Thermodynamics Chemistry

Thermodynamics The study of energy changes in physical and chemical processes.

Energy The capacity to do work or produce heat.

Units for measuring energy or work English unit: calorie (cal) : The energy required to raise the temperature of one gram of water by one degree Celsius. SI Unit: Joule (J): one newton x meter or kg. m 2 /s 2 1 cal = J

Food calories A food calorie (Cal) is actually a kilocalorie (kcal). 1 Cal = 1000 cal

Forms of Energy Potential Energy: Energy stored in an object due to composition or position. Kinetic Energy: Energy of motion KE = ½ mv 2

PE can be changed to KE.

Forms of energy Mechanical Electrical Chemical Electromagnetic Radiation Heat

First Law of Thermodynamics The total amount of energy in the universe is constant. Law of Conservation of Energy Energy can neither be created nor destroyed but can be changed from one form to another.

Heat (q) Energy that flows from one object to another due to a difference in temperature. Heat is the least useful form of energy because it is the hardest to recover. q

Heat energy causes an objects molecules to gain kinetic energy. Temperature is a measure of the average kinetic energy of the molecules of a substance.

Chemical Energy The energy that a substance posses is due to the arrangement of the atoms that make it up. During a chemical reaction atoms are rearranged, so that energy can either be absorbed or released during a reaction. EndothermicExothermic

Enthalpy of Reaction (  H) Also called the heat of reaction. It is the energy change during a reaction. It is equal to the difference in energy of the reactants and products.

Exothermic Reactions Energy is given off or released.  H is negative. The products have less energy than the reactants. Xe(g) + 2F 2 (g)  XeF 4 (s) kk J This reaction gives off 251 kJ of energy as it reacts

Endothermic Reactions Energy is taken in or absorbed.  H is positive. The products have more energy than the reactants. N 2 (g) + 2O 2 (g)  2NO 2 (g) kk J This reaction absorbs 68 kJ of energy as it reacts

To calculate  H for a certain amount of reactant: 1. Change the mass of given to moles. 2. Set up a proportion to find the energy being used.  H rxn =X Coefficient moles of of givengiven 3. Cross multiply and check units and SF.

Example: Calculate the energy given off when 5.8 g of methane is burned. CH 4 (g) + 2O 2 (g)  CO 2 (g) + 2H 2 O(g)  H= -890 kJ

Potential Energy Diagrams A visual representation of how the energy changes during a reaction.

Exothermic Reaction Products have less PE C is the  H (change in energy during reaction) A is the activation energy (difference b/w the initial PE of the reactants and the highest energy level) B is reverse activation energy (if you were to go from products to reactants) Activation energy – the energy required to start a reaction..

Endothermic Reaction Products have more PE A is the PE of reactants D is the PE of products C is the  H (change in energy during reaction) B is activation energy E would be the energy of the activated complex. C  A + B

Catalyst A catalyst is a substance that helps the reaction start, but is not used up during the reaction. It does this by lowering the activation energy. The highest point on the curve is lowered. This makes it easier for the reaction to occur.