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KEY PHRASE---> "EFFECTIVE COLLISIONS"
How does a reaction occur? What is collision theory? When two chemicals react, their molecules have to collide with each other with sufficient energy and the correct orientation for the reaction to take place. This is collision theory. What does orientation mean?
What does nature of reactants mean? Chemists believe that the type, strength and number of chemical bonds or attractions between atoms determines with what speed the particles have to collide with each, to create an effective reaction. This is termed the activation or threshold energy for the reaction. If this activation energy is relatively high, typically the reaction is slower given other factors being equal. If it is low the reaction is typically faster. 1. Reactions involving simple ions are most often instantaneous. This is due to the fact that the positive and negative charges attract each other and no bonds have to be broken in creating the new substances. IONS=FAST Pb2+ (aq)+ 2I-(aq)-->PbI2(s) 2. Reactions between molecules are usually slower than ions. In molecules, bonds have to be broken and new bonds reformed. This slows down reaction rates. MOLECULES=SLOW 2CO(g)+O2(g)-->2CO2(g) How does temperature affect the rate of a chemical reaction? The two molecules will only react if they have enough energy. By heating the mixture, you will raise the average energy levels of the molecules involved in the reaction. Increasing temperature means the molecules move faster and have more energetic collision and therefore more "EFFECTIVE COLLISIONS" that can overcome the activation energy required for a chemical reaction. As a rough approximation, for many reactions happening at around room temperature, the rate of reaction doubles for every 10°C rise in temperature. Experiment Temperature effect on Alka Seltzer (Hot vs Cold)
How does concentration affect the rate of a reaction? Increasing the concentration of the reactants will increase the frequency of collisions between the two reactants. There are more reacting particles in a given volume. So this are more collisions and therefore more "EFFECTIVE COLLISIONS".
Experiment Reaction rate vs. Concentration
How does surface area affect a chemical reaction? Increasing surface area has NO EFFECT on the energy of the particles. It does however increase the number of sites available for a reactions to occur. So more collisions can occur and therefore more "EFFECTIVE COLLISIONS" can happen. Solids with a smaller particle size (e.g. powders or small chips) react more quickly than solids with a larger particle size (e.g large chips). Diagrams of Particle Size and Collisions
Note the perimeter of the large cube has 12 sides for a possible reaction. The Small cubes combined have 36 (9*4)sides for a reaction to occur. Experiment
How do catalysts affect the rate of a reaction? Catalysts speed up chemical reactions. Only very minute quantities of the catalyst are required to produce a dramatic change in the rate of the reaction. This is really because the reaction proceeds by a different pathway when the catalyst is present. Adding extra catalyst will make absolutely no difference. The only other important thing you need to remember about catalysts is that they are not consumed in the course of the reaction and therefore do not appear in the overall reaction. How does a catalyst work? A catalyst provides a surface on which the reaction can take place. This increases the number of collisions between the particles of the substances that are reacting.
Ammonia oxidized on the surface of the hot Pt wire according to the
equation: Or Elephant tooth paste 2H2O2---> 2H2O + O2
What does a catalyst do when a reaction is at EQUILIBRIUM? REGENTS LOVES THIS CONCEPT-A catalyst increase the reaction rate. At equilibrium the increases the rate is equal in both the forward and reverse directions equally. Does increasing pressure count as increasing reaction rate? I would say YES and NO. My reasoning...the pressure is directly related to the concentration of the gases. Therefore, an increase in pressure leads to an increase in reaction rate. Lets file that under concentration. Past Regents Questions-
August 2007-46
Given the balanced equation representing a reaction:
June 2007-16
A 5.0-gram sample of zinc and a 50.-milliliter sample of hydrochloric
acid are used in a chemical reaction. Which combination of these samples
has the fastest reaction rate?
June 2008-46 At 20.°C, a 1.2-gram sample of Mg ribbon reacts rapidly
with 10.0 milliliters of 1.0 M HCl(aq). Which change in
conditions would have caused the reaction to proceed more slowly?
Aug 2009-24 Why can an increase in temperature lead to more
effective collisions between reactant particles and an increase in the
rate of a chemical reaction?
August 2010-43
As the temperature of a chemical reaction in the gas phase is increased,
the rate of the reaction increases because
June 2009-41
Given the balanced equation representing a reaction:
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