Chapter 16 - Rates Of Reactions Flashcards
Rate of reaction
The change in concentration per unit time of any one reactant or product
Average rate of reaction
The average rate of the reaction over the entire experiment
Instantaneous rate of reaction
The rate of the reaction at a particular moment in time
What are the factors affecting the rates of reactions?
Nature of reactants Particle size Concentration Temperature Catalysts
Why does the nature of the reactants affect the rate of reaction?
Reactions involving the breaking of covalent bonds have a higher activation energy as energy is required to break the bonds before new bonds can be formed. Ionic reactions just involve the coming together of charged particles and will not involve the breaking of bonds and so will have higher reaction rate and lower activation energy
Why does particle size affect the rate of reaction?
The smaller the particle size, the greater the surface area exposed and so greater number of collisions, and also, the number of these collisions that are effective increases. Therefore there is increased reaction rate
Why does concentration increase reaction rate?
If concentration increases so to does the number of collisions because particles are closer together. As a result, so will the number of effective collisions. Therefore there is an increase in reaction rate. Also, as the reaction proceeds, the concentration of reactants will decrease and so the rate of reaction will decrease over time
Why does temperature affect the rate of reaction?
At an increased temperature, there is an increase in the number of collisions due to the increased energy of the particles. The increased energy of the collisions means more collisions reach the activation energy.
Catalyst
Since that alters the rate of a chemical reaction but is not consumed in the reaction
Negative catalysts/inhibitors
Slow down reaction
Homogeneous catalysts
Both the reactants and the catalysts are in the same phase, i.e. there is no boundary between the reactants and the catalyst
Phase
Specific state of matter that has uniform chemical and physical properties
Heterogeneous catalysts
The reactants and catalysts are in different phases, i.e. there is a boundary between the reactants and the catalysts
Autocatalysts
One of the products of the reaction acts as a catalyst for the reaction
Give an example of a homogeneous catalyst
The decomposition of hydrogen peroxide solution by iodide ions; they are all in the same, liquid phase
Give an example of a heterogeneous catalyst
The decomposition of hydrogen peroxide (liquid) by manganese dioxide (solid)
Give an example of an autocatalyst
The reaction between potassium permanganate (KMnO4) and Fe2+ ions leads to the formation of Mn2+ ions which act as an autocatalyst for the reaction
What are the two general mechanisms by which catalysis takes place?
- The Intermediate Formation Theory
2. The Surface Adsorption Theory
The Intermediate Formation Theory
States that a catalyst works through the formation of an intermediate compound. In the reaction, one (or sometimes more compounds) combine with the catalyst to form the intermediate compound. This then reacts with the other reactant to form the final product and regenerate the catalyst
Give an example of a reaction the demonstrates the Intermediate Formation Theory
The oxidation of potassium sodium tartrate using hydrogen peroxide. Catalysed by the presence of Co2+ ions (pink)
The Surface Adsorption Theory
The accumulation of substances only at the surface of another substance (heterogeneous)
What are the stages of the Surface Adsorption Theory?
- Adsorption stage
- Reaction on the surface
- Desorption stage
Catalytic converter
Device fitted in the exhaust system of a motor vehicle which contains catalysts to convert pollutants in the exhaust gases to less harmful substances
What does a catalytic converted look like?
It consists of a ceramic honeycomb inside a stainless steel case. The honeycomb is lined with platinum, palladium and rhodium catalysts
What is the benefit of the honeycomb structure of the catalytic converter?
It provides a large surface area on which the reactions can occur
Catalyst poison
Substance that makes a catalyst inactive
What can poison the catalysts in a catalytic converter?
Lead (Pb)
What are the assumptions of the Collision Theory?
○For the reaction to occur, reacting particles must collide with each other
○Collision only results in the formation of products if a certain minimum energy is exceeded in the collision
Effective collision
One that results in the formation of new products
Activation energy
The minimum energy that colliding particles must have for a reaction to occur, i.e, the minimum energy required for effective collisions between particles to occur
Reaction profile diagram
A graph which shows the change in energy of a chemical reaction with time as the reaction progesses
Activated complex
Intermediate substance formed by the collision of molecules with enough energy to react
Exothermic or endothermic reaction? : products have less energy that the reactants
Exothermic
Exothermic or endothermic reaction? : products have more energy that the reactants
Endothermic
Why do catalysts affect the rate of a reaction?
They provide an alternative reaction pathway with a lower activation energy. Therefore, more reactant molecules now possess the energy required for effective collisions to occur and the rate of reaction incfreases
Why do transition metals make good catalysts?
They have valence electrons in two shells instead of one and they can easily lend and take away electrons from other molecules
