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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…

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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

The standard enthalpy change of reaction…

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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…

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Measuring enthalpy changes
Enthalpy changes of combustion
The type of apparatus used in the laboratory is shown below:

The following measurements are

1. Mass of cold water (g)
2. Temperature rise of the water (K)
3. The loss of mass of the fuel (g)

We know that it takes…

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3. Measure temperature change using at thermometer that reads to at least 0.2oC
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…

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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…

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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…

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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…

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Any dynamic equilibrium can be described in terms of its equilibrium constant,

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…

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Carbon is unable to take part in ionic bonding since it is not energetically possible to form
either C4+ or C4. It must therefore bond covalently by sharing electrons.

To achieve its maximum valency of 4. The 2s electrons must become uncoupled to give the
electronic structure: 2s1 2px1 2py1…



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