states
standard state of a substances
pure form of the substance under stnadard condtions
temperature
- meaure of the average kinetic energy of the particles
- Kelvin scale; measures relationship between kinetic energy and temperature (298K usually given as standard)
what does the increase of temperature of an object when heated depend on?
- mass of the object (shared vaue of kinetic energy among particles)
- heat added (amount of kinetic energy)
- nature of the substance
specific heat capacity
- property of a substance which gives the heat needed ot icnrease the tempreature of unit mass by 1 k
- depends on number of particules in unit mass sample
heat change formula
q=mc@T
q= heat change in joules m= mass in grams c= specific heat capacity @T= change in temperature (in kelvin)
specific heat capacity of water
4.18 J g^-1 K^-1
heat capacity formula
heat change (q)/temperature change (@T)
standard enthalpy of combustion
enthalpy change for the complete combustion of one mole of a substance in its standard state in excess oxygen under standard conditions
some reasons for differences in values of combustion reactions + ib data booklet values?
- not all heat produced by combustion transferred to water
- incomplete combustion due to imited oxygen
- experiment not performed under standard conditions
how can the enthalpy change of a solution be measured?
carrying out reaction in insulated system (e.g. polysterene cup) as heat release or absorved by reaction can be measured from the water temperature change
hess law
states that the enthalpy change for any chemical reaction is independent of the route, provided the starting conditions and final conditions, and reactants and products are the same
what happens when you reverse the direction of a reaction?
you reverse the sign of the enthalpy value
standard enthalpy of formation
enthalpy change that occurs when one mole of the substance is formed from its elements in their standard states under standard condtions of
- 298 K (25 C)
- 1.00 x 10^5 Pa
standard enthalpy change of formation in its stable form?
its zero, as no enthalpy change occurs when an element is formed from itself
Hf formula
AH of reaction= AH of Products- AH of Reactants
bond enthalpy
energy needed to break one mole of bonds in gaseous molecules under standard conditions
why do we use average bond enthalpies?
- to take into account bond enthalpies and compare them which exist in different environments
- different enthalpies of atoms bonded in different ways
average bond enthalpies
energy needed to break one mole of bonds in gasesous molecules under standard conditions averaged over similar compounds
oxygen forms
- O2; double bond
- O3; oxygen to oxygen bond is between a single and double bond
- both protect earths surface by UV light radaition ((form protective screen))
oxygen
- double bond in O2 stronger than 1.5 bonds in izone; broken by a radiation of higher energy and shorter wavelengths
ozone
- 1.5 bond length
- longer wavelength radiation needed to break bonds
Ephoton
plance equation; hv
Free radicle
species with an unpaired election
natural formation and depletion of ozone
- UV breaks down oxygen and ozone covalent bonds for form free radicles
- oxygen and oxygen radicals react to form radical (exxoterhmic)
- continous reactions of ozone with free radicles and ozygen with free radicals creates Chapman Cycle
