AL Chemistry Summer Project Standard enthalpy changes

Group 6 (produced by Chen William and Lam Yu Wing)

What is enthalpy? Enthalpy (H,or heat content) is the

amount of heat energy possessed by substances.

It is the sum of internal energy (U) and the product of the pressure and volume of a system.(H=U+PV)

Enthalpy change, ΔH As the enthalpy of a system cannot be

measured but changes in enthalpy can be measured experimentally.

Enthalpy change in a process then corresponds to the heat change of the system when the process occurs at constant pressure:

ΔH= H products － H reactants

Endothermic reactions Endothermic reactions are those in

which heat is added, or flows, into the system, the heat change of the process has a positive value.(ΔH > 0)

Exothermic reactions Exothermic reactions are those in which

heat is withdrawn, or flows, from the system, the heat change of the process has a negative value.(ΔH < 0)

Standard enthalpy change Standard enthalpy of a reaction (ΔH Θ

298 or ΔH Θ ) is the enthalpy change when the molar quantities of reactants as stated in the balance equation completely react together to give the products under

standard conditions (298 K,1 atm.)

Standard Condition 298k Gas: 1 atmosphere Solution: 1 mol dm -3

Solid or liquid: normal physical state at 298 K and 1 atmosphere

Types of standard enthalpy of reaction There are four types of reaction: The standard enthalpy change of

neutralization

The standard enthalpy change of formation

The standard enthalpy change of solution The standard enthalpy change of

combustion

Standard enthalpy change of neutralization Standard enthalpy change of

neutralization(HnΘ ) is the enthalpy change

when 1 mole of water is formed from the neutralization under standard conditions.

e.g. H+(aq) + OH-(aq) H2O(l)

Hn Θ = -57.3 kJ mol -1

Standard enthalpy change of formation Standard enthalpy change of formation(Hf )

is the enthalpy change when 1 mole of the compound is formed from its elements in their standard states at 298K and 1 atmosphere.

2C(s) + 3H2(g) + ½ O2(g) C2H5OH(l)

Hf Θ= - 276 kJ mol -1

Standard enthalpy change of formationHf

Θ of all elements are 0, e.g. HfΘ [O2(g)] = 0.

For elements with allotropes, the most stable allotrope is used,

e.g. graphite is the most stable allotrope of carbon then Hf

Θ [graphite] = 0 but HfΘ [diamond] = +1.9 kJ mol-1.

Standard enthalpy change of formation (HfΘ) is a measure

of energetic stability, more –ve =more energetcially stable with respect to the element.

Standard enthalpy change of solution Standard enthalpy change of solution (Hs

Θ ) is the enthalpy change when 1 mol of a substance is completely dissolved in a infinite amount of solvent under standard conditions.

e.g. CuSO4(s) + water Cu2+(aq) + SO42-(aq)

HsΘ = -57 kJ mol -1

CuSO4.5H2O(s) + water Cu2+(aq) + SO4

2-(aq)

HsΘ = +10 KJ mol -1

Standard enthalpy change of combustion Standard enthalpy of combustion (Hc

Θ )is the enthalpy change when 1 mole of it is burnt completely in excess oxygen in the standard condition.

e.g. C(graphite) + O2(g) CO2(g)

HcΘ = -3934 kJ mol -1

CH4(g) + 2O2(g) CO2(g) + 2H2O(l)

HcΘ = - 890 kJ mol -1

EXPERIMENTAL DETERMINATION OF ENTHALPY CHANGES BY SIMPLE CALORIMETRY

Enthalpy change of neutralization Enthalpy change of combustion

Enthalpy change of neutralization

Heat evolved = [(Macid+Mbase)Sproduct + C]*T

HneuΘ = - heat evolved/no. of moles of water formed

Specific heat capacity (usually taken as 4.18 J g-

1 K-1 for dilute solution

Heat capacity of apparatus

NOTE that if H is -ve

PrecautionPossible errors Improvements

Heat loss to the surroundings (due to evaporation & convection

Use a cover or lid to prevent heat loss

Heat capacity of vacuum flask or expanded polystyrene cup may be neglected

Find the heat capacity of the vacuum flask or expanded polystyrene cup

Specific heat capacity of the product solution is assumed to be the same as that of water

Find the specific heat capacity of the solution

Ordinary thermometer is not precise enough

Replace it with a more precise one ( which can read to 2 decimal places), e.g. Beckmann thermometer