Chemistry - Enthalpy and Fuels

Types of energy

-Kinetic: Gaseous molecules have high motion/kinetic energy
-Potential: Forces of attraction/repulsion between electrons and positively charged nuclei's. The potential to form a bond with something

Enthalpy

 

-stored chemical energy

-when enthalpy changes that means heat energy is being exchanged in some form. In reactions there are two steps:
 1. Breaking the bonds: This is the activation energy, it absorbs energy
 2. Forming the bonds: This releases energy

Types of reactions

-Endothermic
-Exothermic

Exothermic

-More enthalpy is being released as heat
-It will make a thermometer hotter/increase change in temp as it releases heat 

Endothermic

-More heat is being absorbed for the products to increase enthalpy, making the surroundings colder.
-It will make a thermometer colder/absorb heat as energy to do the reaction
 

Fossil fuels

-The formation of coal, oil and natural gas from ancient organic matter that retains it's chemical energy and can then be used
-The burning of such fuels is an exothermic reaction, meaning they are burned because they release energy

Coal

-From wood and plant material, primarily carbon. Burned to produce carbon dioxide

Natural gas

-The burning of gasses (particularly methane, but also ethane and propane)
-Propane and butane can be liquified to become high quality LPG (liquefied petroleum gas)

Crude oils/Petroleum

 -An oil that contains multiple hydrocarbons (mostly alkanes)
 and by using fractional distillation can be separated as petrol, diesel, kerosene, LPG e.g.

Cons of fossil fuels

-Limited availability/amount
-Cost of production

-Not sustainable and renewable

Biofuels

-a fuel produced from biomass

-Bioethanol

-Biogas

-Biodiesel

Bioethanol

-Comes From: When yeast converts to glucose and produces energy

-Useful For: Useful for E10 petrol, only 10% ethanol. Produces less carbon dioxide and reduces petrol consumption.

Biogas

-Comes From: Decomposing organic matter, where the bacteria digesting it releases biogas

-Useful For: Small-scale heating and the generation of electricity, it's organic matter is good fertiliser

Biodiesel

-Comes From: The reaction between vegetable oil/animal fat and an alchohol/methanol

-Useful For: Diesel vehicles that need to do heavy work as it's fuel consumption is very optimal.

Thermochemical equations

You are solving for the enthalpy change per mole of a molecule
-Unit: Change in Enthalpy = kJ mol^-1
-The molecule specified (that the enthalpy change is desired) always has to have a coefficient of 1. E.g. for H2
-2H2 + O2 -> 2H2O would turn into H2 + (1/2)O2 -> H2O
-So that the enthalpy change for H2 is analyzed by a per mole quantity

Change of state

-Just put enthalpy in as a value

-E.g. For Ice -> Water/ H2O (s) -> H2O (l)
-If it has an enthalpy change of 6 kJ mol^-1 then add that to equation
-H2O (s) + 6 kJ mol^-1 -> H2O (l)

Calculation for fuels

-Usually measured at 25 degrees for a constant pressure

-The heat of combustion is the same as the enthalpy change (which is solved at 1 mol)
-So energy released = amount of moles * change in enthalpy

Steps to solve for energy per mole of fuel

 

1.Solve for the amount of moles of the fuel
2.Then multiply that by the energy change of heat of combustion per mole (which will be given)
 

To solve for heat of combustion per gram: Heat of combustion per mole/molar mass = kJ g^-1

-Note that molar mass = grams per mole, so by dividing that by amount of energy your getting per mol, you solve for how much energy / gram

-E.g. 10 energy per mol / 5 grams per mol (same substance/mol). So you get 2 energy per gram for that substance
-So heat of combustion per gram * molar mass = heat of combustion per mole