Regents Chemistry Kinetics and Equilibrium

What is Kinetics? Kinetics is the branch of chemistry that deals with rates of chemical reactions Different factors affect how quickly chemical reactions occur In order for a reaction to occur, reactant particles must collide This is called COLLISION THEORY Reactions occur between collisions of particles that are orientated correctly and have sufficient amounts of energy!

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

What Factors Affect Rate of Reaction? The rate of a chemical reaction depends on a number of factors that affect the number of effective collisions between particles Nature of Reactants Concentration Surface Area Pressure Temperature Presence of a Catalyst

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

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

Surface Area When more surface area of a substance is exposed, there are more chances for reactant particles to collide Ex: Lycopodium solid vs. powder phases

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

Presence of a Catalyst Catalysts are substances that increase the rate of a reaction by providing a different and easier pathway for a chemical reaction Catalysts take part in a reaction, but they are unchanged when the reaction is complete So they are present in the reactants and products and only assist in the rate of the reaction

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

Regents Chemistry Potential Energy Diagrams

What’s Potential Energy? Chemical bonds are large sources of chemical potential energy Potential energy has ability to do stuff! Similar to gravitational potential energy Gravitational PE can be increased by raising an object higher from the Earth’s surface And Reduced by lowering closer to the ground

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

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

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

Activated Complex When molecules collide with the proper orientation, an intermediate product is formed Activated 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 HI molecules collide in a reaction and form H2 and I2 products

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

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

Regents Chemistry Agenda Finish Potential Energy Diagrams Lecture YOU NEED HANDOUT FROM WEDNESDAY Worksheet HW: Finish remaining problems

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

Effect of a Catalyst What does 5 represent?

Two Outcomes of PE Diagrams 1. PE of products is lower than the reactants, so energy is lost and released to the environment – Exothermic! (H = -)

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

Practice Problem

Regents Chemistry Physical and Chemical Equilibrium

What’s Equilibrium? The potential energy diagrams typically show a forward reaction – Left to Right Reactants  Activated Complex  Products But the reverse can also happen… Products  Activated Complex  Reactants And they can both happen at the same time! When they occur at the same rate, the system is said to be in equilibrium

Describing Equilibrium A double arrow is used in the reaction in place of a single arrow Equilibrium is a state of balance between the rates of two opposite processes that are taking place at the same rate Exists only in a closed system! vs.

Equilibrium… Is important because many chemical reactions and physical processes are reversible We will look at two kinds: Physical and Chemical Equilibriums Is an equilibrium between rates, not amounts of reactants and products For example…

Equilibrium in a Closed Container Open Container Closed Container Evaporation continues in the closed container but is balanced by condensing vapor. This is an equal process at equilibrium! H2O(l) H2O(g) H2O(l) H2O(g) End

Physical Equilibrium (PE) The evaporation – condensation of water in a closed system is an example of PE Two examples of PE are Phase Equilibrium Solution Equilibrium

Phase Equilibrium… H2O(s) H2O(l) Can exist between the solid and liquid phases of a substance. This is called the melting point of a solid phase or the freezing point of the liquid phase in a closed container Example: Water at 0 Celsius (closed container) Some of the ice is melting and some of the water is freezing Remember, rates are the same.. not amounts! H2O(s) H2O(l)

Solution Equilibrium You’ve seen this before..saturated solutions! Solids in liquids exist in equilibrium in a saturated solution When we add more solute to a saturated solution, the solute may dissolve but some will also recrystallize out of solution KCl(s) KCl(aq)

Solution Equilibrium cont… Equilibrium may also be attained in a closed system between a gas dissolved in a liquid and the undissolved gas For example  In a closed Pepsi can, there is an equilibrium between the gaseous and dissolved state of CO2 CO2(g) CO2(aq)

Temperature and Physical Equilibrium Equilibrium is affected by temperature If the temperature is raised, a solid generally becomes more soluble in a liquid For a short time the rate of dissolving exceeds the rate of crystallization However, as more solid is placed into solution, the rate of recrystallization increases until a new equilibrium is reached

Temperature and Physical Equilibrium…. Opposite is true for gases in liquids As the temperature increases, the rate of the gas escaping from the liquid increases while the rate at which the gas particles dissolves decreases This decreases the solubility of the gas in the liquid As the temperature rises, the solubility of all gases decreases in a liquid decreases

Chemical Equilibrium When reactants are first mixed and no products are present, only the forward reaction can occur For example: water vapor and methane CH4(g) + H2O(g) 3H2(g) + CO(g) As time progresses, the concentrations of the reactants decreases, causing the forward reaction to slow While the concentrations of the products increases, causing the rate of the reverse reaction to increases

CH4(g) + H2O(g) 3H2(g) + CO(g) This process continues until equilibrium is reached This can be represented in a graph summary – p. 115 Remember..the system must be closed! No product or reactant can leave the system If a precipitate or a gas is formed in a system that is not closed, equilibrium will not be reached The effect of any change on equilibrium is explained by Le Chatelier’s principle End

Regents Chemistry Le Chatelier’s Principle

Le Chatelier’s Principle Any change in temperature, concentration or pressure on an equilibrium system is called a stress Le Chatelier’s principle explains how a system at equilibrium responds to relieve any stress on the system We will look at these separately…

Concentration Changes CH4(g) + H2O(g) 3H2(g) + CO(g) If the stress is the addition of more methane (CH4), the rate of the forward reaction will increase and more products will form As more product forms, the reverse reaction will also increase until reactions are equal again Overall, if the concentration of one substance is increased, the reaction that reduces the amount of the added substance is favored

Concentration Changes cont.. CH4(g) + H2O(g) 3H2(g) + CO(g) If the concentration of methane is reduced, the rate of the forward reaction decreases When the concentration of a substance decreases, the reaction that produces that substance is favored Initially the reverse reaction will take place faster than the forward reaction, and the system is said to be shifting to the left, or toward the reactant side

Concentration Changes - Ex #2 Stress: More NH3 4NH3(g) + 5O2(g) 4NO(g) + 6H2O(g) + heat + - + + + Stress: Less NH3 4NH3(g) + 5O2(g) 4NO(g) + 6H2O(g) + heat - - - - +

Temperature Changes Increasing or decreasing the temperature changes the amount of heat going into / leaving the system We can consider heat as a reactant or product, Increasing the amount of heat drives the reaction in the opposite direction of the location of the heat Decreasing the heat drives the reaction towards the location of the heat

Temperature - Example 4NH3(g) + 5O2(g) 4NO(g) + 6H2O(g) + heat Stress: More Heat 4NH3(g) + 5O2(g) 4NO(g) + 6H2O(g) + heat + + - - + Stress: Less Heat 4NH3(g) + 5O2(g) 4NO(g) + 6H2O(g) + heat - - - + +

Pressure Changes Pressure does not affect the rate of reactions of solids and liquids…however Gases are affected! We will look at how pressure affects CO2 gas in aqueous solution (contains 1 gaseous molecule) Nitrogen and hydrogen gases in the production of ammonia gas (contains more than 1 gaseous molecule)

CO2(g) CO2(aq) Pressure increases Pressure decreases 1 gas molecule Pressure increases System shifts to the right forming more CO2(aq) Pressure decreases System shifts to the left forming more CO2(g) We see this when we open a can of pop Pressure decreases, so CO2(g) increases and it comes out of solution An increase in pressure favors the side of the reaction that contains the gas

Systems with More than 1 Gaseous Substance An increase in pressure will increase the increase the concentration of gaseous molecules on both reactant and product sides of the reaction, but the effects will be unequal An increase in pressure will favor the reaction toward the side with fewer gas molecules

N2(g) + 3H2(g) 2NH3(g) Reactants have 4 gas molecules Products have 2 gas molecules An increase in pressure will favor the reaction towards the products A decrease in pressure will have the opposite effect So a decrease favors more N2 and H2 and less NH3

H2(g) + Cl2(g) 2HCl(g) Both sides have the same # of gas molecules Equal gas molecules Both sides have the same # of gas molecules In this case, pressure changes have no effect on the system Also, don’t forget catalysts! End

Regents Chemistry Entropy and Enthalpy

Entropy and Enthalpy Many factors cause and chemical and physical changes to occur.. For example: Pressure, temperature, concentration changes We see how these changes occur, but why exactly do they occur?