Thermochemistry Flashcards
what is thermochemistry?
deals with the change in energy that occurs during a chemical reaction
chemical equations v thermochemical equations
- they are different
- thermochemical must have enthalpy value
what is an enthalpy value?
tell chemist about the reaction participants and about energy change. enthalpy is defined as the amount of energy change per set amount of substance
to use enthalpy in mol what must be present
chemical formula
incomplete combustion
- limited O2
- products CO and H2O
- all product gas
complete combustion
- O2
- products CO2 and H2O
- products gas
how to balance combustion reactions normally?
CHO
how to balance when oxygen is present in the fuel or reactant?
need to subtract O atom total in the products by the number of O atoms in the chemical reactant (not O2) before you balance O2
if you have to halve or double the stoichiometry you must…. the enthalpy
halve or double
how do you reverse a thermochemical equation?
stoich is the same but enthalpy value becomes positive
how does the state of water impact the enthalpy value?
H2O to liquid releases heat
H2O to gas needs heat
what are the thermochemical reaction types?
endothermic or exothermic
endothermic?
- absorb thermal energy from surrounding environment
- prac will indicate a decrease in temp
exothermic?
- release thermal energy to environment
- prac indicates an increase in temp
what is activation energy needed for?
to break bonds
what happens in an endothermic energy profile?
- reactants have lower energy than products
- energy is absorbed
what happens in an exothermic energy profile?
- reactants have higher energy content than products
- there is some energy absorption for bonds to break
what happens at the transition state?
atoms begin to be rearranged
how is enthalpy calculated from an energy profile?
H(product)-H(reactant)
- positive in endo
- negative in exo
how is energy for a set thermochemical equation calculated? do you need a negative or positive sign?
- get enthalpy from data book
- multiply by number of mole of the fuel
- no sign because not saying whether exo or endo
- if in grams, convert to mol
how to calculate the amount of substance from energy?
(the number of mol x the energy)/ enthalpy that correlates to mol
what is specific head capacity?
ability to hold onto heat energy
what is the density of water?
1ml is 0.997g
what is the SHC equation?
energy= SHC x mass x change in temp
how do you use an experiment to calculate enthalpy?
use water’s capacity to hold onto energy to determine the chemical’s energy
how to find energy produced by burnt fuel?
E = (SCH of water) x mass of water x change in temp of water
this gives energy in J which should be converted to KJ
how is the enthalpy of the burnt fuel calculated?
energy/amount (mol or mass)
calculate enthalpy with mass
KJ of energy/mass
- KJ from SHC eqn
- mass from initial-final mass of fuel
calculate enthalpy with mole
- calculate mol with mass and molar mass
- divide energy KJ by mol
calculate % efficiency
experimental enthalpy/ theoretical enthalpy x 100
calculate heat loss
100%-% efficiency
calculate mass lost
mass x heat loss
calculate amount of fuel needed to reach 100%
100%/ your efficiency x quantity or mass of fuel or chemical
