AS Chemistry Unit 2 Energetics

Enthalpy Change (DH), Calorimetry, Hess's Law & Bond Enthapies


Enthaply Change (DH)

DH/ Delta H = Heat Change

DH represents a heat energy change as a constant pressure this is called ENTHALPY CHANGE.

Reaction 1:- DH = -100KJ mol-1. 100 is the amount of energy. KJ is the unit of measurement, kilojoules and mol-1 is per mol. The minus shows energy is given out, so this reaction is EXOTHERMIC. An exothermic reaction is a reaction where there is a temperature rise.

Reaction 2:- DH = +50KJ mol-1. 50 is the amount of energy. KJ is the unit of measurement, kilojoules and mol-1 is per mol. The plus shows energy is taken in, so this reaction is ENDOTHERMIC. An endothermic reaction is a reaction where there is a temperature fall.

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Calorimetry:- Measuring how much heat we get when we burn 1 mole.

Heat Energy Equation:- q = mcDT. Q is the amount of heat energy in joules. M is the mass of water heated in grams. C is the constant 8.14. DT is the temperature change.

Example:- 12g of pentane (C5H12) raises the temperature of 100cm3 of water by 10*C.

Constant = 4.2, Carbon = 12, Hydrogen = 1.

1. Work out the heat given to the water. q = mcDT. q = 100 x 4.2 x 10. q = 4200J.

2. Work out how many moles of fuel were used. n = m/mr. 12/22 = 0.167 mol.

3. 1 mole of pentane = energy/moles. 4200/0.167=25149.701J. DHc = -25.21KJ mol-1.

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Bond Enthapies

Definition:- the standard enthalpy of formation, which basically mean the energy change of something being made.

A2 + B2 = 2A-B.

Reactants = Product. During the reaction A-B and A-B go together.

A-A + B-B - Break bonds between the atoms and energy is put in. Energy put in is called BOND ENTHALPY (bond enthalpy). A + B = A-B - Bond is made and energy is given out.

DH = Energy in - energy out.

Enthaply Change = Bonds broken - bonds made.

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Calculating Energy Change Using Bond Enthalpies

Hydrogen reacts with iodine as follows:-

H2 + I2 = 2HI

Calulate the energy change (DH) fr the reaction; given,

H-H = +100KJ mol-1. I-I = +50KJ mol-1. H-I = +200KJ mol-1.

DH = E IN - E OUT. Enthapy Change = Bonds Broken + Bonds Made.

1. E IN:- H-H = 1 x 100 = 100KJ. I-I = 1 x 50 = 50KJ. Total E IN = +150KJ.

2. E OUT:- 2H-I = 2 x 200 = 400KJ. Total E OUT = +400KJ.

DH = +150 - 400 = -250KJ mol-1

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Hess's Law

The enthalpy change during a reaction is independant of the route taken.

The enthalpy change when 1 mole of substance is formed from its element in their standard state under standard conditions.

Standard Conditions - Temperature 25*C or 298K.

- Pressure 1 atm or 100 kpa.

Standard Enthalpy of Combustion

Enthalpy change when 1 mole of compound undergoes combustion in and excess supply of O2 under standard conditions. *All elements have enthalpy of formation = 0. E.g C, H2, N2, S, Cl2.

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Standard Enthalpy of Combustion

1. Write the equation for the reaction being asked about e.g. Methane (CH4).

CH4(g) + 2O2(g) --DH1--> CO2(g) + 2H2O(l).

C(s) + 2H2(g) + 2O2(g) --DH2--> CO2(g) + 2H2O(l).

C(s) + 2H2(g) + 2O2(g) --DH3--> CH4(g) + 2O2(g).

2. Underneath the equation put the element. DHf CH4 = +100KJ. DHf CO2 = +200KJ. DHf H2O = +50KJ. DHf O2 = 0.

3. Link the DH's Hess's Cycle - End Point and Direct Link.

4. Direct route = others added.

DH2=DH1+DH3. DH1=DH2-DH3. DH1 = (200 + 2 x 500) = 300 -100 = -200KJmol-1

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Hey quick question: To work out moles of fuel, why did you divide 12 by 22?

Is it not the Mr of pentane? :S 

Thanks. :]

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