chemistry

HEFC ASSIGNMENT BRIEF

Notes:

1.  This assignment must be all your own work.  All sources must be acknowledged and attributed in the text and bibliography

2.   This assignment must be accompanied by a completed HEFC Assignment Submission Form

3.   You must meet the submission deadline unless there are personal reasons that prevent you from doing so, in which case you must contact your tutor.  Late submission without authorisation will cap your achievement to a ‘Pass’

4.  You must comply with the regulations of your College/School for submission of coursework

5.  If you fail to meet the LO(s)/AC then you will have ONE opportunity for resubmission

Grading Criteria

The reactions of halide ions

Requirements per student group:

• solutions of sodium chloride, sodium bromide and sodium iodide or the potassium salts will suffice (approx. 0.5 mol dm^-3)
• solids of the above halide salts
• chlorine water (freshly prepared) (toxic)
• bromine water (corrosive and toxic)
• iodine dissolved in potassium iodide (toxic)
• 0.1 mol dm^-3 silver nitrate solution (for diagnostic testing) (toxic)
• 0 2 mol dm^-3 dilute ammonia solution
• 0.88 concentrated ammonia solution (corrosive)
• Cyclohexane

Safety Notes

Chlorine is highly toxic – it is very harmful to the eyes, lungs and respiratory tract.   Always store in a fume cupboard.

Bromine is very toxic and corrosive.   Use a fume cupboard.

Iodine is harmful.

Ammonia solution is corrosive and forms a toxic vapour.   Wear eye protection when this solution is in use.

Method

The reaction between halide ions and halogens

To 2 cm³ of a halogen solution add 10 drops of cyclohexane (this will selectively dissolve the majority of the halogen present, forming the upper layer, and result in a characteristic coloured solution):

Chlorine in cyclohexane – colourless / pale green solution

Bromine in cyclohexane – orange solution

Iodine in cyclohexane – violet solution.

Then add 2 cm³ of the halide solution of your choice and swirl gently.   If a reaction has occurred, another halogen will be formed and the organic layer (the upper if cyclohexane is used) will change colour.

Repeat for each combination

Record your result in the table shown below.

1)

What do the results of the above experiments indicate about the:

a) Order of the oxidising power of the halide ions

b) Order of the reducing power of the elemental halogen

c) Explain your answers to a & b using ideas of electronic configuration

The reaction between halide ions and silver (I) nitrate solution and then ammonia solution.

To the halide solution (2cm³ in each test tube) add 10 drops of silver nitrate solution (toxic) to each test tube.

2)

a)      Record your results in an appropriate table. Where no change occurred, state ‘no visible change’.

b)      Write a full balanced molecular equation including state symbols for the reaction between NaCl and AgNO₃

c)      Write an ionic equation for the above reaction.

3)

a) LiBr is an ionic compound however it has a larger covalent character than KBr. Explain why this is.

b) Why does the melting point of the metals decrease on descending the group?

c) Metals have many useful properties, one of these is their malleability. Explain why metals possess this characteristic.

d) The element chlorine is a gas at room temperature and had to be dissolved in water. Predict the physical state of Astatine at room temperature and explain why you have picked this.

e) Molecular iodine has a very low solubility in water whereas potassium iodide is much higher. Explain this difference.

4)

a) Consider the reaction between aluminium and water. If a source of aluminium metal is burning explain why water should not be used to put out the flame?

b) In a series of laboratory experiments the metals Mg, Ca, Sr and Ba were added to a test tube with cold deionised water and shaken. Afterwards the contents of each test tube was tested using a pH meter and it was found that, on progressing from magnesium to barium the pH increased from ~7 to ~14.

i) Why would the pH in any of these experiments be higher than 7?

ii) Explain the increase in pH on progressing from magnesium to barium.

5)

For the following reactions state the reaction type and explain using either the transfer of protons or electrons to justify your decision.

a) magnesium oxide + hydrochloric acid  à

Complete the word equation and write a balanced equation.

Classification –

Justification -

b) 8 H₂SO₄ + 2 KMnO₄ + 10 FeSO₄ à 5 Fe₂(SO₄)₃ + 2 MnSO₄ + K₂SO₄ + 8H₂O

Classification –

Justification –

6)

Transition metals have extremely valuable roles within society. One of these is acting as a catalyst. Choose two processes where transition metal catalysts are used.  For each write a balanced equation and explain in detail how they catalyse the reaction.

Section B

Determination of the enthalpy change for the thermal decomposition of NaHCO₃

In reaction 1 you will determine the enthalpy change for the reaction for sodium carbonate, Na₂CO₃, with an excess of dilute hydrochloric acid.

In reaction 2 you will determine the enthalpy change for the reaction for Sodium carbonate, NaHCO₃, with an excess of dilute hydrochloric acid.

You will then use your results from reactions 1 and 2 to determine the enthalpy change for the following reaction.

2NaHCO₃(s)     ®     Na₂CO₃(s)     +     CO₂(g)     +     H₂O(l)

Reaction 1

Na₂CO₃(s)     +     2HCl(aq)     ®     2NaCl(aq)     +     CO₂(g)     +     H₂O(l)

Method

1)  Weigh accurately a weighing bottle containing between 2.5g and 3.0g of sodium carbonate, Na₂CO₃.   Record your results in Table 1a.

2)      Using a measuring cylinder measure out 30cm³ of 2M HCl into a polystyrene cup.

3)      Record the temperature of the acid and using a stop watch record the temperature every minute up to and including the third minute, record your results in Table 1b.

4)      At the fourth minute add the sodium carbonate, do not record the temperature at this point.

5)      At the fifth minute and up to and including the tenth minute record the temperature of the solution, record your results in Table 1b.

6)      Reweigh your weighing bottle and record this value in Table 1a to determine exactly how much sodium carbonate was added.

Table 1b

Reaction 2

NaHCO₃(s)     +     HCl(aq)     ®     NaCl(aq)     +     CO₂(g)     +     H₂O(l)

Method

7)  Weigh accurately a weighing bottle containing between 3.2g and 3.7g of sodium carbonate, NaHCO₃.   Record your results in Table 2a.

8)      Using a measuring cylinder measure out 30cm³ of 2M HCl into a polystyrene cup.

9)      Record the temperature of the acid and using a stop watch record the temperature every minute up to and including the third minute, record your results in Table 2b.

10)  At the fourth minute add the sodium carbonate, do not record the temperature at this point.

11)  At the fifth minute and up to and including the tenth minute record the temperature of the solution, record your results in Table 2b.

12)  Reweigh your weighing bottle and record this value in Table 2a to determine exactly how much sodium carbonate was added.

Table 2b

Tasks

1)      For both reactions plot a graph of time (x-axis) against temperature (y-axis).

2)      Draw two lines of best fit, one through the 0-3 minute results and one through the 5-10 minute results.

3)      At the fourth minute use the two lines of best fit to work out the temperature change which took place at this point.

Questions

1)      Using the temperature change you have calculated for reaction work, calculate the heat absorbed / evolved for reaction 1 and state its units.

2)      Calculate how many moles of Na₂CO₃ were used in reaction 1

3)      Calculate the enthalpy change for reaction 1, stating clearly your sign and unit in the answer.

4)      Repeat the above steps to calculate the enthalpy change for the reaction between NaHCO₃ and HCl in reaction 2.

5)      State Hess’s law

6)      Using the equations you were given above for reaction 1 and reaction 2 draw a Hess’s cycle to enable you to calculate the enthalpy change for the following reaction :

2NaHCO₃(s)     ®     Na₂CO₃(s)     +     CO₂(g)     +     H₂O(l)

7)      Calculate the enthalpy change for the above reaction using your Hess’s cycle stating clearly your sign and unit in the answer.

8)       

9) Given the following data: DH_f (CH₄) = -74.8 kJmol^-1, DH_f (CH₃Cl) = -134.5 kJmol^-1, DH_f (HCl) = -92.3 kJmol^-1;

Calculate DH for the reaction CH₄(g) + Cl₂(g) à CH₃Cl(g) + HCl(g)