3.2 Physical chemistry Flashcards
Describe an exothermic enthalpy profile
reactant line above products line
Describe an endothermic enthalpy profile
reactant line below products line
What is activation energy?
the minimum amount of energy required for a reaction to take place
What are the standard conditions?
298 K
100 kPa
What is the enthalpy change of formation?
the energy required to form 1 mole of a compound from its elements
What is the enthalpy change of combustion?
energy required for the complete combustion of 1 mole of a substance
What is the enthalpy change of reaction?
the energy change associated with a given reaction
What is the enthalpy change of neutralisation?
energy required to form 1 mole of water from neutralisation
Define ‘average bond enthalpy’ ?
the mean energy needed for 1 mole of a given type of gaseous bonds to undergo homolytic fission - breaking of a covalent bond
What is an endothermic reaction (in terms of bond breaking/making)
more energy needed to break bonds than make new bonds
reaction takes in energy
What is an exothermic reaction (in terms of bond breaking/making)
more energy released when new bonds formed than needed to break bonds
reaction releases energy
What factors affect a rate of reaction?
temperature
pressure
catalyst
surface area
concentration
Describe the effect of concentration on the rate of reaction, in terms of collisions.
increased concentration - more molecules in same volume
molecules will be closer together so greater chance of them colliding with enough energy to overcome activation energy
so more frequent collisions
rate increased
Describe the effect of pressure on the rate of reaction, in terms of collisions.
increased pressure - molecules are pushed closer together
same number of molecules, in smaller volume
more collisions occur with sufficient energy to overcome activation energy
rate increases (only for gaseous)
Define a ‘catalyst’
a substance that increases the rate of reaction without being used up. it provides an alternative route with a lower activation energy
What is a homogenous catalyst?
a catalyst in the same state/phase as the reactants
e.g. liquid catalyst with liquid reactants
What is a heterogenous catalyst?
catalyst in a different state/phase as the reactants
e.g. liquid reactants with solid catalyst
What are the economic and sustainability benefits of using a catalyst?
catalysts lower energy demands of processes - reduce costs, less fossil fuel needs to be burnt(combusted) to generate required energy, lower CO2 emissions
Name some techniques that can be used to investigate reaction rates.
- titrations (monitoring concentration changes)
- using gas syringe to measure gas volume of products
- monitoring a reaction on a balance - mass of substance formed/decreasing mass of reactants
all over a period of time
Analyze the Boltzmann distribution
The area under the curve = total number of molecules in a sample
There are no molecules with 0 energy
x axis is not touched
only molecules greater then Ea have enough energy to react
x axis = energy
y axis = number of molecules with a given energy
Describe and explain what happens to the Boltzmann distribution if temperature increases
the peak of the curve is lower and moves to the right - a higher energy
Ea section increases vertically, but Ea line does not move
more collisions take place in a certain length of time - molecules have more kinetic energy (moving faster)
higher propertion of molecules have an energy greater than the Ea - more successful collisions occur
rate of reaction increases
Describe and explain what happens to the Boltzmann distribution if a catalyst is added
The curve shape does not change but the catalyst lowers the Ea
more molecules will overcome the new lower Ea of the reaction.
there will be more successful collisions in a certain length of time
rate of reaction will increase
What is dynamic equilibrium?
exists in a closed system when the rate of forward reaction is equal to the rate of the reverse reaction
concentrations of products and reactions stay the same
What is the effect of increasing concentration on equilibrium?
reactant - equilibrium shifts the the right, forming more products
products - equilibrium shifts to left, forming more reactants
What is the effect of pressure on equilibrium?
increasing - equilibrium shifts to the side with fewer moles
decreasing - equilibrium shifts to side with more moles
What is the effect of changing the temperature of an exothermic reaction?
increasing - equilibrium shifts in endothermic direction (left)
decreasing - equilibrium shifts in exothermic direction (right)
What is the effect of changing the temperature of an endothermic reaction?
increasing - equilibrium shifts in endothermic direction (right)
decreasing - equilibrium shifts in exothermic direction (left)
What is the effect of a catalyst on equilibrium?
does not affect equilibrium position
speeds up rate of forward and reverse reaction equally
What is the Haber process equation?
N2(g) + 3H2(g) <=> 2NH3(g)
Explain how there must be a compromise between chemical equilibrium and reaction rate?
What does Kc value indicate?
> 1 = equilibrium towards reactants
1 = dynamic equilibrium
<1 = equilibrium towards products
