7.1. Energetics 1 Flashcards
temperature
measure of the average kinetic E of particles in a system - state function - expressed in °C or K
heat
energy in transfer - expressed in J or kJ
enthalpy change (delta H)
heat transferred from a closed system to the surroundings during a chemical reaction
exothermic vs endothermic reaction (give examples of each)
exo is when heat is released from the system and endo is when it is absorbed by the system
combustion of fuels, acid-base neutralization
formation of NO2, reaction of Ba(OH)2 and NH4Cl
draw and annotate graphs of exo and endo reaction
…
calorimeter
used to measure the actual amount of heat evolved in an exothermic reaction by using he heat released to increase the temperature of a known mass of water
assumptions and errors using a calorimeter
assumptions:
1) the heat released is completely transferred to water
2) calorimeter is an insulator so its c is zero
3) T max reached is equal to heta evolved
4) C(aqueous solution) = C(water)
errors:
1) loss of heat (graph)
formula for heat
Q=mc(delta)T
c=heat capacity
bond enthalpy
(+ formula)
E change when one mole of a covalent bond in the gaseous state is formed from its gaseous atoms
BE = input - output
(bonds broken - bonds formed)
forming bonds is…
breaking bonds is…
exothermic
endothermic
limitations of using bond enthalpies
1) all reactants have to be in gaseous form
2) average bond enthalpies are used
Hess’ law
the heat of a whole reaction is equivalent to the sum of its steps (adding equations)
combustion of:
- metals
- non-metals
- complete combustion of organic compounds
- incomplete combustion of organic compounds (in the absence of sufficient oxygen - CO and carbon soot produced)
what is choice of fuel determined by
the fuel’s specific energy:
E(specific) = (E released from the fuel)/(m of fuel consumed)
fossil fuels
- crude oil, coal, and natural gas
- unrenewable E sources
- produce greenhouse gases (CO2)
biofuels
- renewable E sources
1) direct combustion of plant material
2) biogas (methane and CO2 mixture)
3) fermentation to produce ethanol - biological carbon fixation - production of organic compounds from CO2 (photosynthesis)
fuel cells
hydrogen fuel cell
- converts chemical E directly to electrical E - doesn’t need an external source of E for charging
standard enthalpy change of reaction
the enthalpy change when molar quantities of reactants in their normal states react to form products in their normal states under standard conditions of T and p (25 °C and 1 atm, when element is in its most stable form (O2(g)))
(delta)H<0
(delta)H>0
exothermic
endothermic
standard molar enthalpy of formation
the enthalpy change for the formation of one mole of a substance from its elements in their standard states - (delta)Hf
its value is zero for all free elements (when only on element is present)
