Heat Changes During Chemical Reactions

Heat Changes During Chemical Reactions

Energy changes in chemical reactions result from the breaking and forming of bonds. The energy needed to break a bond is called the bond-dissociation energy. When the energy needed to break bonds is less than the energy produced by the bonds formed the reaction will give off energy.

Bond dissociation energy is the amount of energy needed to break a covalent bond. These energies are different for every bond. The values here are in kilocalories per mole. The smaller the bond energy the more reactive and weaker the bond. Notice the high bond energy for the nitrogen triple bond. Nitrogen does not react as readily as oxygen which has a bond energy that is almost half the nitrogen bond energy.

Covalent bond

Bond dissociation energy

Covalent bond

Bond dissociation energy

N:::N

226 strong bond

O::O

118.4

N::N

100

F:F

36.8 very weak bond

H:H

103

Cl:Cl

58

H:F

135.2

Br:Br

46.2

H:Cl

102.

I:I

35.6

H:Br

86.8

C:H

98.8

H:I

70.6

C:C

83.0

S:S

63.6

C:F

116.

In the table you can see why oxygen in the air reacts to "burn" substances rather than nitrogen. The bond energy for oxygen is very low. The triple bond in N₂is almost twice as strong. This means it is easier to break the bond in oxygen than it is to break the bond on N₂. The weaker the bond the more reactive the bond.Weak bonds are easily broken. Strong bonds are more stable. Teflon contains C:C and C:F bonds which are very strong. This is why teflon on cookware doesn't react with food. The C:F bonds are hard to break.

Exothermic reactions release energy. The energy released by the bonds formed in products is greater than the energy needed to to break bonds in reactants.

Endothermic reactions take in energy. The energy required to break bonds in reactants is more than the energy released by the bonds formed in products.

Every reaction can be written in the reverse sense. The energy changes for the forward and reverse reactions have the same numerical value but opposite signs.

The energy change that accompanies a reaction is called the heat of reaction. This is also called the enthalpy of reaction. The symbol for enthalpy is "H". A change in enthalpy is a delta H, DH.

The sign of the enthalpy change is negative (-) for exothermic reactions. this means the molecules, atoms and bonds "lost" energy to the surroundings.

Endothermic reactions have a positive (+) sign for the enthalpy change. This means the molecules, atoms and bonds "gain" energy from their surroundings.

The Law of Conservation of energy:

The law of conservation of energy says the amount of energy in the universe is constant. Energy is only transfered in chemical reactions .

This means the amount of energy that exists today is essentially the same amount of energy that existed thousands and millions of years ago. The distribution of energy is changing. The pockets of high energy intensity like stars are gradually cooling off. Billions and billions of years from now the hot spots will be cold. The universe will have a nice uniform temperature. When all the energy in the universe is uniformly spread around the average temperature will be 1 or 2 degrees Kelvin. That is -270 ^oC. Some people call this "heat' death. Pretty cold, huh?

Covalent bonds store energy. This is a form of potential energy. The energy change for a reaction equals the energy released when bonds are formed in products plus the energy needed to break bonds in reactants. The total is negative for exothermic reactions and positive for endothermic reactions.

DH = (DH for bonds formed in products) + (DH for bonds broken in reactants)

Example:

What is the energy change for the reaction between hydrogen and fluorine? This was considered by NASA as a fuel system for rocket boosters.

H₂(g) + F₂(g) ---> 2 HF(g)

DH = 2 mole x (H:F bond energy) +1 mole x (H:H bond energy) + 1 mole x ( F:F bond energy)

DH = 2 mole x (-135.2 kcal /mole) +1 mole x (103 kcal /mole) + 1 mole x (36 kcal/mole)

D H = 2 x (-135.2 kcal /mole) + 1 x (103 kcal /mole) + 1 (36 kcal/mole)

DH =-270.4 kcal + 103 kcal +36 kcal = -131.4 kcal EXOTHERMIC: energy is released

The reaction releases energy and is exothermic -131.4 kcal

Covalent bond	Bond dissociation energy	Covalent bond	Bond dissociation energy
N:::N	226 strong bond	O::O	118.4
N::N	100	F:F	36.8 very weak bond
H:H	103	Cl:Cl	58
H:F	135.2	Br:Br	46.2
H:Cl	102.	I:I	35.6
H:Br	86.8	C:H	98.8
H:I	70.6	C:C	83.0
S:S	63.6	C:F	116.