1.6 -ENTHALPY CHANGES (~Hl. H2.. Let's consider a simple reaction (decomposition of ~ gas): To break the bond between the 2 H atoms, CYle~

Transcription

1 Unit ENTHALPY CHANGES (~Hl Bond Energies H2.. Let's consider a simple reaction (decomposition of ~ gas): To break the bond between the 2 H atoms, CYle~ must be C\ d c\ ~ ct to the molecule. Th t- '(e.s.u \-\-\Cj \-\-ctto m -s en n -\-a.,'(\ rno{~ E Let's consider the reverse reaction (synthesis of H2 gas): 2 H(g) ~ H2 (g) As the H atoms combine, they fe.\eas<-,. energy PE The reaction is shown on the diagram below: Y\\(Jhev -e H + H (\e ~s St-Clb\G) / H ~~--~~ \t)vj-v' E lm()v(' sta ok,) Reaction Proceeds ~ Forward: The H atoms combine to form a JO'fJe.{ 8 (more stable) product Reverse: The H2 molecule breaks apart (energy must be added) and the products (H atoms) have a ~ 'n-tr f (less stable),

2 BOND two atoms. ENERGY is the amount of energy required to j)(.-ak a bond between To break a bond, we must add energy (equal to the energy of the bond): CI2 (g) + ~1f3 \\J -7 2CI (g) ~ b00ch E. When 2 atoms combine, an amount of energy equal to the bond energy is released by the atoms: 2CI (g) -7 CI 2 (g) -\- ~ ~ 3 K.:J -e- -\-" Let's look closer at a bond between two atoms (ex: H -H): \\uc\--eus LDn-to.\ (\S C\r\CtV~et\ ~rt)to'(\. ~ e(lch \-\C\;\U((' ha~ \ e, to S~S (bona) - S~('(\~ \ 0.(" chc\r3~\ (~\ <:' 0 o The electrons are shared (covalent bond) and they hold the positively charged nuclei together (art\- let ch"e..., forces) L nuclei ~ each other. L:C~U\ $\ \je,. forces) At the same time, the electrons (ep-e. \ each other and the o The position of the bonded atoms is a compromise between these forces.

3 Energies in a system (PE and KEl PE (Potential Energy) is the energy existing as a result of o An object's (particle/ molecule) '~S \ -\-~DY\ in space o The S3U tj\ of the attractive and repulsive forces that exist among the particles which make up the object PEof a chemical system is directly related to the energy of the e\e c.-h'b f\-s in the bonds AND the ---*-- and ~ atoms in the molecules. of Ex: When breaking the H-H bond in Ho the electrons must have enough energy to separate and break the bond. PE \ Y" execls e.s H+H \j H :-:---~:;...,.". ~ Re\l~~e (X(\ lor-tct~~ H2- b;vtci) Reaction Proceeds KE (Kinetic Energy) is the energy existing as a result of KE :::.t N\~ 2- f'\ I'Oil'\/'\eil'\t-. hi h :::.l (ma~~)cviloc/~l) O ill _!,...;;:\J;...;\I;...;:"C...-:::;.;r:...:... r Ic..=..:,f-'---.,; "-,---, Wit In t e system o; 'J o Ex: motion of the molecules in the system/ motion of the atoms in the molecules 2-

4 ENTHALPY U:!l In a given reaction, a lot of things are happening: bonds are breakinq, new bonds are formed, phases may chanqe. heat may be lost or absorbed... Accounting for each of these energy changes is complex (and beyond Chern 12). Instead, we use a term flenthalpy" that incorporates all the energies in an ~e(\ system at C os\-an't atmospheric pressure. Enthalpy (H) = total E and \<E in a system at constant P. H REACT = combined enthalpies of all the (eclc-\- (lv\t--s H prod = combined enthalpies of all the ----PCb c\.llt--\ YY\-e l V'\ c,..0h OtY\j<:' /1 l\h = J=\ pcoca - t\ 'C-t-ClCt- = change in enthalpy for reaction 1 Here is a diagram showing these terms: Enthalpy (H) / H2 ( \-\ proo\ ) ~~~~~~ Reaction Proceeds The sign of l\h tells us if the reaction is exothermic or endothermic.

5 Exothermic reaction 6H is ~ - ') o Heat EXnS the system (E~O. ~o.a- E'X " t-sj o The products have less energy (enthalpy) than reactants: Enthalpy (H) H+H ~ ~ MI -= ~",,---_--,H2=--- Reaction Proceeds *energy flows from system ~ surroundings Endothermic reaction 6H is ---fx1$( -nv(' ( +) - o Heat EN1(\ the system (EN DO : heo.i- trvte1zs) o The products have more energy (enthalpy) than reactants: Enthalpy (H) J3----,,1H -:::..-+ O 2 Q...±O Reaction Proceeds *energy flows from surroundings ~ system How to write these reactions: (1) the reaction can be written using a heat term (ex: 164 kj) (2) the reaction can be written and ~H is shown on the side

6 EXOTHERMIC: (1) (2) ENDOTHERMIC: (1) (2) he.o\~ ~-\-evs 2N2 + O 2 +\~AKJ~ 2N2 + O2 ~ hea+ a.s e: P«nDUCT 2 HCI + \~y- \<.:r 2 HCI j ~\-\;:::. - \~t\ K:T (\~ ~ -e.d~-\- ~ lz8(\-ctf\i\j\ 2 N N 2 0 j jj\i ~ -+ \bl\ KJ ***T"Ip to remember the sign of ah: Heat term on the LEFT: ~H = > - Heat term on the RIGHT:~H =- Do Hebden Questions p 13 #23, P 16 #24-28