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)
video
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
Activity Lab