Thermal Energy and Phase ChangesThermal Energy and Phase Changes

Which line segment represents a point at Which line segment represents a point at which you would use the heat capacity of which you would use the heat capacity of the liquid to calculate heat exchange?the liquid to calculate heat exchange?

1 2 3 4 5

13%16%

7%

16%

48%1. A->B2. B->C3. D->E4. F->G5. H->J

Enthalpy Changes and Chemical Enthalpy Changes and Chemical ReactionsReactions

H = energy needed to break bonds – energy released forming bonds

Calorimetry is used to measure enthalpy changes Constant Pressure gives H Constant Volume gives E

Hess’ Law- The RulesHess’ Law- The Rules

Enthalpy is a state function.

If a reaction can be written as the sum of two or more reactions, the H for the net reaction is the sum of the H's for the individual steps.

If a reaction is reversed, the sign of H is reversed.

If the coefficients of the reactants/products are multiplied by a number, then the H must be multiplied by that number, as well.

Hess’ LawHess’ Law

If you can add the reactions, you can add the H’s.

Hess’s LawHess’s Law

Hess’ Law- Lab This Hess’ Law- Lab This WeekWeek

Mg(s) + ½ O2(g) MgO(s) H = ???

Constant Pressure Calorimetry (“Coffee Constant Pressure Calorimetry (“Coffee Cup”)Cup”)

Constant Volume Calorimetry (“Bomb”)Constant Volume Calorimetry (“Bomb”)

N2H4 + 3 O2 2 NO2 + 2 H2O

Ereleased = Eabsorbed by water + Eabsorbed by calorimeter

Ewater =

Ecalorimeter =

Total E =

H = energy/moles = 0.500 g N2H4

600 g water

420 J/oC

Calculating Reaction EnthalpiesCalculating Reaction Enthalpies

Use values that have been determined from experiment

Two types of data that can be used Standard enthalpies of formation Bond enthalpies

Standard Enthalpy of FormationStandard Enthalpy of Formation

Standard conditions: pure form, 1 bar pressure, usually at 298K (25°C)

Heat of formation for an element in its most stable form is zero

Standard heat of formation is given in Joules per mole Results in fractional coefficients on occasion

Hrxn H f

products

H f

reactants

Table 5-2, p. 195

Using Standard Enthalpies of Using Standard Enthalpies of FormationFormation

What is the Hrxn for the detonation of nitroglycerin?

How much energy is released when 10g is detonated?

2C3H5(NO3)3(l) 3N2(g)1

2O2(g) 6CO2(g) 5H2O(g)

Bond EnthalpyBond Enthalpy

Remember that bond energy is the amount of energy required to break a bond in a gas phase molecule

Can only use bond enthalpies for reactions in which everything is in the gas phase

When calculating H using bond enthalpies, assume all bonds are broken in the reactants (H= +) and formed in the products (H= -)

Hrxn broken formed

Bond EnthalpyBond Enthalpy

Formation of water

2H2 O2 2H2O

Hrxn broken formed

Chemical Reactions and Enthalpy Chemical Reactions and Enthalpy ChangeChange

C3H8(g) + 5 O2(g) 3 CO2(g) + 4 H2O(g) H = -2045 kJ

What does this mean?

How much energy is released by a 468-g tank of propane?