Thermochemistry Chapter 10 thermo #2.ppt

Today, you will learn… The definition of heat and enthalpy The units of energy The difference between system and surroundings How to identify a process as endothermic or exothermic How to sketch a potential energy curve for an endothermic or an exothermic process

Energy is a Energy is a state function State function – depends only on initial and final conditions - independent of pathway Distance between cities is a state function… Distance you travel is not!

System and Surroundings In thermodynamics, the world is divided into a system and its surroundings. System The part we want to study (such as a reaction) May be closed, open, or isolated. Surroundings Everything else NOT in the system

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)

Heat always flows from warmer to cooler objects. Ice gets warmer while hand gets cooler Cup gets cooler while hand gets warmer

HEAT and THERMAL ENERGY Thermal motion (random molecular motion) is increased by heat energy i.e. heat stimulates thermal motion Heat The flow of thermal energy from one object to another. (Must be at different temp. and in contact.)

Units of Energy SI Unit for energy (often use kJ) Also used to measure work (energy in transit) Joule Energy needed to raise the temperature of 1 g of water by 1 degree Celsius. 1 cal = J Calorie Always capitalized (Calories) Equivalent to 1 kcal Food Calories

First Law of Thermodynamics: the energy of an isolated system is constant (Law of Conservation of Energy) Signs (+/-) will tell you if energy is entering or leaving a system + indicates energy enters a system - indicates energy leaves a system

ENTHALPY (H) (comes from Greek for “heat inside”) At constant pressure: Change in enthalpy = heat (Q) absorbed or released  H = Q Greek letter Delta means “change”

Exothermic and Endothermic Reactions Prefix exo- means “outside” Prefix endo- means “inside, within” Thermal refers to heat Exothermic = heat is released (let out) Endothermic = heat is absorbed (taken in)

Exothermic Heat is released Enthalpy change (  H) is negative Will feel hotExample: burning fuel Endothermic Heat is absorbed Enthalpy change (  H) is positive Will feel coldExample: photosynthesis

Measuring Heat reaction Exothermic reaction, heat given off & temperature of water rises Endothermic reaction, heat taken in & temperature of water drops

Measuring Heat Exothermic reaction, heat given off Endothermic reaction, heat taken in

Enthalpy and Phase Changes Vaporization Energy added Allows particles to overcome IMAF Endothermic Melting Energy added Allows particles to vibrate more quickly until they can flow Endothermic Freezing Energy Removed Allows particles to move more slowly, settle into solid form Exothermic