Unit 4
Chemical Systems and Equilibrium
ARE YOU READY?
(Page 420)
Knowledge and Understanding
1. (a) solid, liquid, and gas
(b) The system is considered closed because matter cannot flow in or out of the system. Removal of the lid allows
water molecules to escape, creating an open system.
(c) The reduction in size of the sugar cube as it dissolves proves that the solution is unsaturated.
(d) The crystal remains the same size because the solution is saturated with sugar. Crystallization and dissolving are
both occurring at the same rate.
(e) The solution is saturated. The crystal would shrink in size if the solution was unsaturated.
2. (a) Mg(s) 2 HCl(aq) → MgCl2(aq) H2(g)
(b)
nHC1 vHC1 cHC1
0.250 L 0.8 mol/L
nHC1 0.2 mol
2:1 mol ratio
∴ nMgC1 0.1 mol
2
0.1 mol of magnesium chloride is produced.
0.1 mol
(c) cMgC1 2
0.250 L
cMgC1 0.4 mol/L
2
The concentration of the magnesium chloride solution is 0.4 mol/L.
3. (a) A strong base, such as sodium hydroxide, NaOH(aq), raises the pH of water far more than a weak base like
aqueous ammonia, NH3(aq).
(b) An acid is a proton donor while a base is a proton acceptor.
–
–
2–
(c) HPO42–
(aq) HSO4(aq) → H2PO4 (aq) SO4(aq)
B2
A1
A2
B1
4. C3H6(g) 4.5 O2(g) → 3 CO2(g) 3 H2O(g)
H [3Hf(CO
2(g))
3Hf(H O )] [Hf(C H
]
3 6(g))
2 (l)
[3 mol (–393.5 kJ/molCO ) 3 mol (241.8 kJ/molH O )] [17.8 kJ/molC H ]
2(g)
2 (1)
3 6(g)
H 1923.7 kJ/molC H
3 6
Inquiry and Communication
5. Procedure
(a)
1. Put on eye protection and a lab apron.
2. Add about 600 mL of water to a 1-L beaker on a hot plate.
3. Raise the temperature of the water to 60°C and maintain the temperature at this level.
4. Add 100 mL of water to a 125-mL flask.
5. Add 0.020 g of calcium hydroxide to the flask.
6. Insert a thermometer into the flask.
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Chemical Systems and Equilibrium
227
7.
8.
9.
10.
Use water bath to warm the contents of the flask up to 60°C.
Note if the calcium hydroxide had dissolved.
If the calcium hydroxide did completely dissolve, add another 0.02 g to the flask.
Repeat steps 7 through 9 until some calcium hydroxide remains.
Safety Precautions
Wear eye protection and lab aprons.
(b) If 0.10 g of calcium hydroxide dissolves in 100 mL, then 1.0 g will dissolve in 1 L of water.
1.0 g
nCa(OH) 2(aq)
74.10 g/mol
nCa(OH) 0.0135 mol (extra digits carried)
2(aq)
0.0135 mol
[Ca(OH)2(aq)] 1.0 L
[Ca(OH)2(aq)] 0.0135 mol/L
Since there are two hydroxide ions in each Ca(OH)2(aq)
– ] 2 [Ca(OH)
[OH(aq)
2(aq)]
– ] 0.027 mol/L
[OH(aq)
The hydroxide concentration is 0.027 mol/L.
(c) The calculation assumes that the ionization of water does not contribute a significant amount of hydroxide.
6. (a) Chemical equation: NaOH(aq) HCl(aq) → H2O(l) NaCl(aq)
– H+ Cl– → H O Na+ Cl–
Total ionic equation: Na+(aq) OH(aq)
(aq)
(aq)
(aq)
(aq)
2 (l)
– H+ → H O
Net ionic equation: OH(aq)
(aq)
2 (l)
(b)
(c)
(d)
(e)
(f)
(g)
(h)
(i)
(j)
7. (a)
neutralization reaction
buret
titration
an acid–base indicator
Procedure
Add three drops of bromothymol blue indicator to the flask.
• Add base in small increments to the titration flask. Swirl the flask carefully during each addition.
• Continue adding base until a bromothymol blue endpoint.
Safety Precautions
Eye protection and a lab apron must be worn.
15.00 mL
7
Test the solution with pH test strips or a pH meter.
+ ] = 1.0 10–7 mol/L
[H(aq)
(0.020)(0.030)
2.3 10–4
(0.10)x
6.0 10–4
2.3 10–4
(0.10)x
6.0
2.3
(0.10)x
60
2.3
x
60
x 2.3
x 26
228
Unit 4
Copyright © 2003 Nelson
(b)
(2x)2
2 49
(x – 0.20)
(2x)2
2 49
(x – 0.20)
2x
7
x – 0.20
7x – 1.4 2x
5x 1.4
x 0.28
(c)
(3x)2
2.0
(x – 1.0)(x – 3.0)
9x2
2.0
(x2 – 4x 3)
9x2 2x2 8x 6
7x2 8x 6 0
b ac
b2 4
x 2a
2 46)
8 8
7(
21
0.52 or 1.7
8. Since the pH scale is logarithmic, a change in only 1 pH unit represents a 10-fold change in acidity. Sourville’s water
is on average over 10 times more acidic than neutral water. The August and September readings were approximately
100 times the acidity of neutral water. Although the acidity from the occasional soft drink or salad may not be a health
risk, prolonged exposure to excess acid is a potential health risk. For example, below a pH of 5.5, tooth enamel begins
to break down. The water commissioner should take the appropriate steps to raise the pH of the town’s water supply.
Copyright © 2003 Nelson
Chemical Systems and Equilibrium
229