Unit 9 Chemistry - Thermochemistry

Thermochemistry

Study of heat transfer (thermal energy). All physical and chemical changes require a change in energy, either absorbed or released.

Law of Conservation of Energy

Energy cannot be created nor destroyed but is conserved

Heat

Energy transferred between 2 objects due to temperature differences

Which direction does heat flow?

Always flows from hot to cold

When does heat transfer stop?

When thermal equilibrium is reached

How is heat measured?

Measured indirectly through temperature changes

Conduction

Transfer of heat by direct contact between objects

Convection

Transfer of heat by bulk movement of matter in liquids or gases

Radiation

Transfer of heat by electromagnetic waves (light)

Temperature

Measures average kinetic energy of particles

How is temperature measures?

Measures in C or Kelvin (K) (K = C + 273)

Specific Heat

Amount of energy required to raise the temperature of 1g of a substance by 1 C (or 1 K)

What is the unit of measurement for specific heat?

J/g C or J/g•K

Insulator

Transfers heat and energy slowly/poorly. High specific heat values. (Wood, cotton, fiberglass, rubber)

Conductor

Transfers heat and energy easily/more quickly. Low specific heat value. (Copper, steel, iron)

Chemical Potential Energy

• Energy is stored in chemical bonds

• Energy is absorbed when bonds broken

• Energy is released when bonds formed

Enthalpy

Amount of energy lost or gained during a chemical or physical change, abbreviated as ΔH, measured in kj/mol

Endothermic

Process where bonds are broken, which requires energy to be absorbed (+ΔH)

Exothermic

Process where bonds are formed, releasing energy (-ΔH)

Endothermic temperature

In an endothermic reaction, heat flows from the surroundings to the system, making the container cold

Exothermic temperature

In an exothermic reaction, heat flows from the system to its surroundings, making the container warm

System (energy transfer)

Process being studied

Surroundings (energy transfer)

Things & conditions around the system

Universe

System + surroundings

Calorimeter

Insulated container that is used to study heat changes in physical and chemical processes

Hess’s Law

The enthalpy change for a reaction is a sum of its parts

Kinetic Molecular Theory

• Matter is composed of tiny particles

• particles are in constant random motion

• Particles interact w/ each other through attractions and repulsions

Solid intramolecular forces

Dominated by strong intramolecular forces (ionic or metallic)

Solids volume, density, & shape

Definite volume, high density, & shape

Liquid intermolecular forces

Covalent with strong IMF

Liquids volume, density, and shape

Definite volume, medium density, and indefinite shape

Gas intramolecular forces

Dominated by weak intramolecular forces (covalent and weak IMF)

Gas volume, density, and shape

Indefinite volume and shape, low density

Phase changes

Energy is used to change the state by breaking or forming intermolecular forces between molecules

Melting

Solid→Liquid, Endothermic

• Energy required is heat of fusion Δ𝐻𝑓

Freezing

Liquid→Solid, Exothermic

• Heat of solidification Δ𝐻solid

Vaporization

Liquid→Gas, Endothermic

• Enthalpy of vaporization Δ𝐻vap

Condensation

Gas→Liquid, Exothermic

• Heat of condensation, Δ𝐻cond

Sublimation

Solid→Gas, Endothermic

Deposition

Gas→Solid, Exothermic

Does endothermic absorb or release energy?

Absorbs

Does exothermic absorb or release energy?

Releases

What is Activation Energy?

The sufficient amount of energy for a reaction to occur, minimum # required

Transition State/Activated Complex

Unstable arrangement of atoms that exists at the peak of activation energy, half way b/w products and reactants

Catalyst

Provides an alternate pathway with a lower activation energy to speed up the process

What is entropy?

Measure of the molecule randomness/disorder of a system, based on freedom of movement w particles

Gibbs Free Energy

Measures how favorable a process is based on thermodynamic conditions (enthalpy, entropy, and temperature)

Thermodynamically favored

Process occurs w/o external intervention

Thermodynamically unfavored

Process does not occur under the current conditons