Exam Practice

K_p Practice Questions

Question 1

In the reaction between ethene and steam to make ethanol, the equilibrium mixture contained
• 95 moles of ethene
• 55 moles of steam
• 5 moles of ethanol

Total Pressure: 60 atm

$CH_2=CH_2+H_2O\rightleftharpoons CH_3CH_2OH$

a) Calculate the k_p for this reaction.

0.00247 atm^-1

b) Will a reaction that has 30 moles of ethene, 20 moles of steam and 2 moles of ethanol and a total pressure of 40atm be at equilibrium?

0.004333 atm^-1Not at equilibrium

Question 2

A gas X dissociates on heating to set up an equilibrium as shown below:

$X_{(g)}\rightleftharpoons Y_{(g)}+Z_{(g)}$

A quantity of X was heated until equilibrium was reached. If 1 mole of X was put in the reaction vessel and 0.2 moles of Z were produced find k_p if the total pressure is 25kPa.

1.042kPa

Question 3

1 mole of hydrogen and 1 mole of methanol were added to a sealed flask. If the mole fraction of methanol at equilibrium is 0.4 and the total Pressure is 100kPa, what would the value of k_p be?

$2H_2+CO\rightleftharpoons CH_3OH$

0.00260 kPa^-1

Question 4 – Hard!!!

When HI is left to decompose in a sealed flask, an equilibrium is reached. This equilibrium has a k_p of 2 and the total pressure is 12kPa. Find the percentage of HI dissociated.

$2HI\rightleftharpoons H_2+I_2$

73.88%

K_c Practice Questions

Question 1

The reaction of iodine gas with chlorine gas, shown here, has a Kc value of 10.20 at 200ºC. If a closed vessel was charged with the two reactants, each at an initial concentration of 0.250 M, but with no initial concentration of ICl, what would be the equilibrium concentration of ICl(g)?

$I_2+Cl_2\rightleftharpoons 2ICl$

0.308M

Question 2

Given a K_c value of 0.43 for the following aqueous equilibrium, suppose sample Z is placed into water such that its original concentration is 0.033 M. No A(aq) or B(aq) were added to this solution. Once equilibrium has occurred, what will be the equilibrium concentration of Z be when K_c is 0.43 mol^-1 dm³?

$A_{(g)}+2B_{(g)}\rightleftharpoons 3Z_{(g)}$

You need to use the cube root to get rid of the x³

[A] = 0.00786M [B] = 0.0157M [C] = 0.00942M

Question 3

When 1.00 mol of A and 1.2 mol of B are placed in a 2.00 L container and allowed to come to equilibrium, the resulting mixture is found to be 0.20M in D. What is the value of K_c at equilibrium?

$A{(g)}+2B{(g)}\rightleftharpoons C{(g)}+D{(g)}$

3.33mol^-1dm³

K_sp Practice Questions

Question 1

What is the solubility of CaCO₃ in water if the k_sp of CaCO₃ is 1.4 x 10^-8 mol² dm^-6.

0.000118 mol dm^-3

Question 2

What is the solubility of AgBr in 0.1M NaBr if the k_sp of AgBr is 1.9 x 10^-10 mol² dm^-6.

1.9 x 10^-9 mol dm^-3

Question 3 – Hard!

How many grams of NaCl should be added to a saturated solution of AgCl so that the solubility is only 3.5 x 10^-8 mol dm^-3? k_sp of AgCl is 1 x 10^-10 mol² dm^-6.

0.17g

K_d Practice Questions

Question 1

Find k_dwhen 0.1 moles of silane are found in 25mL of organic layer and 0.034 moles of silane are found in 50mL of water.

5.88

Question 2

Find k_dof NH₄Cl when 20mL of water and 100mL of ether are used. The 20 mL of water required 25ml of 0.1M NaOH whereas the ether required 50mL of 0.002M NaOH.

0.008

Question 3

What is the number of moles of organic molecule X extracted into the ether layer if k_d is 7.8, the total number of moles is 0.54 and the volumes of water and ether are 25mL and 20mL respectively?

0.465 moles

Question 4
What is the number of moles of organic molecule Z extracted into the ether layers when two consecutive extractions are done, using 10mL of ether each, if k_d is 7.8, the total number of moles is 0.54 and the volumes of water is 25mL.

0.508 moles

Question 5
Compare the results obtained in question 3 and 4

Bond Breaking, Bond Making

Image result for bond enthalpies table

Question 1

What is the energy released when 1 mole of ethanol is burned in excess air?

-1255kJ mol^-1

Question 2

What is the energy released for the following reaction:7

$N_2+3H_2\rightleftharpoons 2NH_3$

-97 kJ

Question 3

What is the Br-Cl bond enthalpy if the enthalpy of formation of BrCl is 14.4 kJ mol^-1?

$Br_2+Cl_2\rightarrow 2BrCl$

203.1 kJ

Hess’s Cycle

Question 1

Find the enthalpy change of the following reaction:

$H_2O_{(l)}\rightarrow H_2O_{(g)}$

43.8kJ mol^-1

Question 2

Find the enthalpy change of the following reaction:

$Cl_2+2HBr\rightarrow 2HCl+Br_2$

-112kJ

Question 3

Find the enthalpy change of the following reaction:

$C_2H_5Br\rightarrow C_2H_4+HBr$

73.7kJ

Question 4

Find the enthalpy change of the following reactions:

$CH_2=CH_2+H_2\rightarrow CH_3CH_3$

$CH_3CH=CH_2+H_2\rightarrow CH_3CH_2CH_3$

Why are the two values similar but not exactly the same?

a) 136 kJ mol^-1 b) 125 kJ mol^-1

Question 5

Which compound releases more energy per gram, burning hydrogen or burning methane?

Born Haber Cycle

Question 1

Use the following data to calculate the enthalpy of formation of strontium chloride. You must write all thermochemical equations for the steps of the cycle.

The enthalpy of sublimation of strontium = +164 kJ/mole
First ionization energy for strontium = +549 kJ/mole
Second ionization energy for strontium = +1064 kJ/mole
The enthalpy of dissociation of chlorine, Cl₂ = +243 kJ/mole
The electron affinity of chlorine, Cl = -349 kJ/mole
Lattice energy of strontium chloride = -2150 kJ/mole

-828 kJ mol^-1

Question 2

Use the following data to calculate the lattice energy of cesium oxide. You must write all thermochemical equations for the steps of the cycle.

Enthalpy of formation of cesium oxide = -233 kJ/mole
Enthalpy of sublimation of Cs = +78 kJ/mole
First ionization energy of Cs = +375 kJ/mole
Enthalpy of dissociation of O₂ (g) = +494 kJ/mole of O₂ molecules
First electron affinity of O = -141 kJ/mole of O atoms
Second electron affinity of O = +845 kJ/mole of O^– ions

-2090 kJ mol^-1

Gibbs Free Energy

Question 1

Predict whether the following reaction is still spontaneous at 500^oC:

$N_2+3H_2\rightarrow 2NH_3$

Assume that ΔH = 92.22 kJ mol ^-1 and ΔS = -198.75 J K^-1 mol^-1

Not feasible

Question 2

Consider the following reaction for the decomposition of calcium carbonate:

$CaCO_3 \rightarrow CaO + CO_2$

Above which temperature will this reaction become favorable?

Assume ΔHr°and ΔSr° are temperature independent and use the following values for the enthalpies and entropies of formation of the reactant and products.

1108K