Topic 11: Further Equilibria Flashcards
Describe an equilibria experiment invoking cobalt compounds and HCl
Two cobalt compounds exist in eqm solution with chloride ions. Changing conditions changes eqm position and therefore the colour as well.
Make up 100ml of pink cobalt chloride solution w 60ml of conc HCl- a purple solution forms.
Adding more HCl produces a blue solution while adding water restores the pink.
Place about 2 cm depth of the purple solution into different boiling tubes. Place the tubes in an ice bath, room temp and hot water baths to see how temperature affects equilibria position.
1.6 moles of NO and 1.4 moles of O2 were mixed in a container w a volume of 4 dm^3. At eqm, 1.2 moles of NO2 formed. Calculate Kc:
2NO + O2 —> 2NO2
First draw a table containing the start moles, change in moles, eqm moles and finally eqm conc for each reactant and product. Fill in known info- the start moles for 2NO2 should be zero because it is NOT a reactant, but for NO2 the change in moles should be 1.2 because the moles at eqm are 1.2.
Change in moles for 2NO and O2= -2x and -x, so the moles at eqm are 0.4 and 0.8
find the concentrations by doing n/v x 1000, substitute values in for Kc.
Answer= 0.045 mol-1dm3
Why does the Kc expression sometimes have no units?
They have no units because the moles of the product is the same as the moles of the reactant
What must NOT be included in the Kc expression?
Solids in the equation don’t contribute to the expression. Solid concentration is effectively fixed by its surface area so it is constant.
Describe Kp
Kp is the eqm constant in terms of partial pressures for GASES only. We can calculate Kp for equilibria which include gases. If aA + bB —> cC + dD,
Kp= (PC)^c (PD)^d / (PA)^a (PB)^b
Where P is the partial pressure of the species
What is the mole fraction? How does partial pressure relate to mole fraction?
The mole fraction (x) of species (A) in a gas mixture is defined as:
xA = moles of A / total gas moles
Partial pressure= xA x total Pa.
Units of partial pressure are either Pa, KPa or atm.
Partial pressure is the pressure that is due to a particular gas in a mixture.
A gas mixture contains three moles of N2(g), one mole of O2. What is the mole fraction of N2?
x(N2) = 3/(3+1) = 0.75
How do you calculate and determine units for Kp?
eg if Kp= p(SO3)^2 / p(SO2)^2 p(O2)
Just like Kc, write the expression for Kc and use partial pressures for each of the reactants and products
Units depend on the expression, eg if
Kp= p(SO3)^2 / p(SO2) p(O2)
units= (KPa)^2 / (KPa)^2 (KPa) = KPa^-1
What factors affect Kc and the eqm position?
Both Kp and Kc are temp dependant constants.
If gaseous reactant is added the value of the denominator in the Kp expression increases. Eqm shifts right to restore the ratio in the Kp expression.
If gaseous product added the value of the numerator in the Kp expression increases. Eqm shifts left to restore the ratio in the Kp expression
Describe an experiment to find eqm constant in the reaction between ethyl ethanoate and water.
Record and label the the mass of 5 bungs and tubes.
W a pipette filler add 5 cm³ of 2M HCl to each tube. Record the masses of the tubes.
Add the volume of ethyl ethanoate to the tubes, record su masses.
From a second measuring cylinder add the volume of water stated in the table, recording the masses. Shake the tubes and place in a cupboard.
Rinse and fill a burette with 1M NaOH.
Pour the content of tube 1 into the conical flask. Rinse the tube with distilled water into the flask. Add phenolphthalein and titre against NaOH
