CALCULATION OF ENTHALPY CHANGES

Molar Enthalpy

the enthalpy change for 1 mole of a substance associated with a chemical, physical or nuclear change

the units will J or kJ per mole

Example

A common refrigerant , Freon-12, has a molar mass of 120.91 g/mol. The molar enthalpy to vaporize Freon is 34.99 kJ/mol. If 500 g of Freon is vaporized, what is the enthalpy change?

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Specific Heat Capacity

the quantity of heat needed to change the temperature of 1 g of substance by 1˚C

the symbol is C the units are J/g˚C or Jg-1˚C-1 EX. C water = 4.18 J/g˚C

 

Specific Heat Capacity

the quantity of heat needed to change the temperature of 1 g of substance by 1˚C

the symbol is C

the units are J/g˚C or Jg-1˚C-1

EX. C water = 4.18 J/g˚C

Specific heat capacity is a measure of how ‘easily’ a substance changes temperature. A substance with a high heat capacity can absorb more energy without changing temperature than a substance with low specific heat.

 

Heat Transfer Problems

ΔH= mCΔT

Enthalpy Change (J)

Mass of the substance changing temperature (g)

Specific Heat Capacity (Jg-1°C-1 )

Change in Temperature Tf –Ti (°C)

Heat Transfer Problems

When Tf is greater than Ti :

-energy is absorbed- the process is ENDOthermic - ΔH is positive

ΔH= mCΔT

Enthalpy Change (J)

Mass of the substance changing temperature (g)

Specific Heat Capacity (Jg-1°C-1 )

Change in Temperature Tf –Ti (°C)

Heat Transfer Problems

When Tf is less than Ti :

-energy is released- the process is EXOthermic - ΔH is negative

ΔH= mCΔT

Enthalpy Change (J)

Mass of the substance changing temperature (g)

Specific Heat Capacity (Jg-1°C-1 )

Change in Temperature Tf –Ti (°C)

Examples

1) A 150 g sample of Pb at 100.0 °C is plunged into a beaker containing 50 g of water at 22.0 °C. The final temperature of the mixture is 28.8 °C.

a) What is the energy change for the water?

b) How much energy is lost by the Pb? c) Calculate the CPb

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Examples

2) While running an adult expends 5.0 x 105 J/mile. If this energy could be transferred as heat to a beaker of water, how many grams of water could be heated from 25.0 °C to 100.0 °C?

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CALORIMETRY

Calorimeter – an insulated device used to determine temperature changes during a chemical or physical process.

the calorimeter is part of the surroundings

 Two Kinds: 1) Simple 2) Bomb

Simple Calorimetry

the process occurs at constant pressure

RECALL that ΔH is + for an ENDOTHERMIC change and – for an EXOTHERMIC

ΔHobserved = mCΔT = - ΔH rxn

Assumptions

for a dilute aqueous solution, use Cwater = 4.18 J/g°C as the heat capacity for the solution

1 mL of solution = 1 g of solution

no heat is transferred to the outside of the calorimeter( i.e., the calorimeter is perfectly insulated)

Diagram

insulated lid

thermometer

insulated cup

reaction mixture

Example

1) 5.00 mL of 1.0 mol/L NaOH and 5.00 mL of 1.0 mol/L HCl are mixed in a simple calorimeter. The initial temperature of each substance is 23.0 °C. After mixing, the temperature rose to 27.5 °C.

a) What is the enthalpy change for this rxn?b) Calculate the energy change per mole of

HCl.On Board

 

Example

2) 1.50 g of NH4NO3 is added to 35.0 g of water in a simple calorimeter. The initial temperature of the water is 22.7°C and the final temperature of the mixture is 19.4 °C. Calculate the enthalpy change per mole of salt.

 

Bomb Calorimetry

Bomb Calorimetry

Not Really !

Bomb Calorimetry

the process occurs at constant volume used to measure the heat released in a

combustion reaction

Diagram

Bomb Calorimetry

the process occurs at constant volume used to measure the heat released in a

combustion reaction

NOTE: q is used in place of ΔH for an energy change at constant volume

q observed = C’ΔT = - q

combustion

J °C -1

Example

A food manufacturer wishes to know the number of calories per serving of a new dessert. They hire a chemist at the Dep’t of Consumer Affairs to help them out.

a) The chemist places a 1.00 g sample of the dessert in a bomb calorimeter. The heat capacity of the calorimeter is 8.151 kJK-1. The temperature increases by 4.937 °C. Calculate the number of food calories (kcal) in the sample of dessert.

b) A single serving of the dessert weighs 30.00 g. How many food calories are in the dessert?

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