National Chemistry

1999 Credit Almost the answers

1. There is not a lot of point reading this complete table. Note the headings and
then look at the question.
(a) Ionic compounds : conduct when solid no
conduct when moltenyes

melting/ boiling points high
</CITE>
<CITE>remember to pick 2 boxes<BR>
</CITE>
<BR>
(b) covalent compound: conduct when solid <CITE>no </CITE>
<BR>
conduct when molten<CITE> no <BR>
</CITE>
<CITE> </CITE>
melting/ boiling points <CITE>molecule low; network high<BR>
</CITE>

A substance will not be covalent and ionic<BR>
<BR><BR>
2. In alphabetical order:-<BR>
fructose<BR>
glucose both monosaccharide C<sub>6</SUB>H<sub>12</SUB>O<sub>6</SUB><BR>
maltose first 3 reducing sugars pass Benedict's<BR>
sucrose both disaccharide C<sub>12</SUB>H<sub>22</SUB>O<sub>11</SUB> last one fails all tests<BR>
<BR>
starch is a large molecule turns iodine blue black<BR>
<BR>
(a) The single one (C<sub>6</SUB>H<sub>10</SUB>O<sub>5</SUB>)<sub>n</SUB><BR>
<BR><BR>
(b) C<SUB>12</SUB> - a mid sized pair<BR>
<CITE>remember to pick 2 boxes<BR>
</CITE>
<BR>
(c) The single one<BR><BR>
(b) cannot be broken down so small<BR>
<CITE>remember to pick 2 boxes<BR>
</CITE>
<BR><BR>
3. Get to page 5 - the solubility table<BR>
(a) identify one o f these as a precipitate - ie insoluble<BR><BR>
(b) Mixing two of the boxes will give a precipitate<BR>
A + B - produces copper sulphate and barium nitrate - both check in table;<BR>
A + C - produces copper carbonate and lithium nitrate - check both in table<BR>
 A + D; B + C; B + D and C + D must all be tried<BR>
<BR><BR>
4.. A time consuming question but should be easy to get correct.<BR>
(a) higher temperature; smallest particles; more concentrated acid; most reactive
metal all increase reaction rate ?<BR>
So pick slowest reaction.<BR><BR>
(b) Changing temperature - place the letter of the box under the appropriate temperature <PRE>
<EM> <U>20<SUP>o</SUP>C</U> <U>30<SUP>o</SUP>C</U> <U>40<SUP>o</SUP>C</U> </EM>
A C E
B D F</PRE><BR>

MUST be boxes from different columns ie not C and D<BR>

Write the concentrations beside each box letter<BR><PRE>
<EM> <U>20<SUP>o</SUP>C</U> <U>30<SUP>o</SUP>C</U> <U>40<SUP>o</SUP>C</U> </EM>
A 1 mol/l C 2 E 2
B 2 D 1 F 2</PRE><BR>
This eliminates some more pairs - cannot be A and C as although different temperatures
they also have different concentrations.<BR>
Write the metal beside each letter to eliminate some combinations.<BR><PRE>
<EM> <U>20<SUP>o</SUP>C</U> <U>30<SUP>o</SUP>C</U> <U>40<SUP>o</SUP>C</U> </EM>
A 1 mol/l iron C 2 Mg E 2 Mg
B 2 Mg D 1 iron F 2 iron</PRE><BR>

possibilities A &amp; D; B &amp; C; C &amp; E; B &amp; E<BR>
Now add in particle size to identify correct pair<BR>

<CITE>remember to pick 2 boxes<BR>
</CITE>
<BR><BR>
5. (a) Same atomic number but different mass number <BR><BR>
At least select two boxes with something in common.<BR>
<CITE>remember to pick 2 boxes</CITE>
<BR><BR>
(b) argon is 2,8<BR>
Then write down the electron arrangement of the six atoms/ions in question.<BR>
eg A is 2,8,1 &#160&#160&#160&#160&#160 B is 2,8,8 etc.<BR>
<CITE>remember to pick 2 boxes</CITE>
<BR><BR>
(c) Mass number - (number of protons) <BR>
number of protons =atomic number<BR>
<BR><BR>
6. Lot of information :-<BR>
One of the metals corrodes - <BR>
Electrons flow from metal B to metal A - what does this tell you ?<BR>
<EM>fe</EM>
rroxyl indicator shows Fe<SUP>2+</SUP> Use page 7 of data book.<BR>
(a) Blue colour around metal B due to<EM> fe</EM>
rroxyl indicator.<BR>
Electrons move down electrochemical series<BR>
On question paper circle the most reactive of each pair of metals<BR>
B, C &amp; E have metal A the more reactive metal.<BR><BR>
(b) Use page 7 of data book<BR>
Biggest gap or smallest gap ?<BR>
<BR><BR>
7. "statement(s)" in the credit paper usually means TWO correct answers.<BR>
<BR>
In this style of question then there are TWO correct answers and three wrong answers
- work through each statement individuaally deciding if it is True/ False.<BR>
Watch for statements that are the opposite of each other - both cannot be correct
and one usually is.<BR>
During distillation fractions are separated due to boiling points - lower boiling
  points coming off first.<BR>
Fraction 6 is darkest deliberately as it has the biggest molecules<BR>
The bigger the molecule the darker, more viscous, heavier, harder to ignite the substance
is.<BR><BR>
<STRONG>A</STRONG>
. "most easily" is an extreme - either 6 or 1<BR>
<STRONG>B</STRONG>
. is fraction 4 bigger than fraction 3 ?<BR>
<STRONG>C. is</STRONG>
 flammability less at 1 ?<BR>
<STRONG>D. f</STRONG>
 fraction 1 is collected first then fraction 4 has lower boiling point than fraction
5<BR>
<STRONG>E</STRONG>
. Are the smallest molecules collected first ?<BR>
<CITE>remember to pick 2 boxes</CITE>
<BR><BR>
<BR>
8. "statement(s)" in the credit paper usually means TWO correct answers.<BR>
<BR>
In this style of question then there are TWO correct answers and three wrong answers
- work through each statement individuaally deciding if it is True/ False.<BR>
Watch for statements that are the opposite of each other - both cannot be correct
and one usually is.<BR>
<BR>
(a) Each statement must be TRUE for both<BR>
Always worthwhile knowing formula of common acids <BR>
hydrochloric acid H<SUP>+</SUP>Cl<SUP>-</SUP> ; sulphuric acid H<SUP>+</SUP>2SO<SUB>4</SUB><SUP>2-</SUP> ; nitric acid H<SUP>+</SUP>NO<SUB>3</SUB><SUP>-</SUP> <BR>
<STRONG>A</STRONG>
. do all acids react with magnesium ? <BR>
If one does both do<BR>
<STRONG>B</STRONG>
. do all acids have a pH less than 7 ?<BR>
If one does both do.<BR>
<STRONG>C. do</STRONG>
 acids not conduct electricity ?<BR>
If one does not both do not.<BR>
<STRONG>D.</STRONG>
 do all acids produce Cl<SUB>2</SUB>(g) ? Cl<SUP>-</SUP> present in only one.<STRONG><BR>
</STRONG>
<STRONG>E.</STRONG>
 do acids have equal numbers of H<SUP>+</SUP> and OH<SUP>-</SUP> ions - this is a neutral substance<BR>
<CITE>remember to pick 2 boxes</CITE>
<BR><BR>
(b) Each statement must be TRUE for Na<SUP>+</SUP>Cl<SUP>-</SUP> and FALSE for H<SUP>+</SUP>Cl<SUP>-</SUP><BR>
Use information gathered for part (a) to help.<BR>
<BR>
<STRONG>A</STRONG>
. do all both react with magnesium ?<BR>
<STRONG>B</STRONG>
. acids and salts have different pH .<BR>
<STRONG>C. do</STRONG>
 ionic solutions conduct electricity ?<BR>
<STRONG>D.</STRONG>
 do both produce Cl<SUB>2</SUB>(g) ? Must have Cl<SUP>-</SUP> present.<STRONG><BR>
</STRONG>
<STRONG>E.</STRONG>
 both have equal numbers of H<SUP>+</SUP> and OH<SUP>-</SUP> ions <BR>
<BR><BR>
<EM>part 2</EM>
<BR>
<BR>
9. (a) When making a statement about a relationship given in a graph you can usually
frame your answer as follows "As the <CITE>data along the way</CITE>
 increases then the <CITE>data up the way</CITE>
 increases or decreases (pick one)"<BR>
In this question you need to isolate a "group " ie all group 1 elements Li, Na K
  etc<BR>
As it moves Li, Na, K the atom size inreases/decreases <CITE>pick one</CITE>
<BR><BR>
(b) When making a statement about a relationship given in a graph you can usually
frame your answer as follows "As the <CITE>data along the way</CITE>
 increases then the <CITE>data up the way</CITE>
 increases or decreases (pick one)"<BR>
In this question you need to isolate a "Li to F" in the graph<BR>
As you move along group Li (3), Be (4) B (5) ...etc the atom size increases/ decreases
<CITE>pick one</CITE>
 <BR><BR>
(c) Look up the atomic number of calcium.<BR>
Page 1 of the data book would help.<BR>
<BR><BR>
10. (a) Electronic configurations on page 1 may help concentrate your mind.<BR>
Na moves from 2,8,1 to Na<SUP>+</SUP> 2,8<BR><BR>
(b) Oxidation is loss of electrons therefore Fe<SUP>2+</SUP> becomes ....<BR>
 Try page 7 of data book.<BR>
<BR><BR>
11. (a)3 C , 1 S and 8 H<BR><BR>
(b) Write the molecular formula for B<BR><BR>
(c) Must be something, present in the compound, with oxygen. ie C, S and H but
 not chlorine oxide.<BR>
<BR><BR>
12. bubbled through water<BR>

bubbled through concentrated sulphuric acid<BR>

Make sure the test tubes are connected<BR>
<BR>
<BR><BR>
13. (a) The repeating unit is<BR>
<IMG SRC="perspexrepeat.gif"><BR>
3 monomer units would be 6 C joined together<BR><BR>
(b) Reaction which releases heat (heat is always "therm")<BR>
releasing is indicated by "exo" as in exit<BR><BR>
(c) Write the molecular formula C<SUB>5</SUB>O<SUB>2</SUB>H<SUB>8</SUB> <BR>
<PRE>How much C = <U>mass of carbon</U> x 100 %
mass of C<SUB>5</SUB>O<SUB>2</SUB>H<SUB>8</SUB>

= <U>5 carbons </U> x 100
  100</PRE><BR><BR><BR>
14. (a) Migration of ions obviously requires potassium nitrate in gel.<BR>
d.c. power supply for electricity; electrodes<BR><BR>
(b) Red, green and purple/blue colours should suggest pH numbers<BR>
 acid and sodium hydroxide (alkali) have pH numbers<BR>
<BR><BR>
15. (a) Remember that you can only put figures in front of the formulae - you CANNOT
change any of the formula you have been given.<BR>
1 Ag on left hand side but 2 Ag on right hand side so place a 2 in front of Ag<BR>
2 oxygens on left hand side but only one on right hand side so place 2 in front of
H<SUB>2</SUB>O<BR>
Check the rest in a similar manner<BR><BR>
(b) (i) Ag<SUP>+</SUP> ions present at start ----&gt; this one should make Ag<BR>
Ag<SUP>+</SUP> ions ----&gt; Ag<BR>
(ii) Top of page Page 7 shows what type of reactions.<BR>
gain of electrons or loss of electrons<BR>
<BR><BR>
16. (a) How does the structure of the five molecules given affect density.<BR>
Effect of more carbons = increasing density<BR>
Effect of increasing number of -OH groups =<BR><BR>
(b) Similar question to (a) working in reverse BUT must have 4 carbon atoms and
some -OH groups. Only one OH group only gives a density of 0.8<BR>
<BR><BR>
17. (a) Nitrogen in the air is N<SUB>2</SUB>(g)<BR><BR>
(b) ammonia comes from ammonium chloride heated with ... ?<BR>
A salt and water is also produced<BR><BR>
(c) Heat is required at start but not after reaction has started.<BR>
But reaction needs heat. Where is it coming from ?<BR><BR>
(d) Answer must come from diagram.<BR>
Each diagram has 1N and 2 H in common, and one other thing.<BR>
<BR><BR>
18. (a) nitrogen dioxide at 21; sulphur dioxide at -10; ammonia at -33<BR><BR>
(b) One of them is liquid at -25<SUP>o</SUP>C while two are gases.<BR>
Where will -25<SUP>o</SUP>C fit in between these three figures.<BR>
There is only one answer.<BR><BR>
<BR>
19. (a) Easy to make hard to break <BR>
Reactive metals make compounds easily.<BR>
<BR>
(b) Underline the chemicals mentioned in the question <BR>
ie copper sulphide <U>CuS</U> and sulphur dioxide <U>SO<SUB>2</SUB><BR></U>
<center> Write under this the number of moles </center>
<BR>&#160&#160&#160ie 2 mole of CuS ------&gt; 2 mole of SO<SUB>2</SUB><BR>
<center>Write under this the mass of these moles </center> <BR>
&#160&#160&#160&#160&#160&#160&#160&#160&#160&#160 2 x (64+32) ------&gt; 2 x 64<BR>
&#160&#160&#160&#160&#160&#160&#160&#160&#160&#160&#160&#160&#160&#160&#160&#160&#160&#160&#160 192 g ------&gt; 128 g<BR>
<BR>
(c) What is wrong with exhaust gas ?<BR><BR>
<BR>
20. (a) Notice that the enzyme does not appear in the chemical equation.<BR>
It has taken part in the reaction but has not been changed by it.<BR><BR>
(b) What experiment measures concentration ?<BR><BR>
(c) When making a statement about a relationship given in a graph you can usually
frame your answer as follows "As the <CITE>data along the way</CITE>
 increases then the <CITE>data up the way</CITE>
 increases or decreases (pick one)"<BR>
As temperature increases the enzyme activity ..... <CITE>does what </CITE>
?<BR>
<BR><BR>
21. (a) Carbon dioxide indicated by lime water turning milky. <BR>
An ion containing C is needed to produce this gas ?<BR><BR>
(b) The blue solution contains Cu<SUP>2+</SUP> <BR>
Changing these to copper atoms would convince.<BR>
<BR><BR>
22. To calculate the concentration of sodium hydroxide you need the volume of sodium
hydroxide (20cm<SUP>3</SUP>) used and the volume (15cm<SUP>3</SUP>) and concentration (0.1 mol/litre) of hydrochloric acid.

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