Enthalpy

In a chemical reaction, the total kinetic and potential energy of a substance is called its heat content or enthalpy.

We use the symbol H to represent enthalpy. Enthalpy is a characteristic of a substance.Enthalpy is measured in J/mol

Enthalpy is temperature-dependent: as the kinetic energy increases with temperature, so does the enthalpy.

EXOTHERMIC & ENDOTHERMIC REACTIONSEXOTHERMIC & ENDOTHERMIC REACTIONS

Enthalpy changes result from chemical bonds being broken and reformed. Breaking a bond requires energy, whereas creating a bond releases energy.

EXOTHERMIC REACTIONSEXOTHERMIC REACTIONS

H = Hproducts - Hreactants

Exothermic process: a change where heat is released. Here the enthalpy of the products is LESS than the reactants

Exothermic process: H < 0 (at constant pressure)

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Endothermic process: a change (e.g. a chemical reaction) that requires (or absorbs) heat.

An input of heat corresponds to the enthalpy of the products being higher than the reactants.

Endothermic process: H > 0 (at constant pressure)

Photosynthesis is an endothermic reaction

(requires energy input from sun)

Forming Na+ and Cl- ions from NaCl is

an endothermic

process

Systems & Surroundings

In thermodynamics, the world is divided into a system and its surroundings

A system is the part of the world we want to study (e.g. a reaction mixture in a flask)

The surroundings consist of everything else outside the system

SYSTEM

CLOSED

OPEN ISOLATED

OPEN SYSTEM: can exchange both matter and energy with the surroundings (e.g. open reaction

flask, rocket engine)

CLOSED SYSTEM: can exchange only energy with the surroundings (matter remains

fixed) e.g. a sealed reaction flask

ISOLATED SYSTEM: can exchange neither energy nor

matter with its surroundings (e.g. a thermos flask)

Examples

Identify the system and surroundings for:1.Solid dissolved in water

2.Two chemicals burned in a calorimeter

Measuring HeatMeasuring Heat

Exothermic reaction, heat Exothermic reaction, heat given off & temperature of given off & temperature of

water riseswater rises

Endothermic reaction, Endothermic reaction, heat taken in & heat taken in &

temperature of water temperature of water dropsdrops

Thermochemical Equations

Written with the energy term as a reactant or product as part of the reaction

C(s) + O2(g) CO2(g) + 394 kJ

)()(221

)(221 5.26 ggg HIkJIH

Thermochemical Reactions

• The enthalpy term can be removed from the equation and written separately

Enthalpy diagrams

VaporisationVaporisation

Energy has to be supplied to a liquid to enable it to overcome forces that hold molecules together

• endothermic process (H positive)

Melting

Energy is supplied to a solid to enable it to vibrate more vigorously until molecules can move past each other and

flow as a liquid

• endothermic process (H positive)

Freezing

Liquid releases energy and allows molecules to settle into a lower energy state and form a solid

• exothermic process (H negative)

(we remove heat from water when making ice in freezer)

Calorimetry

• The measurement of heat transfer is known as ______________________________.

• We use a calorimeter to measure the heat transferred to/from a reaction.

Calorimetry

Types of Calorimeters

Simple Calorimeter• based on a styrofoam cup

(good insulator)

• Use a lid to prevent heat loss to to the air

• Water is typically the surroundings

Types of Calorimeters

• Lab Calorimeters

Types of Calorimeters

• Flame Calorimeters

Types of Calorimeters

• Bomb Calorimeter

Heat of Reaction

• Calorimeters are used to calculate the enthalpy of a reaction, by determining the heat absorbed/released to the surroundings. Recall that

  Q = mc∆T

Heat of Reaction

• As a reaction produces heat, thermal energy is lost by the ____________ and gained by the ____________

• If we can calculate the heat gained by the surroundings ( or lost by the surroundings in the case of an ____________________________ reaction) we can determine the heat lost by the reaction.

system

surroundings

0 gainedlost QQ

endothermic

Heat of Reaction

Once we have determined the heat, Q, we calculate enthalpy from

Example 1: Determine the heat of solution of KOH if 5.6 g of KOH was added to 100.0 g of water and the temperature rose by 130 C

n

QH rxn

• Example 1: Determine the heat of solution of KOH if 5.6 g of KOH was added to 100.0 g of water and the temperature rose by 130 C

Example 2The combustion of 2.4 g of butane, C4H10, in a

calorimeter causes a rise in temperature of 120C. the calorimeter contains 1400g of water and the calorimeter has a mass of 450g. ccalorimeter = 1.39 J/g0C. Find Hcomb of butane.

Why can an ionic solid dissolving in water be either Why can an ionic solid dissolving in water be either endothermic or exothermicendothermic or exothermic??

Lattice EnthalpyLattice Enthalpy

A measure of the attraction between ions (the enthalpy change when a solid is broken up)

∆Hlattice is positive as it requires energy to break the intermolecular forces holding the ions together in a lattice

Enthalpy of hydration Enthalpy of hydration HHhydhyd

Enthalpy of hydration Enthalpy of hydration HHhdr hdr is negative as is negative as energy is released when the ions are energy is released when the ions are surrounded by water moleculessurrounded by water molecules

Enthalpy of Solution

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