P2 6 Rate/extent of chemical change
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describe collision theory
• collision theory: reactant particles must collide with each other (providing particles have sufficient activation energy) to start a reaction • a collision that produces a reaction is a successful collision
define activation energy
• activation energy - minimum amount of energy needed for a collision to be successful • activation energy is diff for different reactions
state factors affecting rate of chemical reactions (5)
• temperature • concentration of reactant • reacting gas pressure • SA:V of solid reactant • presence of a catalyst -> all increase rate when increased
describe how increased temperature increases rate of reaction
particles gain more energy so move around quicker and collide more frequently
describe the method for effect of temperature on rate of reaction by measuring change in colour/ turbidity (RP) using sodium thiosulfate+ hydrochloric acid
• add 50 cm³ of dilute sodium thiosulfate solution to a conical flask • place the conical flask on a piece of paper with a black cross drawn on it • add 10 cm³ of dilute hydrochloric acid to the conical flask • immediately swirl the flask to mix its contents and start a stopwatch • look directly down through the reaction mixture, when the cross can no longer be seen record the time on the stopwatch • repeat steps with different temperatures of sodium thiosulfate solution
describe how increased concentration of reactant increases rate of reaction
more reactant particles in a given volume so more frequent collisions
describe the method for effect of concentration on rate of reaction by measuring volume of gas produced (RP) using calcium carbonate+ hydrochloric acid
• support a gas syringe with a stand, bung and clamp • add 50 cm³ of dilute hydrochloric acid to a conical flask • add 0.4 g of calcium carbonate to the flask, immediately connect the bung • when reactions stops record volume of gas produced in syringe • repeat steps with different concentrations of hydrochloric acid
describe how increased reacting gas pressure increases rate of reaction
more reactant particles in a given volume so more frequent collisions
describe how increased SA:V of solid reactant increases rate of reaction
-> eg. cutting up the solid into smaller pieces/ into a powder to increase SA more outside particles exposed to other reactants so more frequent collisions
describe catalysts and how they increase rate of reaction
catalyst - substance which increases rate of reaction without undergoing a permanent change * they provide a different pathway for the reaction with a lower activation energy * not included in the equation * eg. enzymes used in biological systems
describe what a reaction profile looks like for catalysed reactions (vs normal reactions)
the curve showing activation energy on the reaction profile will be lower (as they provide a different pathway for the reaction with a lower activation energy)
state the 2 equations for calculating mean rate of reaction
quantity of reactant used / time taken OR quantity of product formed / time taken -> quantity measured in g or cm³ or mol -> time measured in secs -> rate measured in g/s or cm³/s (also mol/s) -> the rate is a mean as the rate varies over time
describe reversible reactions
(backwards reaction) ⇌ (forwards reaction) * direction of reaction is changed by reversing the conditions - if the reactant is heated, the product is cooled * if the forwards reaction is exothermic (-kJ) , the backwards reaction is endothermic (+kJ)
state Le Chatelier’s principle
if a system is at equilibrium and a change is made to its conditions, the system will respond to counteract the change
name the three conditions of dynamic equilibirum
• reaction is reversible • reaction is in a closed system • forwards reaction rate = backwards reaction rate
describe how the equilibrium shifts when there is an increase in temperature
equilibrium shifts to the endothermic side/direction (left or right)
describe how the equilibrium shifts when there is an increase in reactant concentration
equilibrium shifts to the products
describe how the equilibrium shifts when there is a decrease in reactant concentration
equilibrium shifts to the reactants
describe how the equilibrium shifts when there is an increase in pressure
equilibrium shifts to the side with fewer gas molecules
describe how the equilibrium shifts when there is a decrease in pressure
equilibrium shifts to the side with more gas molecules
describe how the equilibrium shifts when there is an added catalyst
has no effect on the position - increases rate of reaction on both sides of the equation so is already balanced (but does allow equilibrium to be reached quicker/ at lower temp)