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CHEM2 REVISION
ENERGETICS

Enthalpy ­ energy per mole

ENDOTHERMIC

Endothermic describes a reaction in which heat energy is taken in as the reactants change to products; thus
the temperature drops

The products are less stable than the reactants; they have more energy

H = +ve

EXOTHERMIC

Exothermic describes a reaction…

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Standard enthalpies of combustion almost always have negative values, as burning a substance in oxygen is
almost always exothermic

BOND DISSOCIATION ENTHALPY

The energy required to separate completely the atoms in one mole of covalent bonds

MEAN BOND ENTHALPIES

H = bonds broken ­ bonds made

Mean bond enthalpy means…

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KINETICS


COLLISION THEORY


COLLISION FREQUENCY

If a chemical reaction is to take place between two particles, they must collide first

The number of collisions between particles per unit time is known as collision frequency


COLLISION ENERGY

Unsuccessful collisions happen when the colliding species do not have enough energy to break…

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PARTICLE SIZE

If a sample of a solid is crushed into smaller pieces, the surface area of the solid will increase

More of the solid particles are exposed to the other reactants, and there is more likely to be a collision;
collision frequency increases


TEMPERATURE


COLLISION ENERGY

The particles have…

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EQUILIBRIA


DYNAMIC

The forward and reverse reactions are both taking place simultaneously and at the same rate

EQUILIBRIUM

The reactions take place at the same speed; the concentration of all reactants and products remain
constant

POSITION OF EQUILIBRIUM

Equilibrium lying to the left means there are more reactants than products…

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WHEN THE TEMPERATURE IS INCREASED, THE SYSTEM WILL MOVE IN THE ENDOTHERMIC DIRECTION IN ORDER TO DECREASE THE
TEMPERATURE. WHEN THE TEMPERATURE IS DECREASED, THE SYSTEM WILL MOVE IN THE EXOTHERMIC DIRECTION IN ORDER TO INCREASE THE
TEMPERATURE



PRESSURE

If pressure is increased, equilibrium moves to reduce it by moving…

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CATALYST

Catalysts increase rate of reaction without affecting position of equilibrium; the reaction therefore takes
place in the presence of a V2O5 catalyst, which ceases to be effective at very low temperatures, placing a
further constraint on the minimum temperature which can be used

Optimum conditions are 400° and a…

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Chosen temperature is 300°C and chosen pressure is 65atm, and concentrated phosphoric acid catalyst
used

Yield achieved is only 5% but reactants can be recycled

PRODUCTION OF METHANOL

CO(g) + 2H2(g) CH3OH(g) H forward reaction = exothermic

Best yield obtained by using low temperature and high pressure

Compromise conditions of…

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Oxidation Is Loss (of electrons)

Reduction Is Gain (of electrons)

OXIDATION NUMBERS

Oxidation numbers (or states) tell you how many electrons an atom has lost or gained

Oxidation occurs when the oxidation state of an element increases (becomes more positive)

Reduction occurs when the oxidation state of an element decreases…

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THE RULES FOR WRITING HALF EQUATIONS

Only one element can gain or lose electrons
The half equation must balance for atoms
The equation must balance for charge (by adding e- or H+)

If your constructed half equation doesn't satisfy all these rules, rewrite it

We can add things to our…

Comments

Thivakar

Hey there, this is some really good notes. Thanks for uploading it :) 

fboco

hey 

Awesome notes thanx

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