A2 Physical Flashcards
Thermodynamics, Kinetics, Equilibrium, Electrode Potentials, Acids Bases and Buffers
State Hess’s Law
At constant temperature, enthalpy changes accompanying a chemical reaction will remain constant, irrespective of the way the reactants react to form product
Two properties that determine lattice enthalpy
Small ions, large charges
Two things Born-Haber cycles used for
Determining which compounds are most likely to be formed
Calculating enthalpy changes that cannot easily be determined experimentally for ionic lattices
What are the 5 steps for a Born-Haber Cycle
Atomisation of metal
Atomisation of non-metal
First IE of metal
First electron affinity of non-metal
Enthalpy of lattice formation
What are the three steps to determining the enthalpy of solution
Enthalpy of lattice dissociation
Hydration of cations
Hydration of anions
What is responsible for the discrepancy between experimental values and theoretical values of lattice enthalpy
Polarisation, small highly charged cation pulls the electrons away from the large highly charged anion
What are the three factors that affect the possibility of a reaction occuring
Enthalpy, entropy, temperature
Define rate of reaction
Change in concentration of any reactant per unit time
What species are in the rate equation
Any that occur in a reaction before and including the rate determining step, including catalysts
Why does the rate constant not increase directly proportional to temperature?
Temperature affects both the frequency of collisions and the collisions that have enough energy to collide sufficiently and react
State the Arrhenius equation
K=Ae^-Ea/RT
What is “A” in the Arrhenius equation
The pre-exponential factor
What are the units for A
The same as the units for K
Define partial pressure
The pressure that would be exerted by one of the gases in the mixture if it occupied that same volume on its own
For a reaction, aA+bB ⇌ yY+zZ, give the Kp expression
Kp=pY^y(pZ^z)/pA^a(pB^b)
Which direction do electrons flow in a cell
Most reactive metal to least reactive metal
What is used to complete the circuit for a cell
Salt bridge
What is the name given to a table of EΘ values that allows the theoretical voltage of a cell to be calculated
Electrochemical series
What is used to measure the standard voltage produced by a metal
Standard hydrogen electrode
What is the IUPAC standard representation of a salt bridge
|| Two solid lines
What is the formula for calculating the EMF for a cell using the electrochemical series
EMF= EΘpositive electrode - EΘ negative electrode
How can electrochemical series and cells be used to determine the feasibility of a reaction
The voltage according to the electrochemical series allows the direction of electron flow to be worked out. This means that electrons will flow in that direction but they will not flow positive to negative
Explain how a zinc carbon cell works
A carbon inert electrode is in the centre of a manganese oxide and powdered carbon mixture, within a paste of NH4Cl and water surrounded by a Zn case. Zn becomes Zn2+ ions and NH4+ become NH3 to produce a current
Explain how Lead-acid batteries work
Lead is oxidised, Lead oxide is reduced. This is reversible, so is recharged by the cars generator.
Give the name of the cell that is rechargeable and a standard size
nickel-cadmium
Give the name of the cell that is rechargeable, light and does not leak so is used in laptops and smartphones
Lithium ion
Explain the benefits of a hydrogen fuel cell
Only product is water so clean
Lower temperature than combustion of hydrogen so no nitrous oxides produced
No reactive metals used to make them
Give the bronsted-lowry definition of an acid and a base
Acid - proton donor, base - proton acceptor
Give the equation for calculating pH
pH=-log10[H+]
Give the equation for the Kw expression
Kw=[OH-][H+]
Give the equation for pKa
pKa=-log10Ka
Describe the shape of a titration curve for a strong acid and a weak base
Begins ≈ pH2, s-shaped curve, flattens at 8-10 pH
Explain the importance of the half neutralisation point
Half of the acid has reacted, so [HA]=[A-], so pKa=pH
Also useful for buffers
What are two ways that acidic buffers can be made
Weak acid
½ neutralisation buffer
Give two examples of basic buffers (including the chemicals)
NH4Cl- and NH3 mixture
H+ +HCO3- → CO2 +H2O (in blood)
Give two everyday uses of buffers
Shampoo, detergents
