Collision Theory In order for a reaction to occur, the particles of the reactant must have enough energy, and must collide at the correct angles (proper orientation). The collision theory explains the factors that affect the rate of a reaction. The greater the rate of effective collisions, the greater the reaction rate is The collision theory explains the factors that affect the rate of reaction

Factors that Affect the Rate of a Chemical Reaction The following factors can increase the rate of a chemical reaction by increasing the number of effective collisions that a occur at a given time Concentration Temperature Surface Area Nature of Reactants Catalyst

Heat of Reaction (Enthalpy) The amount of heat given off or absorbed in a chemical reaction. Heat of reaction is the difference in heat content of the products and reactants. Table I lists common reactions, and there heat of reactions. ∆H = Hproducts - Hreactants ∆H = heat of reaction Hproducts = potential energy of the product(s) Hreactants = potential energy of the reactant(s) ∆H is measured in kJ (kilojoules)

Table I Remember that a -∆H does not mean negative energy – it only implies an exothermic reaction has occurred.

Potential Energy Diagrams

Endothermic Exothermic

Equilibrium When the forward and reverse reactions occur at the same rate. The rates are equal, but the quantities (amounts) of reactants and products are not necessarily equal. Types of Equilibrium: Phase (Dynamic) Solution Chemical

Spontaneous Reactions A reaction that takes place under a specific set of conditions spontaneously. Occur in the direction of: Less energy (lower enthalpy): favors exothermic reactions. Greater entropy (disorder): Solids have the least entropy (most order), liquids have more, and gases have the most entropy (disorder). When a solid dissolves in water (salts or sugars), entropy increases. At low temperature, energy is important; at high energy, entropy is important.

Reactions Going to Completion Some reactions go to completion; the reaction goes in only one direction, the reactants form products, products DO NOT form reactants. Some indicators that a reaction has gone to completion when the following are produced: (1) a gas (2) an essentially unionized product (like water) (3) a precipitate is one of the products. Remember that precipitates are insoluble - Table F

Le Chatelier’s Principle Henry Louis Le Chatelier was a French chemist who devised a principle to predict the effect of change in conditions on a chemical equilibrium reaction The principle states that if a system at equilibrium is subjected to stress, the equilibrium will shift in the direction that relieves the stress. Types of stresses include: concentration, temperature, and pressure

CaCO3(s) + heat ↔ CaO(s) + CO2(g) 2SO3(g) + heat ↔ 2SO2(g) + O2(g) CIA (concentration increase away) TIA (temperature increase away) PILM (pressure increase less moles of gas) CaCO3(s) + heat ↔ CaO(s) + CO2(g) 2SO3(g) + heat ↔ 2SO2(g) + O2(g)