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Miracle Learning Centre Science Tuition Question (S4 Chem) (O Level Science Tuition/IP Science Tuition/Secondary Science Tuition)

(a) A student was asked to investigate the rate of reaction of magnesium with dilute hydrochloric acid. In the experiment, he used a clean piece of magnesium ribbon 5 cm long with 20.0 cm3 of 0.500 mol/dm3 dilute hydrochloric acid (in excess).

(i) Why is it essential to clean the magnesium ribbon? [1]

(ii) Write a balanced chemical equation for the reaction. [1]

(b) He measured the volume of hydrogen produced at r.t.p. at regular intervals. The results are shown in the graph above.

(i) State the volume of hydrogen gas produced when the reaction is complete. [1]

(ii) Hence, calculate the volume of 0.500 mol/dm3 dilute hydrochloric acid needed to react completely with the 5 cm long ribbon. [2]

(c) Copy the given graph into the writing paper. Then sketch on the same axes, the graphs you would expect if the experiment was repeated, making the following single changes in each case :

(i) Using half the length of cleaned magnesium ribbon. Label this graph A.

(ii) Using a cleaned magnesium ribbon 5 cm long but cut into very small pieces. Label this graph B.

(iii) Using dilute hydrochloric acid of concentration 0.250 mol/dm3. Label this graph C.

(d) Use the particles collision theory to explain your answer to c(iii).

Miracle Learning Centre Answer to Science Tuition Question (S4 Chem) (O Level Science Tuition/IP Science Tuition/Secondary Science Tuition)

B1(a)(i) 2m Cleaning to remove the magnesium oxide layer from the magnesium ribbon. [1]

(ii) The chemical equation is : Mg + 2HCl  MgCl2 + H2 [1]

(b)(i) 3m From the graph, the volume of hydrogen produce is 55cm3. [1]

(ii) Step 1 : Moles of hydrogen = volume/24dm3 = 0.06dm3/24dm3 = 0.0025 moles [1] Step 2 : Moles Ratio 2HCl : H2 2 : 1 y : 0.0025 y = moles of hydrochloric acid = 0.00125 [1] Step 3 : Volume of hydrochloric acid = moles/moles concentration = 0.00125/0.5 mol/dm3 = 0.0025 dm3 [1]

(c)(i), (ii), (iii) 3

(d) 2m - Since the hydrochloric acid concentration is smaller [1], there are fewer hydrogen ions per unit volume of the acid to collide with the magnesium, causing lesser frequency of ‘effective’ collision and decrease the rate of reaction. [1]

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