Reactivity 1.1 - 1.4 : What drives chemical reactions?

Heat

measure if the total energy of a substance

Temperature

measure of the avergae kinetic energy of the particles ina subtance

Enthalpy change △H

heat absorbed/released at constant temperature (Break-Make)

Standard enthalpy change △Hº

The enthalpy transferred under standard conditions: 100kPa + 298 K (Products - Reactants)

Average bond enthalpy

the energy required to break one mole of bonds in a gaseuous molecule averaged over similar compounds

Bond breaking is

endothermic

Bond making is

exothermic

Enthalpy change of formation △H_f

Enthalpy change when 1 MOL of a compound is formed from its elements in their standard states under standard conditions

Enthalpy change of Combustion △H_c

Enthalpy change when 1 MOL of a substance is complteley burned in oxygen under standard conditions

△Hº_c

Reactants - Products

Born-Haber cycle

energy cycle for formation of an ionic compound

Lattice enthalpy change △Hº_lat

the enthalpy hange when 1MOL of a solid ionic compound is broken down into gaseous ions under standard conditions

△Hº_lat equation for MgCl2

MgCl_{2 (g)} → Mg²⁺_(g) + 2Cl^- _(g)

E. Atomisation △Hº_atom

enthalpy change when 1MOL of gaseuous atoms is formed from an element in its gaseous state under standard conditions

△Hº_f formula

Biofuel

fuel whose energy is obtained from biological carbon fixation (like photosynthesis): Bioethanol, Biodiesel, and Biogas

Biodiesel?

renewbale vegetable oil → esters of methanol (fuel)

Bio Gas?

organic matter/waste → microorganisms in the absence of oxygen → Methane and CO2

Benefits and Negatives of Biofuels

+ves:

• renwable + readily available

• crops regrown : sustainable

• reduces landfill waste

-ves:

• ↓ land for food

• less specififc energy than fossil fuels

• fertilisers

Fuel cell

converts chemical potential energy stored in a fuel into electrical energy

Proton Exchange membrane / Hydrogen-oxygen fuel cell:

Methanol Fuel cell

Alkaline fuel cell

Benefits and negatives of hydrogen fuel cells vs Methanol fuel cells

Pem +ves Water is the only product → non-toxic reaction occurs at room temp (efficient) No nitrogen oxides (NOₓ)	M +ves easier to store + transport works at lower pressure and temp can be made from biofuel
Pem -ves hydrogen is flammable →safety thick storage →expensive hydrogen source unsustainable	M -ves toxic expensive catalyst produces CO2

Factors that affect the entropy of a system:

• temperature

• change in state

• dissolving/crystalising

• change in no. of gaseuous reactants

Spotaneous process:

process that occurs without adding energy other than the energy required to overcome the energy barrier

• △S_total= +ve

• △S_{total =}

system - surroundings

Spontaeinty and Gibbs free energy

If ΔH ....	And if ΔS ....	Then ΔG is	Spontaneous?	Because
is negative < 0 exothermic	is positive > 0 more disorder	always negative < 0	Always	Forward reaction spontaneous at any T
is positive > 0 endothermic	is negative < 0 more order	always positive > 0	Never	Reverse reaction spontaneous at any T
is negative < 0 exothermic	is negative < 0 more order	negative at low T positive high T	Dependent on T	Spontaneous only at low T TΔS < H
is positive > 0 endothermic	is positive > 0 more disorder	negative at high T positive low T	Dependent on T	Spontaneous only at high T TΔS > H

How do fossil fuels form?

from the reduction of biological compounds that contain carbon, hydrogen, nitrogen, sulfur and oxygen →formed > million years ago in anerabic conditions

How does coal form?

• giants plants die in swamps

• trapped under layers of water and sediment

• coal is formed under ↑ heat and ↑ pressure

Rank the coals on carbom content (low to high)

1. Peat

2. Lignite

3. Sub-bituminous coal

4. Bituminous coal

How does Natural gas/oil form?

• Sea creatures die and are buried on the sea floor

• remains burried deeper and ↑ heat + ↑ pressure forms oil + gas

• oil+ gas reached by drilling

Crude oil/ petroleum

complex mixture of straight chain, branched, cyclic and aromatoc hydrocarbons

What is natural gas made up of?

Mainly Methane but also CO2

Coal +ves and -ves:

+ves

• cheapish

• ↑ specific energy and energy density

• can be converted into liquid fuels and gases

-ves

• finite resource

• produced CO2

• produced SO2 (acid deposition)

• mining = env. damage

Petroleum +ves and -ves:

+ves

• cheap

• ↑ specific energy and energy density

• easy to transport (gasoline)

• feedstock for petrochemicals

-ves

• non-renewable

• produces CO2

• drilling for and transporting →env. damage

• uneven distribution worlwide

Clean fuel +ves and -ves:

+ves

• clean fuel

• ↑ER specific energy than coal/oil

• easy to transport: pressurised containers

• no acid deposition

-ves

• non-renewable

• CO2

• ↓ ER energy density than coal/oil

• uneven distribution

Stabdard Gibbs free energy of formation:

change in Gibbs free energy when 1MOL of a compound is formed from its elements in their standard states under standard conditions