CHEMISTRY - 2.6
(Describe principle of chemical reactivity)
ASSESSMENT SCHEDULE NZIC 2006
Evidence
Achievement
Merit
ONE (a)
Kc = [NH3]2
[N2][H2]3
Correct.
(b)
(i) & (ii)
The equilibrium will move to the right to produce more ammonia
to use up some of the added hydrogen that has been added to the
system. Kc will remain unchanged
Direction of movement
only, or Kc value
unchanged.
(c)
(i) & (ii)
The equilibrium will move in the direction of the least number of
gas molecules, that is to the right. Kc will remain unchanged.
Direction of movement
only, or Kc value
unchanged.
(d)
(i) & (ii)
The equilibrium will move in the direction of the endothermic
reaction, that is to the left. The value of Kc will decrease because
the concentration of the reactants has increased and the
concentration of the products (yield) has decreased.
Direction of movement
only, or Kc value
decreases.
TWO
(a)
(b)
(i) & (ii)
Exothermic because heat is released.
Correct answer and reason
Increase the reaction rate. The increased concentration of HCl
means there are more particles able to have effective collisions
with the zinc in the same period of time(increased frequency of
collisions) , leading to an increase in the rate of reaction.
Effect on reaction covered
and related to increase in
no. of collisions or more
particles in same space.
Complete answer
including time factor.
Using powdered zinc instead of lumps of zinc increases the
available surface area for the reaction to occur on, leading to an
increase in the reaction rate because there are more collisions per
unit time.
More surface area in
powdered zinc.
Increase in surface
area. Leads to more
collisions per unit
time.
(c)
Explanation of
movement and
correct statement
about Kc.
Explanation of
movement and
correct statement
about Kc.
Explanation of
movement and Kc.
decreases.
Excellence
Both correct with
explanations
Evidence
THREE
(a)
(b)
Achievement
Merit
Excellence
A base is a proton acceptor.
Correct.
Acid
Conjugate base
H2O
OH–
+
H 2O
H3O
HS–
S2–
HS–
H2S
A strong acid fully dissociates / ionises in water.
Three correct.
(d)
pH is a measure of hydrogen ions concentration in the solution.
The higher the concentration of hydrogen ions, the lower the pH.
HCl is a strong acid; it is fully dissociated in water producing 0.01
mol L-1 hydronium ions.
Ethanoic acid is a weak acid and exists in equilibrium with water.
The concentration of hydronium ions is less than 0.01 mol L-1.
Therefore, 0.01 mol L–1 solution of HCl will contain more
hydronium ions than the ethanoic acid solution at the same
concentration, so the pH of the HCl will be lower than the
ethanoic acid solution.
Ethanoic acid has a lower
concentration of hydrogen
ions than HCl.
OR
HCl is a strong acid
CH3COOH is a weak acid
Links [H3O+]
strength of acid.
(e)
HCl + H2O  H3O+ + Cl–
CH3COOH + H2O H3O+ + CH3COO–
The conductivity of a solution depends on the number of charged
particles in solution.
A strong acid reacts completely, [so a 0.01 mol L-1 solution of
HCl will contain 0.02 moles of ions (0.01 mol L-1 of Cl- and 0.01
mol L–1 of H3O+).]
A weak acid exists in equilibrium with water, with the
equilibrium favouring the molecular acid, so a 0.01 mol L–1
solution of ethanoic acid will contain far fewer charged particles.
Therefore, the strong acid will conduct electricity far better than
the weak acid.
Conductivity depends on
the number of charged
particles in solution,.
Achieved plus strong
acid has lots of
charged
particles
compared to few
charged particles in a
weak acid.
(c)
Correct
to
Discussion explains
different strengths of
acids and links to
different pH.
Explanation includes
balanced equations and
links conductivity to
concentration of charged
particles and strength of
acid.
Evidence
Four
(a)
(b)
Five
(a)
Achievement
Merit
Solution
[H3O+]mol L–1 [OH–]mol L–1
pH
hydrochloric acid
0.01
2.0
1  10–12
(HCl)
ethanoic acid
4.55  10–4
2.19  10–11
3.34
(CH3COOH)
sodium hydrogen
2.511  10–6
8.4
3.98  10–9
carbonate
(NaHCO3)
sodium hydroxide
12.3
4.67  10–13
0.0214
(NaOH)
NaHCO3 + H2O  Na+ + HCO3–
HCO3– + H2O
H2CO3 + OH–
Sodium hydrogen carbonate dissolves in water releasing Na+ ions
and HCO3– ions. (The Na+ ions do not react with water) but the
hydrogen carbonate ions hydrolyse in water (react) to form an
equilibrium system with water.
The hydrogen carbonate ion acts as a base, accepting a proton
from water and forming OH–. HCO3– is a weak base; it does not
react with water very much so only a small amount of hydroxide
ions are formed. Hence the pH of solution a solution of NaHCO3
is basic.
1 column
correct.
Recognises that a basic
solution has
[OH-] > [H3O+]
The hydrogen
carbonate ions act as
a weak base and
reacts with water to
form hydroxide ions.
0.5 = 0.0208 moles gives 12.4 kJ
24
2 moles would give
2 x 12.4 = 1205 kJ
0.0208
So ∆Ho = -1190.4 kJ mol –1
One step covered or
numerical answer correct.
Correct answer with
correct sign and
units.
or
2
rows
Excellence
One error.
Answer includes
equations links [OH-] to
basic solution
Evidence
(b)
(M) MgO = 40g mol–1
nMgO = 0.200 = 0.005 moles
40.0
1190.4 kJ released when 2 mole of MgO formed
Achievement
One step correct.
Merit
Allow one error.
Excellence
All correct with unit and
three significant figures.
0.005  1190.4 = 2.98 kJ of heat energy produced
2
16 Achieved, 11 Merit and 5 Excellence opportunities
Sufficiency Statement:
Achieved A total of NINE opportunities correct at the Achieved level or higher
Merit
A total of TEN opportunities correct; SIX at the Merit level or higher
Excellence A total of TEN opportunities correct; THREE at the Excellence level, THREE at the Merit level or higher