ALL OF OCR AS CHEMISTRY AS

ALL OF OCR CHEMISTRY SUMMED UP IN EASY REVISION NOTES

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Chemistry Alevel: Chemical Energetics
Enthalpy changes
Exothermic and endothermic reactions
Chemical reactions can either release energy to their surroundings, exothermic, or energy
can be transferred to them from the surroundings, endothermic.
Energylevel profile diagrams.
Exothermic reactions are most common,
however, an important example of an
endothermic reaction is photosynthesis in
plants, where the energy supplied is from
sunlight.
Law of conservation of energy: Energy
cannot be destroyed or created but only
transferred from one form to another. The
total energy of a system of reacting chemicals
and surroundings remains constant.
Enthalpy change is the term used to describe
the energy exchange that takes place with the
surroundings at a constant pressure and is
given the symbol H.
Enthalpy is the total energy content of the
reacting materials. It is given the symbol, H.
H = H products H reactants
The units are kilojoules per mole (kJmol1)
An exothermic enthalpy change is always given a
negative value, as energy is lost to the
surroundings.
H = xkJmol1
An endothermic enthalpy change is always given a positive value, as the energy is gained
by the system from the surroundings.
H = + ykJmol1.
Standard enthalpy changes: standard conditions
If we are to compare the enthalpy changes of a various reactions we must use standard
conditions, such as known temperatures, pressures, amounts and concentrations of
reactants or products.

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The standard conditions are:
A pressure of 100kilopascals (102kPa)
A temperature of 298K (25oC)
Reactants and products in physical states, normal for the above conditions.
A concentration of 1.0mol dm3 for solutions.
The o sign indicates standard conditions.
Standard enthalpy change of reaction
Hor
The standard enthalpy change of reaction is the enthalpy change when the amounts of
reactants shown in the equation for the reaction, react under standard conditions to give
the products in their standard states.…read more

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Chemical reactions can either release energy to the surroundings, exothermic or energy
can be transferred from the surroundings, endothermic.
The law of conservation of energy states that energy cannot be destroyed or created.
Enthalpy change is the term used to describe the energy exchange that takes place with
the surroundings at constant pressure. H is the symbol.
H = H products H reactants
The units for enthalpy change are Kilojoules per mole.…read more

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Measure temperature change using at thermometer that reads to at least 0.2oC
accuracy.
4. Calculate energy transfer using: Energy transfer = mcT (joules)
Hess' Law
Energy cycles
Consider the reaction:
A + B C + D
Here we are only observing one possible reaction route from the reactants A and B to
the products C and D. In fact there may be more than one possible route for this
reaction to take.…read more

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H3 = Hc o Hydrogen = 285.8 kJmol1
H4 = Hc o Methane = 890.3 kJmol1
We can also use Hess's law to help calculate the average bond energy for
CH in
H2 = 74.8 kJ mol1
H3 = +715 kJ mol1
H4 = +218 kJ mol1
H1 = H3 + 4H4 H2
H1 = +715 + 4(218) (74.8) = + 1661.8 kJ
mol1
4 CH bonds in methane therefore:
1 CH bond = 1661.8/4 = +415.…read more

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For a reaction where there is an increase in the number of moles from reactants to
products, increasing the pressure moves the equilibrium to the left.
Where there is a decrease in the number of moles from reactants to products,
increasing the pressure moves the equilibrium to the right. The equilibrium constant
remains the same.
The effect of temperature changes on equilibrium
The change that takes place when temperature is changed depends upon whether the
forward reaction is exothermic or endothermic.…read more

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A reversible reaction is one where there is a forward and backwards
reaction occurring:
aA + bB cC + dD
The double arrow signifies a reversible reaction.
If in the above reactions the concentrations of the reactants and products does
not change, although the reaction is still in progress, then the forward rate
must equal the backward rate.
A situation known as dynamic equilibrium is achieved.…read more

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Any dynamic equilibrium can be described in terms of its equilibrium constant,
Kc.
The equilibrium constant is the product of the molar concentrations of the
products raised to the power of the coefficient in the stoichiometric equation,
divided by the product of molar concentrations of the reactants, each raised to
the power of its coefficient in the stoichiometric equation.
For the above reaction,
[ ] represents concentration of species.…read more

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Comments

Aamina2


Helpful notes!

katrina

These are so good! Thank you! 

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