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

EXOTHERMIC REACTIONS  H = H products - H reactants 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 Heat Exothermic reaction, heat given off & temperature of water rises Endothermic reaction, heat taken in & temperature of water drops

Thermochemical Equations Written with the energy term as a reactant or product as part of the reaction C (s) + O 2(g) CO 2(g) kJ

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

Enthalpy diagrams

Vaporisation 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 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 g of water and the temperature rose by 13 0 C

Example 2 The combustion of 2.4 g of butane, C 4 H 10, in a calorimeter causes a rise in temperature of 12 0 C. the calorimeter contains 1400g of water and the calorimeter has a mass of 450g. c calorimeter = 1.39 J/g 0 C. Find  H comb of butane.

Why can an ionic solid dissolving in water be either endothermic or exothermic? Lattice Enthalpy A measure of the attraction between ions (the enthalpy change when a solid is broken up) ∆H lattice is positive as it requires energy to break the intermolecular forces holding the ions together in a lattice Enthalpy of hydration  H hyd Enthalpy of hydration  H hdr is negative as energy is released when the ions are surrounded by water molecules

Enthalpy of Solution Activity Lab