OMEGA TUITION CENTRE

Students name : ____________________________________ Teachers Name : Mr Chew Chin Kuen Subject : Chemistry Form 5 Class Attend : 5 Chemistry 6 Chapter 4 : Thermochemistry lesson 32 (3 August 2013) 2.30 4.30 pm

Mol = mass / RMM @ mol = (68.33 43.33) / 250

mol = 0.10 mol

q = m c ; q = 100 x 4.2 x 1.5

q = 630 J

H = q / mol H = 630 J / 0.1 mol

H = 6300 J / mol

@ H = + 6.3 kJ / mol

CuSO4 . 5 H2O (s) CuSO4 . 5 H2O (aq)

CuSO4 . 5 H2O (aq)

+ 6.3 kJ / mol

CuSO4 . 5 H2O (s)

To reduce the heat lost to the surrounding

Temperature decrease by half, 0.75oC

white

Pb(NO3)2 (aq) + Na2SO4 (aq) PbSO4 (s) + 2 NaNO3 (aq)

Polystyrene cup and thermometer

Mol = MV / 1000 ; mol = (0.5)(25) / 1000

mol = 0.0125 mol

Mol = MV / 1000 ; mol = (0.5)(25) / 1000

mol = 0.0125 mol

H = q / mol ; 50000 = q / 0.0125

q = 625 J

q = m c ; 625 = (25 + 25) x 4.2 x

= 3.0 oC

Zinc powder dissolve in copper (II) sulphate / brown solid is formed /

blue colour of copper (II) sulphate faded

Zn (s) + Cu2+ (aq) Cu (s) + Zn

2+ (aq)

2.5

5.0

7.5

8.0

8.0

8.0

6.0

4.0

2.0

0

This is due to, the reaction is complete. Zinc and copper (II) ion can no more

react

0.8 g of zinc

Mol = mass / RAM ; Mol = 0.8 / 65.0 mol = MV / 1000

Mol = 0.0123 mol mol = (0.2)(50) / 1000

mol = 0.010 mol [limitant]

q = m c ; q = (50)(4.2)(8.0)

q = 1680 J

H = q / mol ; H = 1680 / 0.010

H =168000 J / mol

@ H = - 168 kJ / mol

Amount of heat released when 1 mol of hydrochloric acid reacts with 1 mol of

sodium hydroxide to form 1 mol of sodium chloride and water

HCl (aq) + NaOH (aq) H2O (l) + NaCl (aq)

q = m c ; q = (50 + 50) x 4.2 x (35 28)

q = 2940 J

Mol = M V / 1000 ; mol = (1.0)(50) / 1000

Mol = 0.050 mol

q = m c ; q = (50 + 50) x 4.2 x (34 28)

q = 2520 J

H = q / mol ; H = 2940 / 0.050

H = 58800 J / mol

H = - 58.8 kJ / mol

H = q / mol ; H = 2520 / 0.050

H = 50400 J / mol

H = - 50.4 kJ / mol

5. Figure 3 shows the set-up for an experiment to determine the heat of combustion of ethanol (C2H5OH).

When 0.23 g of ethanol was burned in air, the heat evolved raised the temperature of 100g of water by 16 oC.

(a) Name the products obtained from the combustion of ethanol.

.

(b) Calculate the heat given out when 0.23 g of ethanol is burned (ignore heat loss to the container and

surrounding).

(c) (i) Calculate the number of moles in 0.23 g of ethanol.

(ii) What is the heat of combustion of 1 mole of ethanol?

(d) The heat of combustion of ethanol obtained from this experiment is less than its theoretical value. State two

precautions you can take to improve the accuracy of the experimental results.

(e) Draw an energy diagram for the combustion of ethanol.

(f) If butanol (C4H9OH) is used in the experiment to replace ethanol, would you expect the heat of combustion

of butanol to be higher, lower or the same as the heat of combustion of ethanol?

Give one reason for your answer.

Carbon dioxide and water

q = m c ; q = 100 x 4.2 x (16)

q = 6720 J

Mol = mass / RMM ; mol = 0.23 g / 2(12) + 6(1) + 16

= 0.005 mol

H = q / mol H = 6720 / 0.005

H = 1344000 J / mol

@ H = - 1344 kJ / mol

Use a wind shield

Thermometer is stirred all time

Heat of combustion by butan-1-ol is higher. This is due to butan-1-ol has more

number of carbon compare to ethanol.

6. Two experiments are carried out to compare the heat of neutralisation of a reaction between an acid and an

alkali.

Experiment I 100 cm3 of 1.0 mol dm-3 hydrochloric acid is mixed with 100 cm3 of 1.0 mol dm-3 aqueous sodium hydroxide in a polystyrene container.

Experiment II 100 cm3 of 1.0 mol dm-3 ethanoic acid is mixed with 100 cm3 of 1.0 mol dm-3 aqueous sodium hydroxide in a polystyrene container.

(a) Complete Table 2 by recording the maximum temperature and calculate the rise in temperature in

Experiment I and Experiment II. [2]

(b) Why is a plastic container suitable for use in this experiment? [1]

.........................................................................................................................................................

(c) For experiment I calculate,

(i) the heat given out [Specific heat capacity of solution: 4.2 J g-1 oC-1; density of solution: 1 g cm

-3]

(ii) the number of moles of hydrochloric acid used

(iii) heat of neutralisation of this reaction [5]

(d) The heat released in Experiment I is higher than in Experiment II. Explain why? [2]

.........................................................................................................................................................

........................................................................................................................................................

(e) Figure 7(i) shows the apparatus used in Experiment I. Predict and explain the rise in temperature expected

if Experiment I is carried out using the apparatus in Figure 7(ii). [3]

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Figure 6 shows the thermometer readings for the

maximum temperatures in Experiments I and II.

Experi

ment

Initial

temperature

(oC)

Maximum

temperature

(oC)

Temperature

rise (oC)

I 28.2 34.8 6.6

II 28.2 34.6 6.4

plastic is a good heat insulator. It reduce heat loss to surrounding

q = mc @ q = (100 + 100) (4.2) (6.6)

q = 5544 J

mol = MV / 1000 ; mol = 1.0 x 100 / 1000

mol = 0.1 mol

H = q / mol @ H = 5544 / 0.1 ; H = - 55.4 kJ/mol

HCl is a strong acid which dissociate completely in water, while CH3COOH is a

weak acid which dissociate partially in water. HCl has more H+ to react compare to

CH3COOH