Thermochemistry

Heat

A form of energy

Temperature

The average kinetic energy of the particles in a system

Reasons for Energy gain/loss

(1) something is heading up or cooking down

(2) Something is undergoing a physical change

(3) Something is undergoing a chemical change

• Energy is absorbed when bonds are broken

• Energy is released when bonds are formed

(4) Something is undergoing a nuclear change

Specific heat capacity

• The amount of energy in Joules required to raise one kilogram of a given substance by one degree

• Units: J/ K•kg

• Heat energy : Q

• Equation: Q = mc∆T

Enthalpy

• internal stored entergy

• Enthalpy: H

• Assumption: we are working under constant pressure and do not have to take into expansion or changes in volume

Exothermic vs. Endothermic

• Exothermic:

  • heat gained by surroundings

  • the release of internal stored energy

• Endothermic:

  • heat lost by surroundings

  • the gain of internal stored energy

• Note: You = surroundings (opp. to the system)

Relationship between Enthalpy and Stability

• bonds are formed that are stronger, relasing energy in the process

• lots of energy would be required to perform the oppsoite proccess

• products are more stable

Enthalpy of transformation

• the quantity of heat energy absorbed or released given a change of state

Calorimetry + Limitations

• measuring enthalpy changes

• filled w/ water or solution

• allows us to monitor the “surrounddings” by measuring the temp changes in water

• limitation: assume that the heat capacity of water applies to all aqueous solutions

First law of thermodynamics (law of cons. energy)

• energy cannot be created nor destroyed, only converted from one form to another

• Q_system = - Q_surroundings

Lab Limitations to Calorimetry

• heat lost to/added by environment (unavoidable), mitigated by material of calorimeter

• heat absorbed by calorimeter itself, quantified as a calorimeter constant

Hess’s Law

• In going from a particular set of reactants to a particular set of products, the change in enthalpy is the same whether the reaction takes place in one step or a series of steps.

Ways to Calculate Enthalpy Change

• enthalpy of formation

• bond enthalpy

B

Bond Energy

• the enthalpy change when a specific bond is boren, the process occuring in the gas phase at standard condtitions

Limitations to Bond Energies

• Bond energy values do not take into account any IMF’s that need to be overcome in order for the reaction to happen, or that may release energy when they form.

• these values are typically smaller than the energy change associated with the chemical reaction, but omitting them adds a source of error

Lattice Energy

• the change in energy that takes place when separated gaseous ions that are packed together to form an ionic lattice

• IB definition: the energy that could be required to separate gaseous atoms

Entropy

• is a measure of disorder or randomness

• it referes to the distribution of available energy among the particles, the more ways energy can be distributed, the higher the enthalpy

Gibbs Free Energy

• the energy in a system that is available to do useful work

• it is not the toal energy in a system

Second Law of Thermodynamics

• all changes either directly, or indirectly increase the entropy in the universe

Third law of thermodynamics

• The entropy of a perfectly ordered pure crystalline substance is zero, @ absolute zero

• Therefore, there exists standard entropies

The three main categories of fossil fuels

• Coal = solid

• Crude oil = liquid

• Natural gas = gas

Advantages and Disadvantages of Fossil Fuels

• Advantages:

  • Easy to transport

  • Inexpensive to obtain

  • Supports other industries

  • Abundant in many countries

  • Contribute to econy and jobs

• Disadvantages

  • Mining, transportation and processing are damaging to the environment

  • Burning = harmful to health and environment

  • Non-renewable

Tendency for incomplete combustion

• All fossil fuels are at risk for incomplete combustion if the air to fuel ratio is not optimized

• Consequences:

  • Less energy released per mile

  • Harmful by products ( CO is poisonous)

• Coal has the greatest tendency to undergo incomplete combustion due to the presence of impurities within its solid form

  • The impurities consume oxygen, leading to the rest of the reaction possibly being incomplete

  • Longer chained crude oils need more oxygen than shorter chains

Renewable vs. Non renewable sources

Non renewable :

• Energy that cannot be renewed at a rate equal to or faster than it is consumed

  • Fossil fuels

  • Nuclear

• Renewable: sources that can be renewed faster than it is consumed

  • Solar

  • Wind

  • Hydroelectricity

  • Geothermal

Spontaneity

• the tendency for a physical or chemical change to occur naturally

• It does not refer to the rate at which a chemical reaction occurs

• Can be calculated with Gibbs free energy

Sun + Biofuels

• Our Sun has been the driving force of energy of our planet and all life cycles on Earth

• Without the ability of plants, fungi and organisms to harness and convert the sun into useable energy, life would not exist

• Accomplished through photosynthesis

• Plants can then be used or modified to create biofuels

Plant Materials to Fuels

• Modern biofuels are made from plant material that is transformed via physical and chemical processes to make usable fuels

• Biofuels are made from the breakdown of decomposition of larger organic molecules into combustible short chained hydrocarbons like alkanes, alcohols, and ethyl esters

• Made from things like:

  • Corn

  • Sugar cane

  • Cooking oil

Advantages and Disadvantages of Biofuels

• Advantages

  • Plants that are used to make biofuels must absorb CO2 when performing photosynthesis, offsetting carbon emissions

  • Small chained hydrocarbons are cleaner

  • Synthesis and refinement are less harmful to the environment

  • High energy density

  • Relatively inexpensive

  • Allow us to preserve fossil fuels

• Disadvantages

  • Uses large quantities of plant material which requires a lot of agricultural land

  • Still emit greenhouse gases

  • Taking plant resources that are needed otherwise for basic needs

Enthalpy of Formation

The standard enthalpy of formation is the enthalpy change that occurs when exactly one mole of a compound is formed from its constituent elements in their most stable standard states at specified conditions