Developing Fuels

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Exothermic Reaction

Back 1

A reaction that gives out energy and heats the surroundings.

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Endothermic Reaction

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A reaction that takes in energy and cools the surroundings.

Front 3

3 things for Standard conditions:

Back 3

- 298K / 25 degrees C

- A standard pressure of 1 atmosphere (atm)

- A standard concentration of 1 moldm -3 for solutions.

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Standard States

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The physical state of a substance under standard conditions. This may be a pure solid, liquid or gas.

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Standard Enthalpy Change for a Reaction

∆rHѲ298

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The enthalpy change when one molar quantities of reactants as stated in the equation react together under standard conditions.

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Kelvin (K)

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273K = 0 degrees C.

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Standard Enthalpy Change of Combustion

∆cHѲ298

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The enthalpy change that occurs when 1 mole of a substance is burnt completely in excess oxygen under standard conditions in standard states.

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Standard Enthalpy Change of Formation

∆fHѲ298

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The enthalpy change when 1 mole of a compound is formed from its elements under standard conditions in standard states.

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Standard Enthalpy Change of Neutralisation

∆neutHѲ298

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The enthalpy change when 1 mole of hydrogen ions react with 1 mole of hydroxide ions to form 1 mole of water under standard conditions and in solutions containing 1 moldm-3.

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Specific Heat Capacity of a substance (c)

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The amount of energy needed to raise the temperature of 1g of a substance by 1K.

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Aromatic Compounds

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Compounds that contain one or more benzene rings.

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Aliphatic Compounds

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Compounds that do not contain any benzene rings.

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Functional Group

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Modifiers that are responsible for the characteristic chemical reactions of molecules.

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Saturated

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Hydrocarbons containing the maximum number of hydrogen atoms possible, with no double or triple bonds between carbon atoms.

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Molecular Formula

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The number of atoms of each element in a compound.

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Homologous Series

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A series of compounds in which all members have the same general molecular formula.

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Average Bond Enthalpy

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The average quantity of energy needed to break a particular bond.

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Cracking

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Any reaction in which a larger molecule is made into smaller molecules.

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Unsaturated

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Any organic compound that has a double or triple bond between carbon atoms.

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Hess's Law

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The enthalpy change for a particular reaction will always be the same and is independent of the pathway taken providing that starting and finishing conditions are the same.

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Catalyst

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A substance that speeds up a reaction but can be chemically unchanged at the end. A catalyst always lowers the activation enthalpy and can provide an alternative pathway for the reaction.

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Catalysis

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The process of speeding up a chemical reaction using a catalyst.

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Homogeneous Catalysis

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When the reactants and products are in the same physical state as the catalyst.

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Heterogeneous Catalysis

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When the reactants and products are in different physical states to the catalyst.

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Catalyst poison

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Poison adsorbs (sticks) more strongly than reactants

So reactant molecules can't bond with the surface of the catalyst.

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Electrophile

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A species which can act as an electron pair acceptor and is attracted to a region of high electron density such as a double bond. They are usually + or delta +

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Carbocation

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An ion with a positively-charged carbon atom.

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Addition reaction

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A reaction where two or more molecules react to form a single larger molecule.

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Polymerisation

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Small molecules called monomers join together to produce long chain polymers.

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Hydrocarbon

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A compound which contains hydrogen and carbon atoms ONLY.

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Arene

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An aromatic hydrocarbon.

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Test for Unsaturation

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Shake the substance with bromine water.

The bromine is decolourised (red-brown to colourless) if the substance is unsaturated.

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Structural Isomer

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Same molecular or chemical formula

Different structural arrangement of atoms

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Stereoisomerism

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Occurs in compounds with C=C double bond.

Owing to the lack of free rotation (locked by the pi bond)

E/X pair where each C of the C=C has a hydrogen atom

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Elastomer

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A polymer which can be deformed and later return to its original size and shape

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Steps in Heterogeneous Catalysis

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Gas molecules are adsorbed (stick) onto the catalyst surface.

Bonds in reactants weaken and break

New bonds and molecules (products) form

Products diffuse away from the catalyst surface.

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Bond-breaking

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Endothermic

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Bond-making

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Exothermic

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Why a reaction is exothermic or gives out heat?

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Bonds broken in reactants - energy in

New bonds formed in products - energy released

Greater energy released than taken in so overall there is an output of energy = exothermic.

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Why experimental enthalpy values are different from those in a data book?

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Bond enthalpies are only average values and vary between molecules

Chemicals are not in their standard states in the given example.

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Sigma bond

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The end-on overlap of electron orbitals (s with s, s with p, p with p) so that the shared pair of electrons is an area of increased electron density along the internuclear line between the atoms.

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Pi bond

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The lateral overlap of p electron orbitals so that the shared pair of electrons consists of two areas of increased electron density above and below the internuclear line between the atoms.