Energetics. 5.1 Endothermic and Exothermic Reactions. Endothermic rxn heat is taken in from the surroundings ( rxn vessel gets cooler) Exothermic rxn rxn that result in the release of heat ( rxn vessel gets warmer)
Define Activation Energy
J g-1 °C-1
kJ kg-1 K-1
J kg-1 K-1
H2(g) 2H(g) ΔHΘ= +436 kJ mol-1
How many H-H bonds were broken?
Br2(l) 2Br(g) Br-Br= 193 kJ mol-1
What is the ΔHΘ? +224 kJ mol-1
Why is this higher than the bond enthalpy?
The reactants are not in a gaseous state
We must also account for the energy required for vaporisation of the reactants
This process is called atomisation
endothermic or exothermic?
Endothermic! What does that mean about ΔH?
Positive! What will bond making be?
Exothermic with a negative ΔH
C2H4(g) + Br2(g) C2H4Br2(g)
What bonds are broken?
What bonds are made?
Follow your steps!
ΔHr = ΣΔHc (reactants)- ΣΔHc(products)
ΔHr = ΣΔHf (products)- ΣΔHf (reactants)
X (g) +e- X- (g)
Electrostatic attractions of + and – ions
The higher the ion charge, the greater the lattice enthalpy
The larger the ions the weaker the forces, the smaller the lattice enthalpy
Bonding is solely due to attractive forces between oppositely charged ions
Theoretical value/ kJ mol-1 736
Experimental value/ kJ mol-1 876
What do the values suggest?
Polarisation of the negative ion by the positive one
The polarisation effect is greater
ΔSΘ= ΣSΘproducts -- ΣSΘreactants
ΔSUniverse = ΔSsuroundings + Δssystem
If ΔSUniverseis positive, the entropy of the universe increases and the rxn occurs spontaneously
When heat is given out in a rxn, the entropy of the surroundings get hotter
Outside influence such as; temp, catalysts, etc.