Chemistry Analyze the energy changes (endothermic or exothermic) associated with chemical reactions An endothermic reaction requires the absorption of heat as the reaction takes place. Energy appears on the reactants’ side; Example: 2H2O + energy → 2H2 + O2 Example: Cooking eggs. Crack a couple of eggs into a hot frying pan, and energy from the pan is absorbed into the eggs during chemical reactions called cooking.

Exothermic Reactions Exothermic reactions release heat. Examples: burning propane to run the grill when you grill a burger, burning wood in a campfire to keep warm, and the combustion of gasoline in a car’s engine. Example: 2H2 + O2 → 2H2O + energy

Exothermic Example Consider an exothermic reaction: AB + C → CA + B The reactants start off at a higher energy state than the products, so energy is released going from reactants to products. Ea is the activation energy, the energy you must put in to get the reaction going.

Heat of Reaction The heat of reaction is the quantity of energy absorbed or released as heat during a chemical reaction. Heat of reaction is expressed in kilojoules (kJ) per mole.

Enthalpy Enthalpy is the energy absorbed or released as heat at constant pressure. Enthalpy is usually expressed as the change in enthalpy. ∆H = ∆U + P∆V (for T and P constant) Where ∆H = change in enthalpy between initial and final states; ∆U = change in internal energy; P∆V = pressure x change in volume

Change in Enthalpy If ∆H < 0, then energy is released and the reaction is EXOTHERMIC. If ∆H > 0, then energy is absorbed and the reaction is ENDOTHERMIC.

Spontaneity Whether a reaction is spontaneous or not if often dependent upon the reaction conditions (for example, the temperature and pressure conditions). Sometimes a reaction is spontaneous, but it must first be activated. This activation usually involves some means of raising the temperature of the reactants, such as through a spark or a lighting procedure. Example: Striking a match

Collision Theory – Understanding How Reactions Occur In order for a chemical reaction to take place, the reactants must collide. Collision Theory states that the collision between molecules can provide the energy needed to break bonds so that new bonds can form.

A One-Step Collision Example Even if collisions occur, sometimes not enough kinetic energy is available for a reaction to occurs. Why? Temperature too low (molecules not moving fast enough) Molecules not colliding in the right orientation (not in the “reactive site”)