Regents Chemistry z KINETICS

What is Kinetics? zKinetics is the branch of chemistry that deals with rates of chemical reactions zDifferent factors affect how quickly chemical reactions occur zIn order for a reaction to occur, reactant particles must collide yThis is called COLLISION THEORY xReactions occur between collisions of particles that are orientated correctly and have sufficient amounts of energy-ACTIVATION ENERGY!!!!!

1- Correct orientation 2- Sufficient activation energy!!! 1- Wrong orientation or not enough activation energy. 2- ACTIVATION ENERGY: the minimum amount of energy needed to form the activaion complex and start a reaction. : Kinetic energy is converted to potential energy

Why is Kinetics Important? zKinetics allows chemists to predict how fast a reaction will occur zImportant in the synthesis of all kinds of compounds zIn manufacturing, it is essential to making products, ytiming rates between chemical reactions to get a desired product

What Factors Affect Rate of Reaction? zThe rate of a chemical reaction depends on a number of factors that affect the number of effective collisions between particles yNature of Reactants yConcentration ySurface Area yPressure yTemperature yPresence of a Catalyst

Nature of Reactants zReactions involve the breaking of existing bonds and the formation of new bonds zGenerally, covalently bonded substances are slower to react than ionic substances as they have move bonds to be broken yBreaking more bonds requires that the particles must have more energy when they collide

Concentration zMost chemical reactions will proceed at a faster rate if the concentration of one or more of the reactants is increased zEx: Combustion of Paper yNormal air is 20% oxygen, if we use pure oxygen it burns much faster! yWhy? KMT says that more collisions between oxygen and paper particles = faster rate of reaction!

1- The greater concentration of reactants, the more collisions 2- Increase concentration = increase collisions = increase rate of reaction!!! Chemical reaction is faster!!!

TEMPERATURE 1- Increase in temperature = increase in rate of reaction 2- WHY? ANS: increase in kinetic energy and motion of particles- more collisions of particles

Surface Area zWhen more surface area of a substance is exposed, there are more chances for reactant particles to collide zEx: one centimeter of a cube of Mg reacting with HCl reacts more slowly then increasing the surface area by breaking Mg down into smaller pieces

INCREASING SURFACE AREA

Pressure zPressure has little or no effect on rates of reactions between liquids and solids… zIt does have an effect on gases! zAn increase in pressure has the effect of increasing the concentration of gaseous particles yTherefore, it increases the rate of a reaction that involves only gases

Presence of a Catalyst zCatalysts are substances that increase the rate of a reaction, by providing an alternative pathway zCatalysts take part in a reaction, but they are unchanged when the reaction is complete ySo they are present in the reactants and products and only assist in the rate of the reaction

Temperature zBy definition, temperature implies that the greater the temperature, the faster the molecules will move (higher kinetic energy) zWhen particles are moving faster, more collisions occur and increasing the likelihood of a reaction yHigher Temp = particles with more kinetic energy = more effective collisions

Regents Chemistry z Potential Energy Diagrams

What’s Potential Energy? zChemical bonds are large sources of chemical potential energy (when bonds are formed) zCHEMICAL ENERGY: energy associated with the breaking and forming of chemical bonds zThis is a form of Potential energy!

Chemical Bonds Have PE! zChemical bonds have large supplies of potential energy zBonds are broken in chemical reactions and new bonds form in products zEnergy can be released or gained in these chemical reactions, so… zPotential energy can be increased or decreased depending on the reaction..

-ENERGY IS ON REACTANT SIDE! -ENERGY RELEASED: ON PRODUCT SIDE!!!!

What’s a PE Diagram?..First look zA PE diagram illustrates the potential energy change that occurs during a chemical reaction --- Potential energy  Reaction Coordinate Products Reactants Activated Complex Reaction Coordinate = progress of reaction

Potential Energy Diagrams zIn order for a reaction to occur, the reactants must have sufficient energy to collide effectively zAs reactant particles approach each other, kinetic energy is converted into potential energy zThe molecules must also have proper orientation to come together…this leads to..

Activated Complex zWhen molecules collide with the proper orientation, an intermediate product is formed zActivated Complex – is a temporary, intermediate product that may either break apart and reform the reactants or rearrange the atoms and form new products

Example of Collisions zHI molecules collide in a reaction and form H 2 and I 2 products

A + B  C + D + Heat Heat is a product because the PE is lower for the products than in the reactants Difference In PE Reactants and Products on a PE Diagram  H = Heat of reaction Activation energy

Activation Energy and Heat of Reaction zThe amount of energy needed to form the activated complex from the reactants is called the activation energy zThe diagram is typically read from left to right and vice versa ySo..we specify by saying forward or reverse zHEAT OF REACTION – is the heat required to form products or reactants in this specific reaction yCan be negative or positive depending on viewing the reaction as forward or reverse

The Effect of a Catalyst zCatalysts are added to a reaction to lower the activation energy, which in turn speeds up the reaction zThe catalyst speed up the reaction by providing a new pathway zIn turn, the activation energy of the reverse reaction is also lowered…but zThe  H remains unaffected..see why

Effect of a Catalyst What does 5 represent?

Two Outcomes of PE Diagrams z1. PE of products is lower than the reactants, so energy is lost and released to the environment – Exothermic! (  H = -)

zPE of the products is greater than the reactants, so energy is absorbed to make the products – Endothermic (  H = +) Two Outcomes of PE Diagrams