Unit AThermochemical Changes

The study of energy changes by a chemical system during a chemical reaction is called thermochemistry.

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

Recall that an isolated system does not exchange matter or energy with its outside environment.

No calorimeter is 100% sealed and insulated, so they only approximate an isolated system.

Analyzing Energy Changes

Thermal energy is the total kinetic energy of the entities of a substance.

Heat refers to the form of energy that is transferred from an object at a higher temperature to an object at a lower temperature.

Q = mcΔt

Q = quantity of thermal energy (J)

m = mass (g)

c = specific heat capacity (J/g·°C)

Δt = temperature change (°C)

The S.I. unit for energy is the joule (J).

The specific heat capacity of a substance is the quantity of energy required to raise one gram of a substance by one degree Celcius.

The change in temperature of the water is used to determine the quantity of heat energy released or absorbed by the chemical system.

Heat Transfer and Enthalpy Change

Kinetic energy (energy of motion) of a chemical system includes:

• moving electrons within atoms

• the vibration of atoms connected by chemical bonds

• the rotation of molecules

• the translation of molecules (moving from one place to another)

The temperature of a chemical system is a measure of the average kinetic energy of the entities that make it up.

So a change in temperature means a change in kinetic energy.

Potential energy (stored energy in chemical bonds) includes:

• covalent and/or ionic bonds between the entities (intramolecular)

• intermolecular forces between entities

covalent bondinghydrogen bonding

The enthalpy (H) of a system is the sum of the kinetic and potential energy within it.

WE CANNOT MEASURE ENTHALPY DIRECTLY!

We can calculate the quantity of heat that is released or absorbed by the surroundings of a chemical system by measuring a change in temperature of the surroundings.

ΔH = Hproducts – Hreactants

An enthalpy change, ΔH, is the difference between the enthalpy of the products and the enthalpy of the reactants for a system under constant pressure.

ΔH = Q (system) (calorimeter)

The change in potential energy of the chemical system equals

the change in kinetic energy of the surroundings.

Zn(s) + 2 HCl(aq) → H2(g) + ZnCl2(aq)

For an exothermic reaction (chemical system releases heat), ΔH (the enthalpy change) is negative.

For an endothermic reaction (chemical system absorbs heat), ΔH (the enthalpy change) is positive.

Molar Enthalpies and Calorimetry

Enthalpy of reaction (or enthalpy change of reaction) refers to the energy change for a whole chemical system when reactants change to products.

Δr H = n Δr Hm

Δr H = enthalpy of reaction (kJ)

n = chemical amount (mol)

Δr Hm = molar enthalpy of reaction

(kJ/mol)

Molar enthalpy of reaction is the enthalpy change in a chemical system per mole of a specific chemical in a system at constant pressure.

The molar enthalpy of combustion of propane (producing water vapour) is –2 043.9 kJ/mol.

In a calorimeter, the change in enthalpy of the chemical system is equal to the change in thermal energy of the calorimeter.

n Δr Hm = mcΔt

ΔH = Q

Your task is to design, build and use a device to determine the gravimetric change in enthalpy of combustion of corn chips.

In your lab report, you must include:

• A labeled diagram of your device.

• A brief summary of how it works.

• A clear step-by-step procedure of your lab.

• All data collected in the lab.

• Calculations using proper symbols, conventions and significant digits.

Read pgs. 485 – 493

pgs. 487, 492 Practice #’s 1 – 13

pg. 494 Section 11.2 Questions #’s 1 – 7