REaction Rates and Equilibrium How Fast and How Far

Objectives To understand collision model of chemical reactions To understand activation energy To understand how a catalyst speeds up a reaction To explore reactions with reactants or products in different phases To learn how equilibrium is established To learn about characteristics of chemical equilibrium To understand collision model of chemical reactions To understand activation energy To understand how a catalyst speeds up a reaction To explore reactions with reactants or products in different phases To learn how equilibrium is established To learn about characteristics of chemical equilibrium

How Chemical reactions occur Molecules react by colliding with each other Reactions go faster if concentration is increased Reactions go faster with increased temperature Molecules react by colliding with each other Reactions go faster if concentration is increased Reactions go faster with increased temperature

Conditions affecting reaction rates Concentration - more molecules lead to more collisions Why temperature? Collisions must have enough energy to break bonds Minimum energy to break bonds = activation energy Concentration - more molecules lead to more collisions Why temperature? Collisions must have enough energy to break bonds Minimum energy to break bonds = activation energy

Temperature and Reaction rate Higher Temp Higher Speed More High- energy Collisions More bond breakage Faster Reaction

Reaction mechanism Step by step sequence of elementary reactions by which an overall chemical change occurs For example: CO + NO2 --> CO2 + NO The reaction mechanism for this might be: 2NO2 --> NO3 + NO (slow) NO3 + CO --> NO2 +CO2 (fast) Each step has its own rate Rate of overall reaction is limited by rate of slowest reaction mechanism step Step by step sequence of elementary reactions by which an overall chemical change occurs For example: CO + NO2 --> CO2 + NO The reaction mechanism for this might be: 2NO2 --> NO3 + NO (slow) NO3 + CO --> NO2 +CO2 (fast) Each step has its own rate Rate of overall reaction is limited by rate of slowest reaction mechanism step

Catalysts Change reaction rate without changing temperature or concentration Enzymes in your body are catalysts Provides new pathway for reaction - lowers activation energy Catalyst not used up in reaction Change reaction rate without changing temperature or concentration Enzymes in your body are catalysts Provides new pathway for reaction - lowers activation energy Catalyst not used up in reaction

How do catalysts work? They increase the rate of effective collisions How? Two main ways: Help provide proper orientation of molecules so collisions “work” Provide an alternate reaction mechanism that requires lower activation energy Catalyst not used up or changed = can go on to catalyze more reactions They increase the rate of effective collisions How? Two main ways: Help provide proper orientation of molecules so collisions “work” Provide an alternate reaction mechanism that requires lower activation energy Catalyst not used up or changed = can go on to catalyze more reactions

Catalyst example Ozone Layer Destruction Ozone absorbs high-energy radiation from the sun (e.g. UV light) O + O 3 --> 2O 2 Chlorine catalyzes the reaction One chlorine atom can catalyze the destruction of 1 million ozone molecules/second Chlorine in the atmosphere from pollution by certain molecules called chlorofluorocarbons (e.g. CF 2 Cl 2 - freon) Ozone Layer Destruction Ozone absorbs high-energy radiation from the sun (e.g. UV light) O + O 3 --> 2O 2 Chlorine catalyzes the reaction One chlorine atom can catalyze the destruction of 1 million ozone molecules/second Chlorine in the atmosphere from pollution by certain molecules called chlorofluorocarbons (e.g. CF 2 Cl 2 - freon)

EQUILIBRIUM The exact balancing of two processes, one of which is the opposite of the other For example - Physical equilibrium - Gas vaporizing over a liquid and gas condensing into liquid H2O(g) H2O(l) This is a reversible process, indicated by double arrow Equilibrium is reached when both processes are occurring at the same rate The exact balancing of two processes, one of which is the opposite of the other For example - Physical equilibrium - Gas vaporizing over a liquid and gas condensing into liquid H2O(g) H2O(l) This is a reversible process, indicated by double arrow Equilibrium is reached when both processes are occurring at the same rate

Chemical Equilibrium 2NO 2 (g) N 2 O 4 (g) Dynamic ConditionDynamic Condition (process always happening) Analogy - rate of cars leaving (reverse reaction) island on bridge = rate of cars arriving (forward reaction) on island on bridge - # of cars on island remains the same Concentration of reactants and concentration of products remain the same indefinitely as long as the conditions are not changed 2NO 2 (g) N 2 O 4 (g) Dynamic ConditionDynamic Condition (process always happening) Analogy - rate of cars leaving (reverse reaction) island on bridge = rate of cars arriving (forward reaction) on island on bridge - # of cars on island remains the same Concentration of reactants and concentration of products remain the same indefinitely as long as the conditions are not changed

Questions What must happen for two molecules to react together? Name two factors that affect the rate of a reaction. What is a catalyst? Give an example. How do catalysts increase the rate of a reaction? Define equilibrium. Give an example of physical equilibrium. Give an example of chemical equilibrium. What must happen for two molecules to react together? Name two factors that affect the rate of a reaction. What is a catalyst? Give an example. How do catalysts increase the rate of a reaction? Define equilibrium. Give an example of physical equilibrium. Give an example of chemical equilibrium.

What if conditions Change? Le Chatelier’s Principle