Rates of Reaction - B PDF

Rates of Reaction – B 𝑅𝑎𝑡𝑒 = Production of Oxygen Experiment 𝐶ℎ𝑎𝑛𝑔𝑒 𝑖𝑛 𝑐𝑜𝑛𝑐. 𝑇𝑖𝑚𝑒 𝑇𝑎𝑘𝑒𝑛  Hydrogen Peroxide breaks down to form oxygen as follows: H2O2 H2O + ½ O2  This reaction is slow & so manganese dioxide is used as a catalyst.  The instantaneous rate of a reaction is the rate at a particular...

Rates of Reaction – B 𝑅𝑎𝑡𝑒 = Production of Oxygen Experiment 𝐶ℎ𝑎𝑛𝑔𝑒 𝑖𝑛 𝑐𝑜𝑛𝑐. 𝑇𝑖𝑚𝑒 𝑇𝑎𝑘𝑒𝑛  Hydrogen Peroxide breaks down to form oxygen as follows: H2O2 H2O + ½ O2  This reaction is slow & so manganese dioxide is used as a catalyst.  The instantaneous rate of a reaction is the rate at a particular point in time during the reaction.  𝑟𝑎𝑡𝑒 = Factors a ecting rates of reaction 1. Concentration of reactants 2. Temperature 3. Particle Size 4. Nature of Reactants: Ionic/Covalent 5. Presence of a Catalyst Definitions  A catalyst is a substance that alters the rate of a chemical reaction, but is chemically unchanged at the end of the reaction.  HETEROGENEOUS CATALYSIS: This is where the catalyst is in a di erent physical phase to the reactants.  HOMOGENOUS CATALYSIS: This is when the catalyst is in the same physical phase as the reactants.  AUTOCATALYSIS: This is where a product of a reaction becomes a catalyst for the same reaction.  The rate of reaction is the change in concentration in unit time of any one reactant or product.  Activation energy (EACT) is the minimum energy with which colliding particles must have for a reaction to occur.  An e ective collision is one which has su icient activation energy & results in product being formed. 1. INTERMEDIATE FORMATION THEORY:  This involves homogeneous catalysis.  An unstable intermediate is formed between a reactant & the catalyst.  This intermediate quickly breaks down to form the product, with the catalyst being regenerated in the process. 2. SURFACE ADSORPTION THEORY:  Adsorption means the accumulation of one substance at the surface of another.  This type of catalysis is heterogeneous.  The reactants are usually gases & the catalyst a solid.  The gaseous reactants di use onto the surface of the solid catalyst (ie. Adsorption takes place).  The reaction between the gases takes place at an active site on the catalyst.  The products di use away leaving the active site free for further reactions.  The more finely divided or porous the catalyst, the greater the reaction rate due to the availability of catalytic sites.  Certain particles (eg. lead, arsenic) can block these active sites; these are called catalytic poisons.  Ex; Methanol & oxygen react together on a hot platinum catalyst: If ∆H is negative  heat has been lost during the reaction ∴ exothermic reaction If ∆H is positive  heat has been taken in during the reaction ∴ endothermic reaction An increase of 10K on a reaction mixture would have the following e ects on:  Number of collisions: Small (50%) increase  The Activation energy: No change Instantaneous Rate of Reaction found by drawing tangent to curve at the point. Exp. to show e ect of concentration on rate: Concentration is directly proportional to rate. Exp. To show e ect of temperature on rate;  Again Na2S2O3 is reacted with HCL to produce a sulphur ppc.  The sodium thiosulphate is heated to a range of temperatures before the HCl is added:  Exponential – not directly proportional The E ect on reaction rate of a catalyst;  Add manganese dioxide to a beaker containing hydrogen peroxide.  Bubbles of oxygen gas are immediately observed. 2H2O  O2 + 2H2O