Apologies for the formatting issues. When I have time this will be fixed.
1(a)(i)(a) The zinc would dissolve and the blue colour of Cu2+(aq) would fade as a brown / red-brown solid (Cu(s)) is deposited at the bottom of the cup.
1(a)(i)(b) It indicates that Zn is above Cu in the series.
1(a)(i)(c) In an exothermic reaction DHreaction is negative and energy is given out to the surroundings. In an endothermic reaction energy is absorbed from the surroundings and DHreaction is positive.
1(a)(ii)(a) Final thermometer readings : 30.7, 34.5, 36.9, 37.9
Temperature change : 2.7, 6.5, 8.9, 9.9
1(a)(ii)(b) Title of graph: “Graph of temperature change / °C vs mass of zinc added / g.”
Statement of scale: x axis : 1 cm = 0.2 g
y axis : 1 cm = 0.5 °C
1(a)(ii)(c) 7.9 °C
1(a)(iii) Moles of CuSO4 in 100cm3 CuSO4 = [(0.2/1000) ´ 100] mol CuSO4
= 0.02 mol CuSO4.
Therefore this volume contains 0.02 mol Cu2+(aq) and requires 0.02 mol Zn(s) for complete reaction.
Mass of Zn needed = (0.02 ´ 65)g Zn
= 1.3 g Zn.
1(a)(iv) In experiments 4 and 5 the zinc is in excess so the maximum amount of zinc reacting in both cases is the same.
1(a)(v) Silver is below copper in the reactivity series and therefore no reaction (displacement) occurs, so no heat is evolved.
1(b)(i) Test 1 - First Inference - Na+(aq) or NH4+(aq) present.
Second Inference - NH3(g) present; NH4+(aq) confirmed.
Test 2 - First inference - Cl-(aq) present .
Second inference - Cl-(aq) confirmed.
Ionic Eqation - Ag+(aq) + Cl-(aq) ® AgCl(s)
1(b)(ii) Ammonium chloride (NH4Cl).
1(c)(i) Dip a glass rod in aqueous ammonia and place it in the air above one of the acid samples. Repeat this with the other acid sample.
1(c) Glass rod, aqueous ammonia.
1(d) The HCl would produce white fumes with the glass rod dipped in aqueous ammonia, while this would not be observed with the vinegar.
General Discussion Of Relevant Theory
Before starting your answer, read through the entire question, to get a better understanding of what is going on. This is a general rule that must be followed for every exam, and every question. Sometimes, information in one part of the question, or the paper can help in answering another part.
1(a)(i)(a) and (b)
The experiment involves solid zinc in powdered form and copper (II) sulphate solution reacting in a polystyrene cup. Because the data in part (a)(i) includes a volume of copper (II) sulphate solution used and its concentration, we can expect later on to be asked to do a calculation using this data.
The given equation must first be analysed as a whole, and then we should focus on each species involved.
Whole Equation
Inspection of the entire equation shows that it is a displacement reaction, where the zinc metal displaces copper (II) ions. The copper (II) ions are displaced from solution, or from the copper (II) sulphate. At this point the reaction taking place can be termed a displacement reaction.
How do we know that it is a displacement reaction, and not some other type?
There are several types of reaction with which you should be familiar. These ought to have been taught at the lower form level (Forms 1 to 3). A brief summary is given here:
Combination Reactions
In combination reactions, two or more substances (either elements or compounds) react to produce one substance. This reaction type can be represented by the general equation:
A + B ® AB
Decomposition Reactions
In decomposition reactions, one substance undergoes a reaction to form two or more substances. The substance broken down is always a compound, and the products may be elements or compounds. Heat is often necessary for the process. This type of reaction can be represented by the general equation:
AB ® A + B
Substitution (Displacement) Reactions
In substitution or displacement reactions, one element or group in a compound is replaced by another element or group. This reaction type can be represented by the general equation:
A + BX ® AX + B
where A and B may be metals or non-metals. There are two general types of substitution or displacement reactions:
1.Where a metal displaces (replaces) ions of another metal from is salts or replaces the hydrogen ion in an acid. It is possible to arrange metals in a series, the Reactivity Series in which the most reactive metal is placed first. The general order of the series is:
K > Na > Mg > Al > Zn > Fe > Pb > (H) > Cu > Ag > Au
For a metal M to displace a metal X from a solution of its salts, M must be more reactive than X. Thus zinc metal will displace copper from a solution of its salt because zinc is more reactive. Similarly, copper displaces silver from a solution of its salts.
Zn0(s) + CuSO4(aq) ® ZnSO4(aq) + Cu0(s)
Cu0(s) + 2AgNO3(aq) ® Cu(NO3)2(aq) + 2Ag0(s)
The superscript ‘0’ was used above to show when the uncombined metal was present. You should never include it when answering questions.
For a metal to displace hydrogen ions from an acid, the metal must be above hydrogen in the Reactivity Series. Thus magnesium and zinc will both react with hydrochloric acid to produce hydrogen gas, but copper will not react with dilute hydrochloric acid.
Mg0(s) + 2HCl(aq) ® MgCl2(aq) + H2(g)
Zn0(s) + 2HCl(aq) ® ZnCl2(aq) + H2(g)
2. One non-metal displacing another non-metal.
F2(g) + 2NaCl(aq) ® 2NaF(aq) + Cl2(g)
Cl2(g) + 2KI(aq) ® 2KCl(aq) + I2(g)
The zinc reaction above, is exactly that given to us, so we therefore have a substitution or displacement reaction in our question.
Double Decomposition Reactions
Double decomposition reactions can be represented generally by the equation:
AB + CD ® AD + CB
In this reaction type two compounds exchange radicals. This reaction type is also called metathesis or double replacement. One example of a double decomposition reaction is:
AgNO3(aq) + NaCl(aq) ® AgCl(s) + NaNO3(aq)
Neutralization Reactions
All neutralization reactions involve reaction between an acid (or acidic oxide) and a base (or basic oxide). Water is produced in neutralization reactions; heat is also give out, i.e. neutralization reactions are exothermic.
Reversible Reactions And Dynamic Equilibrium
At this point it is sufficient to note that these reactions can be recognized by the use of the arrow “⇌” instead of the arrow “®” in the equation. Hence the reaction given in our question is not of this type.
Redox Reactions
These are reactions in which oxidation and reduction occur simultaneously. Some of the other previously mentioned reaction types may be redox reactions as well. Consider the reaction between zinc metal and aqueous copper (II) sulphate. The equation for the reaction is:
Zn(s) + CuSO4(aq) ® ZnSO4(aq) + Cu(s)
The ionic half equations are:
Zn(s) - 2e- ® Zn2+(aq) Zinc has been oxidized: electron loss.
Cu2+(aq) + 2e- ® Cu(s) Copper has been reduced: electron gain.
So the reaction we are dealing with in this question is also a redox reaction.
Individual Species
Zinc : Zinc metal produces aqueous zinc ions. This change can also have observable consequences. Zinc is a bluish-white, lustrous metal while aqueous zinc ions are colourless. Hence, in this reaction we will observe the metal dissolving, and the resulting solution, would be colourless. If there is any colour in the solution, it will not be due to Zn2+(aq).
Sulphate : The sulphate ion remains unchanged in this reaction. It is a spectator ion, which is also colourless. This can be proven by writing the ionic equation, in which sulphate is not present.
1(a)(i)(c)
Definitions of endothermic and exothermic reactions are as follows:
A reaction absorbing heat from the surroundings is endothermic. ΔH is written as positive for this reaction type.
A reaction giving out heat to the surroundings is said to be exothermic. ΔH is written as negative for this reaction type.
Exothermic reactions result in a temperature rise of the reaction mixture (and sometimes the reaction vessel), while endothermic reactions result in a temperature drop.
1(a)(ii)(a)
It is important to note that all upper and lower temperatures on the four thermometer stems shown are not the same. Additionally, we must establish that each graduation represents 0.1 °C.
When filling out the final thermometer readings, be sure that:
1. All readings are to the same number of decimal places. In this case it will be one decimal place, We can determine this also from the reading given for experiment 5.
2. All temperature changes are to the same number of decimal places.
3. No units are placed with the readings filled in, since it is standard procedure to include units in table headings ONLY.
4. All temperature changes are calculated using the formula:
DT = Final temperature – Initial temperature
such that all temperature changes are positive.
1(a)(ii)(b)
When plotting the graph, one should remember:
1. Denote data points by either an “x” or a dot inside a circle. Nothing else is accepted by CXC.
2. Data point symbols must not be too large or too small. A good guide is to make them the same size as the smallest squares on the graph paper.
3. Do not forget to join the points by an appropriate line or curve. All curves must be smooth.
4. Lines and data points must not be too dark or light, or too thin or thick. A 0.5 mm pencil with a 2B point will be sufficient.
5. Include a title with units.
6. Include and a statement of scale.
1(a)(ii)(c)
When asked to determine a reading from a graph, you must ‘show working’ which in this case means that you must include dashed lines, intersecting both axes, and the line or curve drawn.
1(a)(iii)
When given space for a calculation, remember to show all statement (working) since this gains marks. In this question, a final numerical answer should be awarded one of the two marks, if statement is absent. Always remember units.
Reasoning Behind Calculation
From the given balanced equation, one mole of zinc will displace (completely react with) one mole of copper (II) sulphate i.e. Cu2+(aq) ions.
Therefore if we can find out how many moles of Cu2+(aq) ions are present in the 100cm3 of CuSO4(aq) used. This is why the first step of our answer will be to find the moles of CuSO4 in 100 cm3 of solution (0.02 mol).
We will then make the link, that this same number of moles of zinc is required. Now having a number for moles of zinc required (0.02 mol) we can convert moles of zinc required to mass of zinc required. This is done using the formula:
Mass of Zn = (moles of Zn) ´ (molar mass of Zn)
Recall that any mass must be given units of grams, unless it is a relative mass. Hence the final answer here, is in grams.
1(a)(iv)
Upon adding varying masses of zinc to the copper (II) sulphate solution, we would run through three different situations.
1. The moles of zinc added is less than that required to react with all the Cu2+ present. The more zinc added under these conditions, the greater will be the temperature rise, since there is still Cu2+ available to take part in the exothermic reaction. Therefore we can now understand why the 2nd, 3rd, and 4th points on the graph are higher than the ones before them.
2. The moles of zinc added is equal to that required to react with all the Cu2+ present. At this point, the maximum amount of heat energy possible is released, and no Cu2+ remains to react and give off more heat.
3. The moles of zinc added exceeds that required to react will all the Cu2+ present. Heat energy will be evolved, and the temperature will rise, once there is still a zinc-copper reaction occurring. There will then be a point reached where all the Cu2+ has been consumed, and some unreacted zinc remains. This unreacted zinc will have nothing with which to interact and evolve more heat. No further temperature rise will therefore occur when the zinc is in excess.
1(a)(v)
The reactivity series is relevant here. The system under investigation is now:
Ag(s) + CuSO4(aq) ® no reaction
According to the reactivity series, silver cannot displace copper. No reaction will occur, so no temperature change can be observed.
1(b)(i)
The only way to work out this part, is to learn the qualitative analysis tests, which can be found in any standard chemistry textboook.
1(b)(ii)
Solid R would consist of a compound containing both the cation and anion confirmed from tests in 1(b)(i). The main issue here is to correctly write the formula, taking into account charges on both ions.
1(c)(i)
In order to design an experiment as required, we must find some chemical way in which hydrochloric acid and vinegar differ. This will be based on knowledge of the properties and reactions of both substances. Note that vinegar is CH3COOH(aq) i.e. a solution of a weak acid.
Vinegar | HCl | |
1.Higher pH | Lower pH | |
2.Dissociates to give CH3COO- and H+ | Dissociates to give Cl- and H+ | |
3.Organic acid so reacts with alcohols to give ester. | Mineral acid which does not react with alcohol to give ester. | |
4.Does not give white fumes when reacting with ammonia. | Reacts with aqueous ammonia which liberates ammonia gas to form white fumes with HCl only. |
Distinguished by
1. Universal indicator or pH meter. HCl will have the lower pH.
2. Test both for Cl- ions. Vinegar will give no result.
3. React both with alcohol and see which one forms ester.
4. React both with aqueous ammonia which liberates ammonia gas to form white fumes with HCl only
Please contact me here http://www.facebook.com/pages/CXC-Chemistry-Lessons/147920055257221
if you find any typographical errors, or have any comments.
Reference:
1. Chemistry for CXC, Lambert and Mohammed, 1993 edition.
No comments:
Post a Comment