CHEMICAL KINETICS!

CHEMICAL KINETICS!

Collision Theory • If you have sodium (Na) in one beaker and water (H2O) in another, will they react? 2 Na (s) + H2O (l) ---> 2 NaOH (s) • No – not only do both components have to be present, they have to be in contact • When atoms and/or molecules collide, there is a chance that a reaction will take place

Every Collision is a Good Collision? • Not true – the vast majority of collisions don’t cause reactions. Those that cause reactions are called successful collisions. • A reaction has to have sufficient activation energy in order to cause a reaction • Some factors in the collision process are: • Temperature • Concentration • Catalysts • Nature of the Reactants

Activation Energy (Ea) • Svante Arrhenius coined the term in 1889 (You’ll love Arrhenius when you learn his equation.) • The minimum energy required to cause a reaction between two reactants • Activation energy is different for every reaction • Imagine your alarm is going off – how often do you hit the snooze button? Each time you do, you don’t have enough activation energy to wake up, so you sleep a little longer.

Activation Energy… again • Activation energy graphs look like hills • The beginning of the hill is the energy of the reactants, the top of the hill is the energy needed to start the reaction (activated complex) • The end of the hill is the energy of the products • If the end of the hill is higher than the beginning, the reaction is ENDOTHERMIC. If the end is lower, the reaction is EXOTHERMIC.

Activation Energy Graphs… again • A reaction with more than one step will have multiple hills • The first ‘hill’ applies to the first reaction, and so on • Each peak is a different ‘activated complex’ and each trough is an ‘intermediate’ – intermediates are products that are formed along the way, but are used up in reactions that follow

Temperature • What is the difference between a solid and a gas? What is the difference between a hot particle and a cold particle? • SPEED • Particles at higher temperatures move at higher speeds. The faster it moves, the more energy is has – remember physics? More energy means more energetic collisions. • Increasing the temperature allows collisions to overcome activation energy.

Concentration • Imagine a cardboard box with 10 bouncy balls that never stop bouncing, 5 red and 5 green. • Next, imagine the same box with 55 bouncy balls, 50 red and 5 green. • Last, imagine that same box with 100 bouncy balls, 50 red and 50 green. • In which case will there be the most collisions of red and green balls? The least? • Higher concentration means more collisions, and more collisions means a better chance for more successful collisions. • http://www.kscience.co.uk/animations/collision.htm

Catalysts • Catalysts are substances that are added to a reaction to lower the activation energy ‘barrier’. • We’ll be going overcatalysts more lateron in this unitwhen we look atreactionmechanisms.

Nature of Reactants • With atoms and small molecules, where the collision takes place doesn’t matter • If you have a big blob of atoms stuck together, though, and the only place to react is on the bottom, will all collisions be successful? • This is why a lot of collisions don’t react – they have to collide just right

Nature of Reactants… again • Two kinds of reactions: • Homogenous phase – all reactants are in the same phase (solid, liquid, gas, aqueous) • Faster • Heterogeneous phase – reactants are in different phases • Slower

Svante Arrhenius • Swedish scientist – studied physics and chemistry • Came up with the idea of activation energy in 1889 • The Arrhenius equation, first proposed by JacobusHenricusvan’t Hoff (say that ten times fast!), describes the temperature dependence of the rate of a reaction

Arrhenius Equation • Are you ready? OR

CHEMICAL KINETICS!