7.1. Chemical equilibria: reversible reactions, dynamic equilibrium
A subsection of Chemistry, 9701, through 7. Equilibria
Listing 10 of 211 questions
Alcohols and esters are important organic compounds which are widely used as solvents. Esters such as ethyl ethanoate can be formed by reacting carboxylic acids with alcohols. CH3CO2H + C2H5OH CH3CO2C2H5 + H2O This reaction is an example of a dynamic equilibrium. Explain what is meant by the term dynamic equilibrium. Write the expression for the equilibrium constant for this reaction, Kc. For this equilibrium, the value of Kc is 4.0 at 298 K. A mixture containing 0.5 mol of ethanoic acid, 0.5 mol ethanol, 0.1 mol ethyl ethanoate and 0.1 mol water was set up and allowed to come to equilibrium at 298 K. The final volume of solution was V dm3. Calculate the amount, in moles, of each substance present at equilibrium. Alcohols may be classified into primary, secondary and tertiary. Some reactions are common to all three types of alcohol. In other cases, the same reagent gives different products depending on the nature of the alcohol. In the empty squares below give the structural formula of the organic compound formed in each of the reactions indicated. If no reaction occurs, write ‘no reaction’ in the space. alcohol reagentand conditions CH3CH2CH2CH2OH CH3CH2CH(OH)CH3 (CH3)3COH red phosphorus and iodine heat under reflux concentrated H2SO4 heat Cr2O7 2–/H+ heat under reflux
9701_s07_qp_2
THEORY
2007
Paper 2, Variant 0
View
Ethanoic acid can be reacted with alcohols to form esters, an equilibrium mixture being formed. CH3CO2H + ROH CH3CO2R + H2O The reaction is usually carried out in the presence of an acid catalyst. Write an expression for the equilibrium constant, Kc, for this reaction, clearly stating the units. Kc = units In an experiment to determine Kc a student placed together in a conical flask 0.10 mol of ethanoic acid, 0.10 mol of an alcohol ROH, and 0.005 mol of hydrogen chloride catalyst. The flask was sealed and kept at 25 °C for seven days. After this time, the student titrated all of the contents of the flask with 2.00 mol dm–3 NaOH using phenolphthalein indicator. At the end-point, 22.5 cm3 of NaOH had been used. Calculate the amount, in moles, of NaOH used in the titration. What amount, in moles, of this NaOH reacted with the hydrogen chloride? Write a balanced equation for the reaction between ethanoic acid and NaOH. Hence calculate the amount, in moles, of NaOH that reacted with the ethanoic acid. Use your results from to calculate the amount, in moles, of ethanoic acid present at equilibrium. Hence complete the table below. CH3CO2H ROH CH3CO2R H2O initial amount / mol 0.10 0.10 equilibrium amount / mol Use your results to calculate a value for Kc for this reaction. Esters are hydrolysed by sodium hydroxide. During the titration, sodium hydroxide reacts with ethanoic acid and the hydrogen chloride, but not with the ester. Suggest a reason for this. What would be the effect, if any, on the amount of ester present if all of the water were removed from the flask and the flask kept for a further week at 25 °C? Explain your answer.
9701_s11_qp_22
THEORY
2011
Paper 2, Variant 2
View
Questions Discovered
211