IB SL Chemistry - Reactivity 1.2 + 1.3

Bond Enthalpy (BE)

The average energy required to break one mole of bonds by homolytic fission in one mole of gaseous covalent molecules under standard conditions.

True or False: Homolytic fission (the breaking of bonds) is an exothermic process.

False- due to the fact that bond breaking occurs, energy is absorbed from the surroundings and into the system.

What is the equation to calculate the enthalpy change of a reaction with bond enthalpy data?

ΔH = ∑(BE of bonds broken) - ∑(BE of bonds formed)

True or False: Bond enthalpies are averaged values, so the calculated bond enthalpy will differ from its actual values.

True

True or False: Bond enthalpy data takes intermolecular forces into account.

False

What is Hess’s Law?

An application of the conservation of energy law: Regardless of the route by which a chemical reaction proceeds, the enthalpy change will always be the same, as long as the initial and final states of the system are the same.

What are the added reactants and products in combustion reactions?

Burned in oxygen. Produces CO₂ and H₂O.

What is the product in a combustion reaction between a reactive metal and oxygen?

A metal oxide

True or False: In a combustion reaction between a reactive metal and oxygen, the metal is oxidized and the oxygen is reduced.

True

What is the product in a combustion reaction between a non-metal and oxygen?

A non-metal oxideTrue

True or False: The saturated hydrocarbon Alkane is relatively inert.

True- because of their low polarity bond and strong covalent C-C + C-H bonds.Vo

Volatility

The tendency of a substance to change state from a liquid to a gas.What

What is the relationship between carbon chain length, boiling point and volatility?

As the length of the carbon chain increases in the alkane series (stronger LDFs), the boiling point also increases, and therefore volatility decreases.

True or False: Complete combustion is an endothermic reaction.

False

Biological carbon fixation and an example

The production of organic compounds from carbon dioxide.

Ex: Photosynthesis.

When oxygen is the limiting reactant…

…there is an incomplete combustion of the fuel.

What species are produced in incomplete combustion reactions?

Carbon monoxide (CO) or soot (C)

True or False: Incomplete combustion reactions can occur simultaneously with complete combustion reaction in different ratios.

True- incomplete reactions are less exothermic than complete reactions.

What greenhouse gases are examples of the consequences of using fossil fuels?

A greenhouse gas traps heat energy inside the Earth’s atmosphere. Ex: CO₂, CH₄, nitrous oxide, water vapor and hydrofluorocarbons.

What is carbon dioxide’s role in the greenhouse effect?

A CO₂ molecule will absorb infrared radiation resulting in the vibration of its bonds. Then, the molecule will emit in infrared radiation back into the atmosphere or towards the Earth.

Specific energy (with regards to fuel)

The amount of heat energy released per mass of the fuel.

Renewable energy sources

Sources that depend on the heat or motion of the Earth or the sun’s radiation.

List key attributes of primary (volatic) cells

• Converts chemical energy to electrical energy

• Mostly irreversible

• Used for low-current applications

• Has finite amounts of reactants within a cell

List key attributes of secondary cells

• Converts chemical energy to electrical energy

• Reversible reaction

• More environmentally friendly

• Infinite amounts supplied from an external source

List key attributes of hydrogen fuel cells

• Reactants: H₂ and O₂ gas as fuel

• Products: Water, electricity and heat

• Proton Exchange Membrane (PEM)

• Clean energy source, but H₂ gas is not abundant on Earth

What is the purpose of PEM in a hydrogen fuel cell?

Selectively allows hydrogen ions to diffuse between the cathode and anode but prevents the passage of other ions, molecules or electrons between these electrodes.

Electrolysis of water

The process of using electricity to split water into O₂ and H₂ gas. (The reversible process that occurs in a hydrogen fuel cell.)

Direct-Methanol Fuel Cell (DMFC)

The source of the hydrogen ions is methanol rather than hydrogen gas. (Note: Methanol is “cleaner” than hydrogen because of its smaller environmental impact and has a far greater energy density tan hydrogen gas.)