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Unit 2 Module 3 Notes

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CHEMISTRY

2.3.1 ENTHALPY CHANGES ENTHALPY, H

Heat content that is stored in a chemical system

CHEMICAL SYSTEM

The reactants and products

SURROUNDINGS

Outside the chemical system

CONSERVATION OF ENERGY



No energy is lost Energy changes is measured as heat Heat loss in a chemical system = heat gain to surroundings Heat gain in a chemical system = heat loss from surroundings

ENTHALPY CHANGE, ΔH


The heat exchange with the surroundings during a chemical reaction, at constant pressure
ΔH = H products - H reactants All chemical reactions either release or absorb heat

EXOTHERMIC REACTIONS




ΔH, negative Enthalpy of products is smaller than the enthalpy of the reactants Heat loss to the surroundings E.g. chemicals reacting together in an inner chamber of self-heating cans CaO (s) + H2O (l)  Ca(OH)2 (aq) ΔH = -ve

ENDOTHERMIC REACTIONS




ΔH, positive Enthalpy of products is greater than the enthalpy of the reactants Heat gain from the surroundings E.g. evaporation of water to absorb heat from beer in self-cooling bear cans H2O (l)  H2O (g) ΔH = +ve

EXOTHERMIC REACTIONS
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Oxidation of fuels Respiration

OXIDATION OF FUELS



Oxidation of fuels by combustion Oxidation of methane in natural gas to form CO2 and H2O Products have less enthalpy than the reactants Excess energy released as heat

OXIDATION OF METHANE


CH4 (g) + 2O2 (g)  CO2 (g) + 2H2O (l) ΔH = -890 kJ mol-1 1 mol CH4 (g) reacts with 2 mol 2O2 (g) to form 1 mol CO2 (g) and 2 mol 2H2O (l) Releases 890 kJ of heat

RESPIRATION

Sugars, such as glucose C6H12O6, are oxidised to CO2 and H2O C6H12O6 (aq) + 6O2 (g)  6CO2 (g) + 6H2O (l) ΔH = -2801 kJ mol-1

ENDOTHERMIC REACTIONS


Thermal decomposition of limestone Photosynthesis Require an input of heat energy

THERMAL DECOMPOSITION OF LIMESTONE


Limestone contains calcium carbonate, CaCO3 CaCO3 (s)  CaO (s) + CO2 (g) ΔH = +178 kJ mol-1 Heat energy must be provided

PHOTOSYNTHESIS


Light from the Sun provides energy 6CO2 (g) + 6H2O (l)  C6H12O6 (aq) + 6O2 (g) ΔH = +2801 kJ mol-1 Reverse of respiration

ENTHALPY PROFILE DIAGRAMS

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H products H reactants (endothermic reactions)

ACTIVATION ENERGY, Ea

Energy barrier that prevents many reactions from taking place spontaneously Energy equired to break bonds in the reactants

Ea OF EXOTHERMIC REACTIONS

Energy is required to break the first bond and start the reaction Once the energy barrier has been overcome, the net out output of energy provides more energy that can be used to overcome the activation energy for the reaction to continue Once an exothermic reaction begins, the activation energy is regenerated and the reaction becomes self-sustaining

Ea OF ENDOTHERMIC REACTIONS

The reaction has to overcome the energy barrier No excess energy to break more reactant bonds, a sustained amount of energy needs to be continually supplied to keep the reaction going

STANDARDS

Chemists use standard enthalpy changes, measured under standard conditions Ensures that all reactions and enthalpy changes are carried out under same conditions

STANDARD CONDITIONS


A pressure of 100 kPa (1 atmosphere) A stated temperature at 298 K (25 ⁰C) is usually used A concentration of 1 mol dm-3 (for reactions with aqueous solutions)

STANDARD ENTHALPY CHANGE, ΔHƟ


H - enthalpy
Δ - change
Ɵ
- under standard conditions
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STANDARD CONDITIONS

Standard conditions are a pressure of 100 kPa (1 atmosphere), a stated temperature, usually 298 K (25⁰C) and a concentration of 1.0 mol dm-3 (for reactions with aqueous solutions)

STANDARD STATES


For a standard enthalpy change, any substance must be in its standard state The physical state of a substance under standard conditions E.g. Mg (s), H2 (g), H2O (l)

STANDARD ENTHALPY CHANGES


Standard enthalpy change of reaction Standard enthalpy change of combustion Standard enthalpy change of formation

STANDARD ENTHALPY CHANGE OF REACTION, ΔHrƟ

The standard enthalpy change of reaction, ΔHrƟ, is the enthalpy change that accompanies a reaction in the molar quantities expressed in a chemical equation under standard conditions, all reactants and products being in their standard states 1 E.g. H2 (g) + 2 O2 (g)  H2O (l) ΔHrƟ = -286 kJ mol-1

STANDARD ENTHALPY CHANGE OF COMBUSTION, ΔHcƟ

The standard enthalpy change of combustion, ΔHcƟ, is the enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standard conditions, all reactants and products being in their standard states

ΔHcƟ FOR ETHENE 1 2

C2H6 (g) + 3 O2 (g)  2CO2 (g) + 3H2O (l) ΔHcƟ = -1560 kJ mol-1

The complete combustion of 1 mol of C2H6 (g)

STANDARD ENTHALPY CHANGE OF FORMATION

The standard enthalpy change of formation, ΔHfƟ, of a compound is the enthalpy change that takes place when one mole of a compound is formed from its constituent elements in their standard states under standard conditions

ΔHfƟ FOR WATER

1

H2 (g) + 2 O2 (g)  H2O (l) ΔHfƟ = -286 kJ mol-1 1 mol of H2O (l) being formed from its elements

ΔHfƟ OF AN ELEMENT

No chemical change All elements have a standard enthalpy change of formation of 0 kJ mol-1

DETERMINATION OF ENTHALPY CHANGES

Q = mcΔT Joules
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