Reactions rates and equilibrium chp 10 Flashcards
whats the formula for the rate of reaction and what units are used
when is the rate of reaction fastest and when is it slowest
- 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 concentrations stops changing and the rate of reaction is 0
what 4 factors effect the rate of reaction
-Concentration (pressure when reactants are gases)
-temperature
-use of a catalyst
-surface are of solid reactants
what is collision theory
collision theory states that 2 reacting particles must collide for a reaction to occur.
in most collisions, the molecules collide but then bounce off each other and remain chemically unchanged
Does every partical that collides react
Usually only a small proportion of collisions result in a chemical reaction
in most collisions, the molecules collide but then bounce off each other and remain chemically unchanged
why are some collisions effective and why are some ineffective
An effective collision is one that leads to a chemical reaction
A effective collision needs 2 conditions to be met:
-the particles collide with the correct orientation
-the particles have sufficient energy to overcome the activation energy barrier of the reaction
How does increasing the concentration affect the rate of reaction
- higher conc higher number of particles in same space
- particles collide more frequently
^more effective collisions, increase rate of reaction
How does increasing the pressure of a gas affect the rate of reaction
- ↑ pressure = ↑ rate of reaction
- The concentration of the gas molecules increases as the same amount inhabits a smaller volume.
- The gas molecules are closer together and collide more frequently, leading to more effective collisions in the same time period.
how can the progession of a reaction be followed
-Monitoring the removal (decrease in concentration) of a reactant
-following the formation (increase in concentration) of a product
-other measurable properties that might change as the reaction proceed include gas volume and colour.
if a reaction produces a gas, which 2 methods can be used to determine the rate of the reaction
- monitoring volume of gas produced at regular time intervals using gas syringes
- monitoring loss of mass of reactants using a balance
what do catalysts do
a catalyst is a substance that changes the rate of a chemical reaction without undergoing any permanent change itself
what are the characteristics of catalysts
-the catalyst is not used up in the chemical reaction
-the catalyst may react with a reactant to form an intermediate or may provide a surface on which the reaction can take place
-at the end of the reaction that catalyst is regenerated
How do catalysts work on a basic level
A catalyst increases the rate of a chemical reaction by providing an alternative pathway or lower activation energy
what is a exothermic reaction
A reaction is exothermic when the products have less energy than the reactants and so the enthaply change is negative
what is an endothermic reaction
a reaction is defined as endothermic when it absorb energy and so the enthalpy change is positive
what does a catalyst look like on a enthaply change diagram
what is a homogenous catalyst
A homogeneous catalyst has the same physical state as the reactants.
The catalyst reacts with the reactants to form an intermediate, the intermediate then breaks down to give the product and regenerates the catalyst
whats an example of a reaction that involves the use of a homogenous catalyst
-Making esters with sulphuric acid as a catalyst
-The reactants (ethanol and ethanoic acid) and the catalyst (sulphuric acid) are all liquids
what is a hetrogeneous catalyst
- different physical state than reactants
How do hetrogeneous catalysts interact with reactant molecules.
Reactant molecules are absorbed (weakly bonded) onto the surface of the catalyst, where the reactions take place.
After reaction, the product molecules leave the surface of the catalyst by desorption.
what are some examples of reactions that need catalysts and what are those catalysts
why are catalyst important for sustainability and the economy
90% of all chemical materials are produced using a catalyst
if a chemical process requires less energy, then less electricity or fossil fuels are used, making the product faster and using less energy can cut costs and increase profitability
what is autocatalysis
a chemical reaction is said to have undergone autocatalysis if a reaction product acts as a catalyst for that reaction. typically the reaction starts slowly and then speeds up as the products are formed
if a collison is described as elastic what does it mean
the molecules do not slow down as a result of a collision and no energy is lost
what is the spread of molecular energy between gaseous molecules called
the boltzmann distibution
what does a boltzmann distribution look like and what does it represent
it represents the spread of energy between all the molecules in a given substance and also shows the amount of molecules with activation energy
what are some important features of the boltzmann distribution
- no molecules have 0 energy
- the curve starts at the origin
- area under curve is equal to total number of molecules
- there is no maximum energy for a molecule
- what is the effect of increasing temperature on the Boltzmann distribution curve
- explain the effect
- As temp increases, average energy molecules increases
^more molecules have Ea - small proportion of molecules will have below Ea
- increased rate of reactions
^more molecules have Ea and collisions occur more due to rise in kinetic energy
vice versa
what is the effect of catalysts on the boltzmann distribution
- catalyst provides alternative reaction route with lower activation energy
- greater proportion of molecules would have activation energy
^more success full collisions
what are reversible reactions
reactions that take place in both forward and reverse directions (an example is the haber process)
what is a equilibrium system and what are its features
in an equilibrium system:
- rate of forward reaction is equal to rate of reverse reaction
- concentrations of reactants and products do not change
- they are dynamic
what needs to occur for a reaction to remain in equilibrium
for a reaction to remain in equilibrium, the system must be closed
what is meant by a closed system
A closed system is isolated from its surroundings, so the temperature, pressure and concentrations of reactants and products are unaffected by outside influences
what does le chatelier’s principle state
states that when a system in equilibrium is subjected to an external change the system readjusts itself to minimise the effect of that change
what does the position of equilibrium indicate
a measure of the concentrations of the reacting substances at equilibrium .
what is the effect of changing concentration on the position of equilibrium
- The equilibrium would shift towards the side of the reaction that hasn’t experienced the increase in concentration
How does an increase or decrease in temperature effect the position of equilibrium
- ↑ temperature shifts equilibrium towards endothermic reaction
- ↓ temperature shifts equilibrium towards exothermic reaction
what is an example of a reaction that can be used to illustate changing concentration
what is the effect of increasing and decreasing pressure
-increasing the pressure of the system will shift the position of the equilibrium to the side with the fewer molecules, reducing the pressure of the system
-Decreasing the pressure shifts the position of equilibrium to the side with more gaseous moles
the pressure of a gas is proportional to its concentration
what is the effect of a catalyst on the equilibrium
-a catalyst does not change the position of equilibrium; it merely speeds up the rates of the forward and reverse reactions equally
-it will increase the rate at which an equilibrium is established
what are the conditions used for the haber process
-temperature of 350-500°C
-pressure of 100-200 atm
-an iron catalyst
what is the formula for the the equilibrium constant
small letters = balancing numbers
big letters = equilibrium concentrations of substance
what does the equilibrium constant (Kc) represent
-a Kc value of 1 indicates a position of equilibrium that is halfway between reactants and products
-a Kc value >1 indicates a position of equilibrium that is towards the products
-a Kc value <1 indicates a position of equilibrium that is towards the reactants
- the larger the value of Kc the further the position of equilibrium lies to the right-hand side and the greater the concentration of the products compared to the reactants
