1.8 Thermodynamics Flashcards
State the meaning of the term enthalpy change.
Heat energy change at a constant pressure
Define lattice enthalpy of dissociation
the enthalpy change needed to convert 1 mole of solid ionic compound into its gaseous ions under standard conditions
Equation for lattice dissociation of CaCl2 (s)
CaCl2 (s) -> Ca (s) + Cl2 (g)
Magnesium chloride dissolves in water.
Give an equation, including state symbols, to represent the process that occurs when the enthalpy of solution of magnesium chloride is measured.
b) define enthalpy of solution
MgCl2 (s) -> Mg^2+ (aq) + 2Cl^- (aq)
b) the enthalpy change when one mole of of ionic solid is dissolved in water to infinite dilution so that the ions no longer interact under standard conditions.
The enthalpy of hydration of Ca2+(g) is –1650 kJmol^1
Suggest why this value is less exothermic than that of Mg2+(g)
Ca^2+ is a bigger ion as it has more shielding and a larger ionic radius
- Has a weaker attraction to the delta negative oxygen in H2O
Explain why the standard entropy value for carbon dioxide is greater than
that for carbon.
CO2 is more disordered than Carbon
What is the entropy gibbs calculation
Delta G = ( - T x Delta S ) + Delta H
which compounds will have the largest enthalpy change values between MgO, BaCl2 and MgCl2
MgO greater than MgCl2,
as Oxygen has a greater charge than chlorine, -2 vs -1
MgCl2 is greater than BaCl2 because Mg has less shielding than Ba so MgCl2 is smaller and has a stronger electrostatic attraction to outermost electrons.
State why there is a difference between the theoretical and experimental
values.
experimental takes covalent character into account.
Explain why the enthalpy of hydration becomes less exothermic from Li+
to K+
size of ion increases as charge stays same
less electrostatic attraction to delta negative oxygen in H2O
Define enthalpy of formation
b) show enthalpy of formation of CsI caseium iodide
a) enthalpy change when one mole of a substance is created under its standard conditions from its pure elements.
Cs (s) + 0.5 I2 (s) -> CsI (s)
The enthalpy of lattice formation for caesium iodide in Table 1 is a value
obtained by experiment which was +107 kJmol^-1.
The value obtained by calculation using the perfect ionic model is −582 kJmol−1
Deduce what these values indicate about the bonding in caesium iodide
Caesium iodide is almost perfectly ionic
Lattice enthalpy values can be obtained from Born–Haber cycles and by
calculations based on a perfect ionic model.
Which compound shows the greatest percentage difference between these two
values?
A CsF
B CsI
C LiF
D LiI
D
Look for compound with most covalent character
look for smallest positive ion & largest negative ion
Define the term enthalpy of lattice formation.
Enthalpy change when 1 mole of solid ionic compound is formed from its gaseous ions
A calculation of the enthalpy of lattice formation of silver iodide based on a
perfect ionic model gives a smaller numerical value than the value in the born - harbor cycle
Explain this difference.
silver iodide contains covalent character
forces holding lattice together are stronger
Identify a reagent that could be used to indicate the presence of iodide ions in an aqueous solution and describe the observation made.
AgNO3, yellow ppt
Cl2 or Br2, brown solution / black solid formed
Anhydrous magnesium chloride, MgCl2, can absorb water to form the hydrated
salt MgCl2.4H2O
MgCl2(s) + 4H2O(l) ⟶ MgCl2.4H2O(s)
Suggest one reason why the enthalpy change for this reaction cannot be determined directly by calorimetry
Not possible to prevent some dissolving
it is soluble and it dissolves
Describe how you would carry out an experiment to determine the enthalpy
of solution of anhydrous magnesium chloride.
You should use about 0.8 g of anhydrous magnesium chloride.
Explain how your results could be used to calculate the enthalpy of solution.
- measure water with appropriate apparatus
- pour solution into insulated container / polystyrene cup
- add known mass of MgCl2
- weigh polystyrene cup before and after mass is added and find difference to find mass of MgCl2
- Record initial temperature and continue to measure temperature at suitable intervals of every 5 minutes until trend is seen
- Plot a graph of temperature against time
- extrapolate lines to when solid is added to find initial and final value of temperature
- final temperature - initial = change in temperature
- use of equation q=mc delta t
A reaction is exothermic and has a negative entropy change.
Which statement is correct?
A The reaction is always feasible
B The reaction is feasible above a certain
temperature
C The reaction is feasible below a certain
temperature
D The reaction is never feasible
C
Define the term electron affinity for chlorine.
the enthalpy change for formation of one mole of chloride ions from chlorine atoms
Explain why there is a difference between the hydration enthalpies of the Mg^2+ and Na^+ ions.
- magnesium is a smaller ion and has a greater charge
- magnesium ion attracts water more strongly
A 5.00 g sample of potassium chloride was added to 50.0 g of water initially at 20.0 °C. The
mixture was stirred and as the potassium chloride dissolved, the temperature of the
solution decreased.
Describe the steps you would take to determine an accurate minimum temperature
that is not influenced by heat from the surroundings.
- start a clock when KCl is added to water
- Record the temperature every subsequent minute for about 5 minutes
- Plot a graph of temperature vs time
- Extrapolate back to time of mixing = 0 and determine the temperature
A 5.00 g sample of potassium chloride was added to 50.0 g of water initially at 20.0 °C.
The temperature of the water decreased to 14.6 °C
Calculate a value, in kJ mol−1, for the enthalpy of solution of potassium chloride.
You should assume that only the 50.0 g of water changes in temperature and that
the specific heat capacity of water is 4.18 J K−1 g−1.
Q = mc x delta t
change in temperature = 5.4 degrees
Q = 50 x 4.18 x 5.4 = 1128.6 J
moles = 5 / 74.6 = 0.0670
energy change per mole = 1128.6 / 0.0670 = 16839 J mol^-1
= + 16.8 kJ mol^-1
Write an equation for the process that has an enthalpy change equal to the electron affinity
of chlorine
Cl(g) + e- → Cl-(g)
Part b) next slide
Part a) previous slide
Use figure in part a) and data in table 1 to calculate a value for the enthalpy of lattice dissociation of calcium chloride
Use equation and the data in table 1 to calculate minimum temperature in Kelvin at which this reaction becomes feasible
Part c) use data from table 1 and table 2 to calculate a value for the enthalpy change when one mile of magnesium chloride dissolves in water
Part c) use data from table 1 and table 2 to calculate a value for the enthalpy change when one mile of magnesium chloride dissolves in water
The enthalpy of solution of calcium chloride is -82.9 kJmol^-1. The enthalpies of hydration for calcium ions and chloride ions are -1650 and -364 kJ mol^-1. Use these values to determine a value for lattice enthalpy if lattice dissociation of calcium chloride
The enthalpy of hydration for the potassium ion is -322 kJ mol^-1. The lattice enthalpy of dissociation for potassium bromide is +670 kJ mol-1. Calculate the enthalpy of solution of potassium bromide. Enthalpy of hydration of bromide ion is -335 kJ mol^-1
