C10 Reaction rates and equilibrium Flashcards
Formula for rate of reaction
rate = change in concentration / time
( mol/dm^3/s )
What is the rate of a reaction?
Change in concentration of a reactant/product in a given time
Describe a concentration-time graph
Rate of reaction fastest as each reactant highest in concentration
How can you change the rate of a chemical reaction?
-Concentration/pressure
-Temperature
-Catalyst
-SA of reactants
What’s collision theory?
Two reacting particles must collide in the correct orientation, with sufficient energy (to overcome activation energy) for a reaction to occur
What is an effective collision?
Collision leading to a chemical reaction
Affect of increasing the concentration on rate of reaction?
-When conc of reactant inc, rate inc
-Inc number of particles in same volume
-Particles closer together so collide more freq
-More effective collisions
How does increasing pressure of gas affect rate of reaction?
-Rate inc
-Conc of gas molecules inc as same number of gas molecules occupy smaller volume
-Closer together, more freq collisions, more effective collisions
How can you follow the progress of a reaction?
-Monitor removal of a reactant (dec in conc)
-Monitor formation of a product (inc in conc)
How can you determine rate of a reaction that produces gases?
-Monitor volume of gas produced at regular time intervals (using gas collection)
-Monitor loss of mass of reactants using balance
What is a catalyst?
Substance that increases the rate of reaction without being used up
By providing alternative route of reaction with lower activation energy
What is an intermediate?
Species formed during a reaction that reacts further and isn’t present in final products
Features of catalyst
-Not used up
-May react with reactant to form an intermediate
-May provide surface for reaction to take place
-Regenerated at end of reaction
Draw enthalpy change profile with catalyst
What are the two types of catalyst?
Homogenous
Heterogenous
What’s a homogenous catalyst?
Catalyst has same physical state as reactants
Two examples of homogenous catalysis
-Esters made by a sulfuric acid catalyst (liquid)
-Ozone depletion with chlorine radicals as catalysts (gasses)
What’s a heterogenous catalyst?
Catalyst has a different physical state from the reactant
What is adsorption?
Process that occurs when a gas/liquid/solute is held to the surface of a solid
weakly bonded
How does a homogenous catalyst work?
Catalyst reacts with reactants to form an intermediate
Intermediate breaks down to give products and regenerates catalyst
How does a heterogenous catalyst work?
Reactant molecules adsorbed onto surface of catalyst, where reaction takes place
Product molecules then leave by desorption
What’s desorption?
Release of an adsorbed substance from a surface
Examples of heterogenous catalysts
Fe (s) - Haber process/making ammonia
Pt (s) / Rh (s) - reforming
Ni (s) - hydrogenation of alkenes
V2O5 (s) - Contact process/making SO3
How can catalysts improve sustainability?
-Inc rate of reaction by lowering energy requirements
-Reduces temperature and energy req
Economic importance of catalysts
If a process requires less energy, costs of fuel and electricity can be cut down
Inc profitability
What is the Boltzmann distribution?
Spread of molecular energies
What does Ea represent
Activation energy (small proportion, area shaded under graph on right)
What are features of the Boltzmann distribution?
-No molecules have zero energy (starts at origin)
-Area under curve is equal to total number of molecules
-No maximum energy for molecule (curve doesn’t meet x-axis)
Affect of temperature on Boltzmann distribution curve (graph)
Avg energy of molecule increases
-Peak is lower and shifted towards right
-Ea same but has greater proportion of molecules
Affect of temperature on Boltzmann distribution curve
-More molecules have an energy greater or equal to the activation energy
-Greater proportion of collisions lead to reaction, inc rate of reactiom
-Collisions become fore freq
Affect of catalyst on Boltzmann distribution curve (graph)
Shifts Ea to left and becomes Ec
Affect of catalyst on Boltzmann distribution curve
-Greater proportion of molecules have energy greater/equal to the lower activation energy
-More molecules have effective collisions, inc rate of reaction
What are reversible reactions?
Reaction that takes place in both forward and reverse directions
What does this <=> indicate
Reversible reaction
What is dynamic equilibrium?
Equilibrium that exists in a closed system
where rate of forward and reverse reaction is the same
and concentrations do not change
What is a closed system?
System isolated from its surroundings
What is the position of equilibrium?
Relative quantities of reactants and products, indicating extent of a reversible reaction at equilibrium
What is le Chatelier’s principle?
When a system in dynamic equilibrium is subjected to an external change, the system readjusts itself to minimise the effect of the change and to restore equilibrium
What will increasing concentration of reactants do?
-Equilibrium shifts to right, making more products
What will increasing concentration of products do?
-Equilibrium shifts to left, making more reactants
What will changing temperature do?
-Inc temp shifts equilibrium position in endothermic direction
-Dec temp shifts equilibrium position in exothermic direction
If forward reaction is exothermic, what is the effect of changing temp?
Inc temp - position shifts to left, more reactants made
Dec temp - position shifts to right, more products made
If forward reaction is endothermic, what is the effect of changing temp?
Inc temp - position shifts to right, more products made
Dec temp - position shifts to left, more reactants made
What will increasing pressure do?
-Equilibrium position moves to side with fewer gaseous moles
What effect does a catalyst have on equilibrium?
Does not change equilibrium position, just speeds up the rate of reaction
What is an equilibrium constant?
Measure of the position of the equilibrium
-Magnitude indicates whether there are more products/reactants
(Kc)
What is equilibrium law?
aA + bB <=> cC + dD
Kc = [C]^c [D]^d / [A]^a [B]^b
-[] mean conc of
What does the value for Kc tell us
-Value of 1 indicates equilibrium is halfway between reactants and products
-Value>1 indicates equilibrium towards products
-Value<1 indicates equilibrium towards reactants
