enthalpy changes in chemical reactions #4
DESCRIPTION
Enthalpy Changes in Chemical Reactions #4. Enthalpy. Enthalpy (H) The “heat content” of a substance It is the total KE and PE of a substance at constant pressure KE + PE = constant (enthalpy). Kinetic & Potential Energy. KE: it is the energy of MOTION … - PowerPoint PPT PresentationTRANSCRIPT
EnthalpyEnthalpy (H)• The “heat content” of a substance
• It is the total KE and PE of a substance at constant pressure
• KE + PE = constant (enthalpy)
Kinetic & Potential Energy
• KE: it is the energy of MOTION…
ex; atoms and electrons moving within the molecules, or in the entire system (reaction)
• PE: energy existing due to position and the sum of all attractions/repulsions between particles. PE is directly related to BOND energies.
Bond energy
• Bond energy is the amount of energy required to break/form a bond between two atoms.
Ex:
Endothermic Reactions
• CH3OH C(s) + 2H2(g) + ½ O2(g) H = + 201 KJ • positive H means endothermic
• CH3OH + C(s) + 2H2(g) + ½ O2(g)
Kinetic Energy Distributions
• In general, molecules at R.T and pressure undergo about 10^10 collisions/second! Yet, there are only so many successful collisions…
KE Distributions• Increased Temperature = increased number of
molecules that have enough energy to react/cross the activation energy barrier successfully.
Yes, collisions would also increase, but only 1% more collisions for every 10C increase in Temp.
Activation Energy
• The minimum potential energy needed in a collision before a reaction can take place.
• It can also be defined as the minimum energy colliding particles must have in order to have a “successful” collision.
• At the start of a rxn, most energy is stored as kinetic energy.
• As molecules approach each other, electrons on the outer shells start to repel one another; slowing down and causing an increase in PE
• If enough PE is gained, bonds can be broken/made and we form an activated complex.
• After the rxn, the new products start repelling each other, moving away, increasing speed and KE, therefore decreasing PE.