Page 57 - PRE-U STPM CHEMISTRY TERM 1

Chemistry Term 1 STPM
(ii) Using the expression: (b) The order of reaction with respect to a given substance
[H ] = K C is the power to which the molar concentration of the
+
b
(7.9 × 10 ) = (1.8 × 10 )C substance is raised in an experimentally determined
–5
–4 2
C = 3.5 × 10 mol dm –3 rate equation.
–2
[CH COOH] = 3.5 × 10 mol dm –3 (c) (i) Rate = k[NO Cl]
–2
3
(c) CH COOH is a weak acid that dissociates partially in (ii) Mechanism: 2
3
water. As a result, more of it must be present to NO Cl : NO + Cl Slow (Rate-determining
provide the same H concentration. 2 2 step)
+
(d) (i) H9C≡C9H + NH : H9C≡C + NH 3
–
–
2
(ii) In the above reaction, ethyne acts a proton NO Cl + Cl : NO + Cl Fast
2
2
2
donor (Bronsted-Lowry acid) while the NH 2 – (iii) During the reaction, the colour of the mixture
ion acts as a proton acceptor (Bronsted-Lowry changes from yellow (NO Cl) to brown (NO ).
2
2
base). 19 (a) A buffer solution is a solution that resists changes in
pH on the addition of a little acid or base.

–
+
SECTION C (b) CH COOH(aq)  CH COO (aq) + H (aq)
3
3
CH COONa(aq) : CH COO (aq) + Na (aq)
–
+
18 (a) (i) Change in temperature: 3 3
The Boltzmann distribution curve for a Due to common ion effect, the mixture would
chemical reaction at two different temperatures contain a large amount/resoviour of undissociated
–
is shown below: CH COOH molecules and the CH COO ions.
3
3
On the addition of a little acid, the added H would
+
Fraction of molecules react with the CH COO ions to form undisoociated
–
T >T 1 2 with energy, E ≥ E at T 2 CH COOH molecules according to the equation:
3
1
a
Fraction of molecules T 2 a 1 On the addition of a little base, the added OH would –
Fraction of molecules
3
T
with energy, E ≥ E at T
H
+ CH COO (aq) : CH COOH(aq)
+
–
3
3
(added)
–
react with CH COOH molecules to form CH COO
3
3
ions according to the equation:
OH
–
(addad) + CH COOH(aq) :
3
CH COO (aq) + H O(l)
–
E Energy 3 2
–
+
a In both cases, the added H or OH are ‘destroyed’
For a reaction to occur, the reactant molecules and the pH of the buffer solution remains unchanged.
must collide with one another with energy (c) Using the expression for a buffer solution:
greater than the activation energy E . When the [CH COOH]
a
3
temperature increases (T > T ), the fraction of pH = pK – [CH COO ]
a
–
2
1
molecules having energy greater than the 3
activation energy increases and also the Since the molarity of CH COOH and CH COONa
3
3
collisions are now more energetic. This results are the same and the volume of the mixtures is
3
in an increase in the rate of reaction. The constant (total = 50.0 cm ), the concentration of the
reverse is true when the temperature is lowered, species can be equated to their respective volumes in
and the rate of reaction decreases. the mixture. Hence,
(ii) Presence of a catalyst: pH = pK – [V(CH COOH)]
3
A catalyst provides an alternative pathway, with a [V(CH COONa)]
lower activation energy for the reaction to Rearranging: 3
occur.
[V(CH COOH)]
3
pH = –log [V(CH COONa)] + pK a
Fraction of molecules with catalyst Hence, a graph of pH against log [V(CH COONa)]
3
Activation energy
[V(CH COOH)]
3
Activation energy
3
without catalyst
would give a straight line with a negetive slope, and
the intersect at the pH axis gives the value of pK for
Energy ethanoic acid. a
pH 3.92 4.28 4.70
As can be seen from the Boltzmann distribution [Methanoic acid]
curve above, in the presence of a catalyst, more log [Sodium methanoate] 0.95 0.6 0.18
molecules have energy greater than the new
(lower) activation energy. This would result in
more ‘effective collision’ leading to an increase
in the rate of reaction.
380
12 Answers.indd 380 3/26/18 4:06 PM