10- Reaction Rates and Equilibrium Flashcards
What is rate of reaction ?
the change in concentration of a reactant or product in a given time
How do you calculate rate?
change in concentration / time
What makes an effective collision?
-particles must have sufficient energy to overcome the activation energy barrier of the reaction
-particles must collide in correct orientation
What factors can affect the rate of chemical reaction?
-concentration (or pressure when reactants are gases)
-temperature
-use of a catalyst
-surface area of solid reactants
What effect does increasing temperature have on reaction rate?
reaction rate increases
Explain why increase temperature has its effect on reaction rate?
-particles have more energy
-higher speed
-more frequent collisions
-more successful collisions
-rate increases
What is the rule of thumb for reaction rate and temperature?
A 10C increase in temperature doubles the rate of
What effect does increasing concentration/pressure have on reaction rate?
reaction rate increases
Explain why increasing concentration/pressure has its effect on reaction rate
-concentration increases
-more particles per unit volume
-particles closer together
-more frequent collisions
-more successful collisions
-rate increases
How can progress of a chemical reaction be followed?
-monitoring the removal of a reactant
-following the formation of a product
What are the two methods of determining reaction rate, when a gas is produced?
-monitoring the volume of gas produced at regular time intervals using gas collection
-monitoring the loss of mass of reactants using a balance
When is the rate of reaction the fastest?
-at the start of the reaction, as each reactant is at its highest concentration
-slows down as the reaction proceeds, because the reactants are being used up and their concentrations decrease
-once one of the reactants has been completely used up, the concentration stops changing and the rate of reaction is zero
What does collision theory state?
-two reacting particles must collide for a reaction to occur
-only a small proportion of collisions result in a chemical reaction
-in most collisions, molecules collide but then bounce off each other and remain chemically unchanged
How do you calculate rate of reaction from graphs?
-draw a tangent
-calculate its gradient (change in y / change in x)
What is a catalyst?
a substance that change the rate of a chemical reaction without undergoing any permanent change itself
What are the main properties of a catalyst?
-not used up in the reaction
-may react with a reactant to from an intermediate or may provide a surface for the reaction to take place
-the catalyst is regenerated at the end of the reaction
How does a catalyst increase the rate of reaction?
-provides an alternate reaction pathway with a lower activation energy. A greater proportion of molecules have more energy greater than or equal to activation energy
What is a homogenous catalyst?
Has the same physical state as the reactants
How does a homogeneous catalyst interact with the reactants?
-froms an intermediate
-this intermediate then breaks down to give the product and regenerates the catalyst
Give 2 examples of homogenous catalysts?
-sulfuric acid (H2SO4) in production of esters
-chlorine radicals in ozone depletion
What is a heterogenous catalyst?
has a different physical state from the reactants
How does a heterogeneous catalyst interact with reactants?
-Usually solids in contact with gaseous reactants or reactants in solution
-reactants are absorbed onto the surface of catalyst where the reaction takes place
-after the reaction, the products leave the surface of catalyst by desorption
Give examples of heterogenous catalysts?
-iron in making ammonia
-platinum or rhodium in reforming
-nickel in hydrogenation of alkenes
-vanadium oxide in making sulfur trioxide
What are the benefits of using a catalyst?
-increase rate of reaction by lowering activation energy
-reduced temperature, lower energy demand = lower CO2 emissions
-greater atom economy, reduced use of toxic solvents and reactants
-products made faster and cuts costs and increases profit
-less fossil fuels used = cuts emissions of CO2
What is autocatalysis?
A reaction product acts as a catalyst for that reaction
What are the main features of Boltzmann distribution?
-no molecules have zero energy (curve starts at the origin)
-the area under the curve is equal to the total number of molecules
-there is no maximum energy for a molecule (the curve does not meet the x axis at high energy. the curve would need to reach infinite energy to meet the x-axis
How does the Boltzmann distribution curve change when temperature is increased?
-the peak is lower and shifted to the right (peak at higher energy)
-a greater proportion of molecules can overcome the activation energy
-number of molecules is the same so area under earth the curve remains the same
In terms of molecules and collisions what occurs at higher temperatures?
-more molecules have an energy greater than or equal to the activation energy
-therefore a greater proportion of collisions will lead to a reaction, increasing the rate of reaction
-collisions will be more frequent as molecules have greater kinetic energy
How does the Boltzmann distribution show the effect of a catalyst?
-activation energy with a catalyst is lower then normal activation energy
-therefore a greater proportion of molecules exceed the new lower activation energy (Ec)
-(on collision) a greater proportion of molecules will react to form products
