u1: enthalpy Flashcards
at molecular level, potential energy is a result of
- forces between nucleus+electrons and between electrons in each atom (bc. like charges repel)
- forces between protons and neutrons in each nucleus (PEnuclei) (attracted to each other)
- forces between nuclei and shared electron pair in each bond (PEbond)
at molecular level, kinetic energy is a result of
- molecules moving through space (KEtranslational)
- molecules rotating (KErotational)
- bound atoms vibrating (KEvibrational)
- electron moving within each atom (KEelectron)
definition and variable of enthalpy
- (H)
- total energy contained in chemical system inc. many kinetic/potential energies
what kinetic/potential energies are involved the total energy contained in a chemical system (enthalpy)
no KE/PE, j say what
- moving electrons in atoms
- vibration of atoms connected w/ bonds
- rotational/translational motion of atoms/molecules
- nuclear potential energy of protons/neutrons in nuclei
- chemical potential energy of atoms connected by chemical bonds
two types of physical changes
- dissolving
- phase changes
what changes in bonds occur during dissolving? what is this change known as?
- bonds between ions or molecules in solute break so that attraction can occur with solvent
- bonds between molecules in solvent break so attraction occurs w/ solute
- bonds (imfs) form between solute and solvent
- known as enthalpy of the solution (∆Hsolution)
what changes in bonds occur during phase changes?
- changes to intermolecular bonds occur
- more ordered to less ordered: energy added from surroundings to system=endotheric
- less ordered to more ordered: energy released to surroundings from system=exothermic
enthalpy of phase changes and symbols
- solid to liquid: enthalpy of melting (∆H°melt)
- liquid to gas: enthalpy of vapourization (∆H°vap)
- gas to liquid: enthalpy of condensation (∆H°cond)
- liquid to solid: enthalpy of freezing (∆H°fre)
enthalpy of phase change relationships
∆H°melt = -∆H°fre
∆H°vap = -∆H°cond
what indicates a chemical change?
reactions that produce heat/light, precipitate, gas, or are difficult to reverse
what changes in bonds occur during chemical changes?
- intramolecular bonds (ionic/covalent/metallic) in reactants break apart
- new intramolecular bonds occur to form products
- both steps require energy
what is required for a chemical reaction to occur
- bonds within reactants are broken: requires energy = endothermic
- bonds are formed between rearranged atoms to form products: releases energy = exothermic
what is the difference between the energy used to break bonds and the energy released to reform new bonds called?
- enthalpy change of the reaction
-∆Hr
what happens during nuclear decay
atoms release neutrons/protons from nuclei to become new elements
what is necessary for nuclear decay to occur
a difference between
- binding energy between subatomic particles
- potential energy of nucleus
how is enthalpy measured
- it isn’t possible to directly measure enthalpy of a system so enthalpy differences are measured instead
- enthalpy difference comes from difference in potential energy that occurs during change of the system
- this is measured as heat
- ∆H = Q
formula to specifically calculate heat
- ∆H = Q = mC∆T
- Q = heat (J)
- m = mass of system (g)
- C = specific heat capacity (J/g°C)
- ∆T = Tf - Ti (°C)
thermal energy
potential energy and kinetic energy
why are calorimeters used?
- water absorbs and holds onto thermal energy well
- also easier
- prevents thermal energy escaping into surroundings
what is a calorimeter
double-walled vessel filled with water and equipped with a thermometer and stirring rod
how do you find heat/change in enthalpy for calorimeters using ∆H = Q = mC∆T
- use mass and C of water
- find mass of water using D=m/V
- D in g/mL
- m in grams
- V in mL
relationships in calorimetry
since ∆Esurroundings = -∆Esystem,
Qsurr = -∆Hsys
assumptions of calorimetry and q=mC∆T
- thermal energy exchanges between outside wall in surrounds is minimal and can be ignored
- any new solution produced in water of calorimeter is dilute, so C of water does not change
- process in calorimeter occurs under constant pressure
