Thermodynamics, Rate Equations Flashcards
What are the orders for concentration’s influence on rate of reaction?
if no effect —>
Zero order A^0
if rate doubles when concentration doubles (direct proportion) —>
First Order A^1
if rate quadruples when concentration doubles —>
Second Order A^2
The rate constant…
- Different for every reaction
- Varies with temperature
- units are different for every reaction
What is Enthalpy of Lattice Formation?
Enthalpy change when 1 mole of ionic lattice/solid ionic compound is formed from its gaseous ions.
Why may the calculation of enthalpy for lattice formation based on the perfect ionic model give a smaller numerical value than the calculated value? e.g in the case of AgI
The Covalent character of the molecule
They may not be completely ionic.
the forces/bonds holding the matrix together are stronger.
Enthalpy change of formation
The enthalpu change when 1 mole of a compound is formed from it’s elements in their standard states under standard conditions.
(2Cs + 2H2aq –> C2H6aq)
Lattice enthalpy of formation
The enthalpy change when 1 mole of a solid ionic compound is formed from it’s gaseous ions under standard conditions.
Ca2+g + 2Cl-g –> CaCl2s
- exothermic
Lattice enthalpy of dissociation
The enthalpy change when 1 MOLE of solid ionic compound is dissociated into it’s gaseous ions under standard conditions.
CaCl2s –> Ca2+ + 2Cl-g
Enthalpy change of dissociation
The enthalpy when 1 mole of bonds of the same type of molecule in the gaseous state is broken
F2g –> 2Fg
Enthalpy change of 1st ionisation energy
The enthalpy change of when 1 mole of gaseous 1+ ions are made from 1 mole of gaseous atoms
- endothermic
Enthalpy change of atomisation
The enthalpy change when 1 mole of gaseous atoms are made from an element in it’s standard state.
1/2F2(g) –> F(g)
- endothermic
Enthalpy change of 2nd ionisation
The enthalpy change when 1 mole of gaseous 2+ ions are made from 1 mole of gaseous 1+ ions.
Ca+(g) –> Ca2+
- endothermic
1st electron affinity
The enthalpy change when 1 mole of gaseous atoms form 1 mole of gaseous 1- ions.
e.g. O(g) –> O-(g)
- exothermic
Why do we use Born Haber cycles?
Born-Haber cycles are useful to calculate lattice enthalpies. This is because we can’t calculate directly from experiments.
2nd electron affinity
- The enthalpy change when 1 mole of gaseous 1- ions form 1 mole of gaseous 2- ions.
e.g. O-(g) –> O2-(g)
-endothermic
Why may the theoretical Lattice enthalpies and experiment Lattice enthalpies differ?
- The covalent character of the ionic compound ( the amount at which the positive ions polarise the negative ions)
- can result in greater experimental values for lattice enthalpy than theoretical ( when there is high covalent character)
- The larger the ion the greater the distortion
Theoretical Lattice Enthalpies and Experimental Lattice Enthalpies
- Theoretical lattice enthalpies can be calculated from data assuming a perfectly ionic model.
- during experimental testing you will find that you will not get the exact value you get from the theoretical lattice enthalpies
- This means that the compound doesn’t follow the perfectly ionic model and has some covalent characteristics.
- Most of the time the positive ion distorts the charge distribution in the negative ion. ( The positive ion polarises the negative ion + the more polarisation the greater the covalent character)
Enthalpy change of solution
The enthalpy change when 1 mole of an ionic substance is dissolved in the minimum amount of solvent to ensure NO FURTHER enthalpy change is observed upon further dilution.
For a Substance to dissolve… (enthalpy change of solution)
- substance bonds must break (endothermic)
- New bonds formed between the solvent and substance (exothermic)
Why may an ionic compound not dissolve?
For an ionic compound to dissolve the new bonds formed during the hydration of the freely moving ions must be equivalent or greater in strength than the ionic bonds broken.
- If not then substance is unlikely to dissolve
How do you calculate the enthalpy change of solution
- using lattice dissociation enthalpy
- using enthalpy of hydration
(use the hess’ law structure)
The sum of the two is equal to the enthalpy of solution.
Enthalpy of Hydration
- Enthalpy change when 1 mole of aqueous ions is made from 1 mole of gaseous ions. / Enthalpy change when 1 mole of gaseous ions form 1 mole of aqueous ions.
(exothermic)
Define Entropy
Entropy is the measure of disorder in a system.
The more disorder there is the higher the level of entropy
things to remember when calculating gibbs free energy
- when calculating D S make sure it’s S = products - reactants and that you include balancing numbers in calculation
- making sure you convert your entropy value from JK-1mol-1 to KJK-1mol-1 ( or you can convert your enthalpy to KJmol-1 but former is easier)
- making sure your temperature is in KELVIN ( it’s not temp change so it matters that it’s kelvin)
Why may endothermic reactions still be spontaneous?
- a calculated positive entropy can tell you a reaction is feasible
- increasing entropy is energetically favourable and some reactions that are enthalpically unfavourable (endothermic) can still spontaneously (feasibly) react if changes in entropy overcome changes in enthalpy
Why may a reaction be theoretically feasible but not have a spontaneous reaction?
- the reaction may be feasible but not spontaneous
- This could mean it has a high activation energy or it’s an incredibly slow reaction.
When is a reaction feasible?
When Delta G </ 0
- Temperature effects feasibility of a reaction
- this is dependent on whether delta H or delta S is positive or negative
Why does the rate of reaction change when temperature changes, explain in terms of the rate constant.
- the rate constant is fixed at particular temperatures. If the temperature changes the rate constant also changes.
- As we increase the temperature the particles have more energy and they collide more often. This increases the rate.
when calculating the rate constant what is important to remember?
- finding the orders of reaction
- calculating the units using the concentrations.
rate equations
- rate equations can only be constructed by conducting an experiment.
- ## the initial rates can be used to work out the rate equation for a reaction.
Iodine Clock Experiment
H2O2(aq) + 2H+(aq) + 2I- –> 2H2O(l) + I2(aq)
- in this practical you are looking for colour changes
- add sodium thiosulfate and starch (which acts as an indicator)
- the sodium thiosulfate reacts immediatley with the I2
- when there is no more sodium thiosulfate the I2 reacts with starch, resulting in a deep blue/black colour.
Other methods of calculating the rate of an experiment
Colorimeter
- measures the absorbance of light by a coloured sample. The more concentrated a sample is, the darker it’s colour and hence the more light absorbed.
- good for any reaction with a colour change.
Rate-Concentration Graphs VS Concentration-Time Graphs
Rate-Concentration graphs
- help us identify the order
- the rate on a STRAIGHT HORIZONTAL LINE GRAPH is constant so the reactants are ZERO ORDER. (the change in concentration is not affecting the rate)
- the r ate on a straight DIAGONAL line shows a proportional relationship so FIRST ORDER.
- the rate on a curved line shows shows a SECOND ORDER reactant(s)
CONCENTRATION-TIME
- concentration on a straight diagonal line shows the reactant to be ZERO ORDER
- the concentration on a curved but moderately steep line is FIRST ORDER (more working out will likely be needed)
- the concentration on a STEEPLY curved line would be SECOND ORDER.
Rate-determining step
the SLOWEST step in a MULTI-STEP reaction.
- speeding up the slowest step will increase the whole reaction rate.
- substances not in the rate equation won’t be in the rate determining step. THUS all substances in the rate determining step are IN the rate equation.
- catalysts can also be within rate equations
- for rate determining step reactants their rate equation power is their balancing number.
- The reactants that form the reactants in the rate determining step must also be apart of the rate equation.
How can we determine the likeliness of a mechanism using the rate determining step?
- when determining which mechanism is more likely to occur for a reaction you can look at the rate equation.
- If certain molecules/reactants that are within the rate equation (including their balancing numbers shown as powers) show up in one of the mechanisms then that mechanism is more likely to occur.
Arrhenius Equation
- can be used to find activation when plotting
Define overall order of reaction
The sum of the powers in a rate equation
suggest why hydration of chloride ions is an exothermic process
- The water is polar
- the chloride ions attract the slightly positive hydorgen
The bond dissociation enthalpy
mean enthalpy change when one mole of covalent bonds is broken into 2 gaseous atoms
- this is averaged over different compounds of ranging energies
the change enthalpy of atomisation
enthalpy change when one mole of gaseous atom is formed from the/an element in its standard state
the enthalpy change of formation
the enthalpy change when one mole of substance is formed from its constituent elements in their standard states under standard conditions
lattice dissociation enthalpy
enthalpy change when one mole of ionic crystal is broken into its constituents gaseous ions
enthalpy change of hydration
enthalpy change when one mole of gaseous ions become aqueous ions
what is lattice enthalpy dependent on?
- the size of the ion
- the charge of the ion
How do you use the Rate Determining Step to write a mechanism
Conditions
- The reactants in the RDS must only be those in the rate equation (and in the correct proportions)
- All the mechanism steps must combine (with some cancelling down) to give the overall equation
- Using the rate equation and reaction equation make an equation for the first step, including an intermediate as a product
- using the intermediate and the remains of the reaction equation make equation that gives the product of the reaction
Conditions for a Rate Determining Step
- The reactants in the RDS must only be those in the rate equation (and in the correct proportions)
- All the mechanism steps must combine (with some cancelling down) to give the overall equation
How can one find the order of reactants?
- Orders of reaction can only be determined using experimental data
- Ionisation energies are always positive
- Second Ionisation energies are always more endothermic than first ionisation energies.
- as there is less repulsion between the remaining electrons
- so more energy required to remove them
- electron affinity and lattice enthalpy typically
- difficult to measure them directly
- the enthalpy change when one mole of an ionic compound dissolves in excess water is the enthalpy of solution
- the more exothermic the enthalpy of solution, the more likely the compound is to dissolve
- the hydration energy depends on the charge and the size of the ions: the larger the charge the smaller the size, the larger the hydration energy
- Solubility of an ionic compound depends on the hydration enthalpy and the lattice enthalpy
HS = Enthalpy of Hydration + Enthalpy of Lattice Dissociation
OR
HS = Enthalpy of Hydration - Enthalpy of Lattice Dissociation
