Thermochemistry Introduction

Thermochemistry The study of energy changes during chemical reactions or changes of state.

First Law of Thermodynamics Energy is neither created nor destroyed during chemical or physical changes. Also called the Law of Conservation of Energy

What is Energy? Energy is the ability to do work.

Some Types of Energy: Kinetic Potential Energy of motion Energy stored by object due to its position or composition Chemical Potential Energy is stored in chemical bonds

Energy We’re Going to Focus On Chemical Energy (energy in bonds) Heat (energy due to atomic/molecular motion)

Heat (q) The flow or transfer of energy due to a temperature difference. Heat always flows from an object of higher temp to one of lower temp until both are the same temp. Why? Heat flows from the hot pack to the woman’s cooler back.

Units for Energy: calorie (cal) Joule (J) Amount of energy required to raise the temperature of 1g of water 1oC 1000 calorie = 1 kcal = 1 nutritional Calorie (Cal) Joule (J) 4.184 J = 1.000 cal

Energy in Food: Nutrition Labels The United States This is in nutrition (food) calories, Cal. It is actually 453,000 cal, or 453 kcal. 1 Cal = 1000 cal

England

Germany

Australia

New Zealand

China

Unit Conversion Problems Convert 345.7 J into calories. Answer: 82.62 calories

Unit Conversion Problems Convert 325 Calories to Joules Answer: 1.36 x 106 J

Some Basic Definitions Surroundings System: The part of the universe we’re focusing on Often the materials involved in a chemical reaction or phase change Surroundings: Everything else in the universe Surroundings Surroundings System System System Surroundings Surroundings Surroundings

Describing the Direction of Heat Flow: Exothermic: Heat flows out of the system and into the surroundings The system loses energy The surrounding gain energy qsys is negative qsurr is positive We see an increase in temperature in the surroundings. An exothermic reaction would feel hot to the touch because heat is being transferred to your hand. System System System System System System Surroundings Surroundings Surroundings Surroundings

Describing the Direction of Heat Flow: Endothermic: Energy flows into the system from the surroundings The system gains energy The surroundings lose energy qsys is positive qsurr is negative We see a decrease in temperature of the surroundings An endothermic reaction would feel cold to the touch because it is absorbing heat from your hand. System System System System Surroundings Surroundings

General Outcome: qsurr = -qsys Signs are used to indicate the direction of energy flow. According to the law of conservation of energy, the amount of heat lost or gained by the system MUST be exactly the same as the energy gained or lost by the surroundings. However, the signs are opposite.

Chemical (Potential) Energy Think about this: Is the breaking of bonds an endothermic or exothermic process? Endothermic! It requires energy to occur.

Chemical (Potential) Energy Breaking bonds is ALWAYS endothermic (it always requires energy!) So, qsys is positive for breaking bonds.

Chemical (Potential) Energy Think about this: Is the formation of bonds endothermic or exothermic? Exothermic! Energy is released when bonds form. Pow!!!

Chemical (Potential) Energy Forming bonds is ALWAYS exothermic(it always releases energy!) So, qsys is negative for forming bonds. Pow!!!

Your turn Would you expect a synthesis reaction to typically be an endothermic or exothermic reaction? Exothermic because bonds are being formed. Would you expect a decomposition reaction to typically be an endothermic or exothermic reaction? Endothermic because bonds are being broken.

Your turn Is the melting of ice endothermic or exothermic? Endothermic because intermolecular forces are being broken. Is the condensing of steam endothermic or exothermic? Exothermic because intermolecular forces are being formed. (This is why burns from steam are so bad.)

Your turn HW: Thermochemistry Introduction ws