CALORIMETRY

Calorimetry is the technological process of measuring energy changes of an isolated system called a calorimeter

Thermochemistry

The study of energy changes by a chemical system during a chemical reaction

Isolated System

Calorimetry deals with an isolated system, where matter NOR energy can move in or out

Laws of Thermodynamics

1st Law = energy cannot be created or destroyed but transferred

2nd Law = heat flows from hot to cold, and heat loss is equal to heat gained

-mcΔt = nHm

Heat lost = heat gained

“system” “calorimeter”

nHm = -mcΔt

Ep = -Ek

Within the calorimeter there is a known volume of liquid (usually water) in which the chemical reaction to be studied occurs

Energy (as heat) is transferred between the chemical system and the water

Calorimetry requires careful measurements of masses and temperature changes

the chemical energy lost by the chemical system is calculated by the thermal energy gained by the surroundings

remember: thermal energy is the total kinetic energy

q = mcΔt

remember potential energy of a reaction is the enthalpy

ΔH = nHm

Assumptions in Calorimetry

styrofoam is considered a perfect insulator…

energy lost to the calorimeter is ignored

only the water releases or absorbs heat…

so we only need to use the specific heat capacity of water (4.19 kJ/kgC)

volume of water is equal to (and interchangeable with) the mass of water…

mL = g

L = kg

these assumptions allow the idea that energy gained by the calorimeter water equals the energy lost by the reaction and vice versa

nHm = mcΔtcalorimeter

Example 2

When excess zinc is added to 50.0mL of 0.25mol/L aqueous copper(II)sulfate, the calorimeter warms by 12.5°C. What is the molar enthalpy of reduction of aqueous copper(II) ions?

Answer:

-209500J/mol

Example 2

When 50.0mL of 1.0mol/L NaOH, initial temperature 17.4°C is mixed with 50.0mL of 1.0mol/L HCl, initial temperature 24.2°C. What is the molar enthalpy change for the neutralization of the base?

Answer:

28492J/mol

Bomb Calorimetry

Bomb Calorimetry

professionals use a bomb calorimeter to measure the energies released and absorbed by reactions.

bomb calorimetry minimizes heat loss with superior insulation and very exact temperature measurements giving more accurate enthalpies of reaction

Heat Change of the

Calorimeter Includes water, bomb, thermometer stirrir, container…

the masses and heat capacities of all the components of the bomb calorimeter are constants…

therefore we can use the formula

nHm = -CΔT

Example 1

Consider a 1.00g sample of benzoic acid burning in a bomb calorimeter. The initial temperature is 25.00°C and the heat capacity of the calorimeter is 5.43kJ/°C. The combustion causes the temperature of the calorimeter to rise to 29.86°C. Using this data, what is the molar enthalpy of the combustion of benzoic acid?

Answer:

-3223kJ/mol

Example 2

A reaction in a bomb calorimeter absorbs 22.4kJ of heat energy. What is the initial temperature of the calorimeter if its final temperature was 24.22°C and its heat capacity is 28.7kJ/°C.

Answer:

+25C