ScholarshipQuantitative chemistryFree

Hokey pokey and impure baking soda

Two independent analyses of a mixture; thermal decomposition energetics and entropy

Hokey pokey is made by stirring baking soda (sodium hydrogencarbonate,

\ce{NaHCO3}

) into syrup heated to about 150 °C. The mixture froths dramatically because the

\ce{NaHCO3}

decomposes into sodium carbonate, water vapour and carbon dioxide. A sample of old baking soda is suspected to contain some

\ce{Na2CO3}

. M (g mol

^{-1}

\ce{NaHCO3}

84.0;

\ce{Na2CO3}

106.0;

\ce{CO2}

44.0;

\ce{H2O}

18.0.

\Delta_fH

(kJ mol

^{-1}

\ce{NaHCO3(s)}

−951;

\ce{Na2CO3(s)}

−1131;

\ce{CO2(g)}

−394;

\ce{H2O(g)}

−242.

Facts you may need

thermal decomposition stoichiometry
simultaneous equations
ΔrH from ΔfH
entropy of system and surroundings
basic salts

(a)

Write a balanced equation for the decomposition, and explain at the particle level why it makes the hokey pokey expand.

(b)

A 2.000 g sample of the old baking soda was heated to constant mass; the final mass was 1.336 g. Calculate the percentage by mass of

\ce{NaHCO3}

, and explain why the sample was heated to constant mass.

(c)

Calculate

\Delta_rH

for the decomposition. The reaction does not occur at room temperature but occurs readily at 150 °C. Discuss this observation in terms of the entropy changes of the system and the surroundings.

(d)

A second analysis: 0.500 g of the same old baking soda was dissolved in water and titrated with 0.250 mol L

^{-1}

HCl using methyl orange; the titre was 25.20 mL. Write equations for the reactions of both components with HCl, and determine the percentage of

\ce{NaHCO3}

(e)

Evaluate the two methods. Show quantitatively how sensitive the titration result is to a titre error of 0.10 mL, and identify one systematic error that would make the mass-loss result too high.

(f)

Hokey pokey made with too much baking soda tastes bitter and soapy. Explain, with an equation, why the residue is basic.