MODULE 2 Flashcards
What is kinetic energy
Energy of motion
What is potential energy
Energy of position - relative to other objects/force
What unit is used for energy
Joules (J)
How to go from J to kj
J divided by 1000
Energy is ……….. in exothermic reactions
Released
Negative change in enthalpy
Energy is …….. in endothermic reactions
Absorbed
Positive enthalpy change
A positive enthalpy change means the reaction is …….
Endothermic
A negative enthalpy change means the reaction is
Exothermic
How to work out change in enthalpy
Enthalpy of products - enthalpy of reactants
What is entropy
Measure of possible arrangements within a system
A high entropy would indicate
Greater amount of possible arrangements within a system, more disorder
A low entropy would indicate
Fewer possible arrangements within a system, more ordered
What is the spontaneous direction os entropy
Increasing entropy
Which state has the highest entropy
Gas
If a gas is forming from a solid is the entropy change likely to be positive or negative
Positive
Does fewer moles mean more or less possible arrangements
Fewer moles = less arrangements
If a reaction has fewer moles of products than reactants (all in the same phase) is it likely to have a negative or positive entropy change
likely to have negative entropy change
If the change in G is negative will the reaction be spontaneous
Yes
negative G = spontaneous
Positive G = Non spontaneous
Zero G = equilibrium
What can be said about the spontaneity of a reaction if change in H is + and change in S is -
The reaction can NEVER be spontaneous at any temperature
What can be said about the spontaneity of a reaction if the change in H is - and the change in s is +
The reaction WILL be spontaneous at all temperatures
If the change in H and S for a reaction have the same sign what can be said about the spontaneity of the reaction
The spontaneity will depend on temperature
If change in both H and S is positive when will the reaction be spontaneous
ONLY at high temperatures
If change in both H and S is negative when will the reaction be spontaneous
ONLY at low temperatures
What is a coupled reaction
When a spontaneous reaction (negative change in G) is used to drive a non spontaneous reaction (positive change in G) by coupling them together
Unit for rate of reaction
mol L-1 s-1
Conc divided by time
Factors affecting rate of reaction
- chemical nature if reaction
- physical states of reactants and products
- concentration of reactants
- temperature
Catalyst = increase reaction rate
Inhibitor = decrease reaction rate
general rate law for a reaction
Rate = k(A) power of x (B) power of y
What does x equal in rate law equation
Order in respect to A
x = 0,1,2 …..
What does y equal in the rate law equation
Order with respects to B
B = 0,1,2…..
What does x + y mean in rate law
Gives the overall order of the reaction
Do you take into account stoichiometry when writing a rate law
No ignore it, THIS IS THE ONLY TIME ITS IGNORED
The unit of k when overall order of reaction is 0
(x+y=0)
mol L-1 s-1
The units of k when overall order of reaction is 1
(X+y=1)
s-1
The units of k when the overall order of reaction is 2
(X+y=2)
L mol-1 s-1
The units for k when overall order of reaction is 3
(X+y=3)
L2 mol-2 s-1
How to calculate k (the rate constant)
K = initial rate divided by
A to power of x times B to power of y
How to find x in rate constant, using initial rates
Hold B constant,
measure initial rates for different values of A
- changing the concentration of A
How to find y in the rate constant using the initial rates method
Hold A constant,
Measure initial rates for different values of B
- different concentrations of B
Does rate constant k increase or decrease with temperature
Increase
If the concentration of A doubles and there is no change to the initial rate what order will the reaction be
Order 0
If the concentration of A doubles and the rate also doubles what order will it be
1
If the concentration of A doubles and the rate increases by more than double
2nd order
What is the concept half life
Time taken for reactant to reach half its initial value
At half life t = 1/2t
What is an elementary step
Series of steps a reaction takes to complete an overall reaction
- stoichiometry IS USED when writing rate laws for elementary steps - ONLY TIME for rate laws
What is a reaction mechanism
Elementary steps describing an overall reaction
What is a reaction intermediate
Species which play a role in the reaction but do not appear in the overall equation
- created and used in the elementary steps
What is a rate determining step
The slowest elementary step, that determines the reaction rate
`if the first elementary step is rate determining how would you write a rate law
Use stoichiometry to write the rate law for that step
Rate = rate determining step = k1(……..)
If the second elementary step is rate determining how would you write a rate law
Write the rate law using stoichiometry
- the rate law may contain intermediates = these must be removed
Write the equilibrium constant for first step
Rearrange equilibrium constant to equal the intermediate needing to be removed
Then substitute this rearranged equilibrium constant into the rate law in place of intermediate
Which elementary step would show the greatest peak on energy profile graph ……… and why
The slowest elementary step as it requires the greatest amount of activation energy
What is molecularity
The total number of species involved in a single collision leading to a reaction
How to get from minutes to seconds
Times by 60
How to get from minutes-1 to seconds-1
Divide by 60
What is activation energy
The energy threshold needed to be overcome for a reactant to be converted into a product
What is transition state
Where bonds are partly broken and partly formed
Where the reaction will either form products (effective collision) or return to reactants (ineffective collision)
How does temperature impact reaction rate
It effects collision frequency and collision effectiveness
- increase in temp = molecules move faster = more energy
Oxidation means that electrons are …..
Lost, cation is formed
Reduction means that electrons are ……
Gained
When atoms exist as elements their oxidation number is …
0
The oxidation number of a monoatomic ion is
The same charge as the ion
Hydrogen in compounds has an oxidation number of …
+1
Oxygen has an oxidation number of -2 in compounds expect which compound
Hydrogen peroxide (H202) - it has an oxidation number of -1 in this
The sum of oxidation numbers in molecules always equals…
0
The sum of oxidation numbers in polyatomic ions is
Equal to the charge of the charge of the ion
What is an oxidising agent
The reactant that is causing oxidation
It itself is being reduced, but it is causing another species to lose electrons
What is a reducing agent
The reactant that is causing reduction
It itself is being oxidised but it is causing another species to gain electrons
The substance with the highest potential energy in spontaneous redox reactions will …..
Be reduced
How to calculate potential difference of a reaction
Use the values given to you
Reduction process - oxidation process
Will the potential energy difference always be positive or negative for a spontaneous reaction
It will always be positive
If Eo cell is positive reduction will occur at the ….. electrode
Right hand side electrode
If Eo cell is negative then reduction will occur at the …….. electrode
Left hand side electrode
What does Eocell measure
How much each side wants to gain electrons
How do you work out Eocell
Eo of right hand side - Eo of left hand side
Reduction always occurs at the ……..ode
Cathode
Oxidation always occurs at the ….ode
Anode
How do you know if a reaction has a pH dependent reduction potential
If the reaction contains H+
What is biological standard state
When all species are in standard conditions
1 mol L-1 except hydrogen
H+ has concentration of 1x10-7 - a realistic biological concentration
An increase in Q means that E will ….
Decrease
Change in G under standard conditions equals what at equilibrium
Zero
What are the issues with chemical oxidants in biological environments
They are often toxic
Require a low pH
Also are un selective (cause anything to oxidise)
Biological oxidants must themselves be able to be….
Reduced and then re-oxidised when they pass on electrons
What is a transition metal complex
Compounds in which a transition metal has a number of ligands bonded to it
What is a ligand
A molecule or ion which has one or more donor atoms that have a lone pair of electrons
A ligand is a Lewis ………
Lewis base
Common donor atoms in ligands are ….
Nitrogen and oxygen
Transition metals are Lewis….. and will always be a …ion
Lewis acids and will always be a cation
When ligands bond to transition metals they change the ….
Reduction potential of the transition ion
If a very electronegative element is bonded to the outside of a cytochrome it will….. the reduction potential
Increase the reduction potential of the transition metal
If a transition metal complex has strong electronegative elements bonded to the outside of it will the reduction potential of the transition metal be reduced or increased and why
The reduction potential will increase because the electronegative element will pull electron density away from the transition metal, making it want to gain more electrons
How to work out the reduction potential, under standard conditions when given the pH
E(standard conditions) - 8.314 x temp / n x 96485 x In (1 / (1x10 to the power of negative ph) to the power of the number of hydrogens in the equation
