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Equilibria seminar

University of Lincoln presentation

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Le Chatelier’s PrincipleWhen an external change is made to a system in equilibrium, the

system will respond to oppose the change

1. BiCl3(aq) + H2O(l) ↔ BiOCl(s) + 2HCl(aq)

2. Cr2O72-(aq) + 2OH-(aq) ↔ 2CrO4

2-(aq) + H2O(l)

How does reaction 1 respond to addition of hydrochloric acid?How does reaction 2 respond to addition of alkali?How does reaction 2 respond to addition of acid?

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N2(g) + 3H2(g) ↔ 2NH3(g)

CO(g) + 2H2(g) ↔ CH3OH(g)

2NO2(g) ↔ 2NO(g) + O2(g)

PCl5(g) ↔ PCl3(g) + Cl2(g)

H2(g) + I2(g) ↔ 2HI(g)

CO(g) + H2O(g) ↔ CO2(g) + H2(g)How do the above equilibria respond to:An increase in pressureA decrease in pressure

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N2(g) + 3H2(g) ↔ 2NH3(g) rH = -92.2 kJ mol-1

H2(g) + I2(g) ↔ 2HI(g) rH = -9.4 kJ mol-1

CO(g) + H2O(g) ↔ CO2(g) + H2(g) rH = -41.2 kJ mol-1

PCl5(g) ↔ PCl3(g) + Cl2(g) rH = 87.9 kJ mol-1

How do the above respond to anIncrease in temperatureDecrease in temperature

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CH3COOH (aq) + H2O(l) ↔ H3O+ (aq) + CH3COO-

(aq)

• CH3COOH: CH3COO- at pH = 4.77 ?

• CH3COOH: CH3COO- at pH = 3 ?

• CH3COOH: CH3COO- at pH = 7 ?

ionised][un[ionised]

logpKpH a

pKa= 4.77

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CH3NH2(aq) +H2O(l) ↔ CH3NH3+

(aq) + OH-(aq)

pKa = 10.66 (of conjugate acid)

pH = 10.66 [B]=[BH+]pH =8 what happens to CH3NH3

+(aq): CH3NH2(aq)

pH =13 ?

][

][log

ionised

ionisedunpKpH a

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Acidic drugs

How does this molecule ionise?

pKa=4.5

pH =3 (stomach pH)?

pH=6 (intestine)?

ibuprofenCH3

CH3

CH3

O-

O

H+

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amphetamine (C6H5CH2CH(NH2)CH3)Write an equation for the reaction of

amphetamine with water.The pKa of the conjugate acid is 9.8. What

will happen to the ratio of ionised to unionised amphetamine at:

pH 7pH 12Why might this be important?

Basic drugsCH3

NH3+

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Making a buffer solution

• Choose a weak acid with a pKa close to the required pH of the buffer.

• Choose an appropriate salt of the weak acid

• Determine [salt]/[acid] ratio needed to give correct pH

What would be the pH of an ethanoate buffer with equal acid and sodium ethanoate concentrations?

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Acid pKa Conjugate base

H3PO4 2.12 H2PO4-

HNO2 3.34 NO2-

H2CO3 6.37 HCO3-

HCN 9.31 CN-

HCO3- 10.25 CO3

2-

pKa values (data tables)

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[acid][salt]

logpKpH a

04.77pH

pH = 5

pH = 4

Ethanoate buffers

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Acknowledgements

• JISC• HEA• Centre for Educational Research and

Development• School of natural and applied sciences• School of Journalism• SirenFM• http://tango.freedesktop.org