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SS2: CHEMISTRY - 2ND TERM

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  1. Water & Solution I | Week 1
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  5. Rate of Chemical Reaction | Week 6 & 7
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SS2: CHEMISTRY – 2ND TERM Rate of Chemical Reaction | Week 6 & 7 SS2: Chemistry Theory Questions – Rate of Chemical Reaction
Lesson 5, Topic 6
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SS2: Chemistry Theory Questions – Rate of Chemical Reaction

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Question 1:

(a) What is meant by the rate of chemical reaction?

(b) Excess dilute hydrochloric acid was added to 1.5g of magnesium trioxocarbonate[IV], and carbon[IV]oxide gas was evolved. If the reaction was completed after 3 minutes, what was the rate of the reaction?

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Question 2:

(a) Draw the energy profile diagram for the reaction 
H2(g) + I2(g) → 2HI(g)      ∆H = -13KJmol -1

(b) If the concentration of the HI increases from 0 to 0.001moldm-3 in 50seconds, what is the rate of reaction? 

(c) Collision theory suggests that for two particles to react, they must collide. What five factors determine whether or not the collision would lead to formation of product.

(d) Explain the observations on adding dilute H2SO4 separately to Zinc dust and Zinc granules of the same mass. The dust produced more vigorous efferverscence than granules?

View Answers

Question 3:

(a) Explain in terms of the collision theory
i. The effect of temperature increases on a reaction rate.
ii. How the rate of a gaseous reaction is affected by an increase in pressure.

(b) consider the reaction represented by the equation:
Mg[s] + 2HCl[aq] → MgCl2[aq]  + H2[g]
i. Name the type of reaction involved
ii. Give two ways by which the reaction could be made faster

(c) What volume of hydrogen gas would be produced from 6.0g of the magnesium? [H = 1, Mg = 24, 1 mole of a gas occupies 22.4dm3 at s.t.p ]

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Question 4:

Draw an energy profile diagram to illustrate a catalysed exothermic reaction and label parts of the curve representing the following.
i. Activated complex [without catalyst]
ii. Activation energy [with catalyst]
iii. Enthalpy change

(a) State two conditions that can lead to ineffective collisions during chemical reaction. 

(b)The photocatalytic reaction between hydrogen and chlorine, when carried out in a dimly lit room, is represented by the following equation.
H2[g] + Cl2[g]  → 2HCl[g]     △H = -XKJmol-1
Explain what happens when the reaction is carried out in sunlight.

(c) i. Define the term catalyst
ii. Give two characteristics of a catalyst
iii. Using energy profile diagrams, show catalysed and uncatalyzed reactions.

View Answers

Question 5:

a. The volume of carbon(IV)oxide produced from calcium trioxocarbonate(IV) and excess hydrochloric acid was measured at selected intervals until two successive readings agree as shown in table 
i. Plot the volume of CO2 gas collected against time taken.
ii. State the time taken for the reaction to reach completion
iii. What was the volume of CO2 collected after 20 seconds?
iv. Why did the reader stop taking readings at 6-second intervals after 36 seconds?
v. Why must the readings continue until three successive readings agree?

Table 2

Time [secs]612182430365060100150250
vol. of CO2 collected [cm3]4060758593100105112115115115

a. Two different samples of calcium trioxocarbonate(IV( of the same mass were put in two separate beakers as shown as in fig5 (diagrams are labelled X and Y). Bench hydrochloric acid were added to each in excess and effervescence colourless gas evolved.

Fig 5

NNsjI3kUrGJ746pRL8BzggWsyxliI8HMoH7kkMXYgBAobNycHgebLFj7pJ4yUlkNxZOZEPTrX0J4ltNF1c fTcP1RLSgS

Fig 5: beakers bearing calcium trixocarbonate[iv] of the same mass.

i. Which of the setups in X and Y will be the first to stop producing effervescence.
ii. Explain your answer in [i] above
iii. Identify the colourless gas evolved
iv. Write a balanced equation for this reaction.

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Question 1

(a)  What is meant by the rate of a chemical reaction?

Answer: This is the number of moles of reactants converted or products formed per unit time.


(b) Excess dilute hydrochloric acid was added to 1.5g of magnesium trioxocarbonate[IV], and carbon[IV]oxide gas was evolved. If the reaction was completed after 3 minutes, what was the rate of the reaction?

Solution:

Rate = \( \frac{mass}{time\:taken} \\ \frac{1.5}{3}\)

⇒ 0.5g/min

Question 2

(a) Draw the energy profile diagram for the reaction
H2(g) + I2(g) → 2HI(g)      ∆H = -13KJmol -1

Answer:

(b) If the concentration of the HI increases from 0 to 0.001moldm-3 in 50seconds, what is the rate of reaction?

The average rate of a reaction over a given time interval is found by determining this ratio:

Rate = \( \frac{\Delta C}{\Delta T} \)

where ΔC is the change in concentration observed and Δt is the length of the time interval.

∴ Rate = \( \frac{0.001\:-\:0}{50} \)

∴ Rate = \( \scriptsize 2 \: \times \: 10^{-5} \normalsize \frac{mol}{dm^3s} \)

Answer:  = 2 x 10-5 mol/dm3/sec.

 

(c) Collision theory suggests that for two particles to react, they must collide. What five factors determine whether or not the collision would lead to formation of product.

Answer:

i. Nature of reactants
ii. Concentration of reactants
iii. Temperature
iv. Catalyst (presence)
v. Pressure


(d) Explain the observations on adding dilute H2SO4 separately to Zinc dust and Zinc granules of the same mass. The dust produced more vigorous efferverscence than granules?

Answer:

Due to increased surface area, there are more zinc molecules to react with H2SO4, thus, greater effervescence is observed.

Question 3

(a) Explain in terms of the collision theory

i. The effect of temperature increases on a reaction rate.

Answer: Temperature increases the kinetic energy of the reactants thus, collision is frequent and the rate of reaction increases.

ii. How the rate of a gaseous reaction is affected by an increase in pressure.

Answer: An increase in pressure decreases the volume, thus, the gas molecules are closer together, thus, collision increases and the rate of reaction increases.


(b) consider the reaction represented by the equation:
Mg[s] + 2HCl[aq] → MgCl2[aq]  + H2[g]

i. Name the type of reaction involved

Answer: Substitution or displacement reaction

ii. Give two ways by which the reaction could be made faster

Answer:

– Increasing the surface area of Magnessium
– Increasing the concentration of Magnessium.


(c) What volume of hydrogen gas would be produced from 6.0g of the magnesium? [H = 1, Mg = 24, 1 mole of a gas occupies 22.4dm3 at s.t.p ]

Solution:

Balanced equation

Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)

No of moles of Mg = \( \frac{mass}{molar\:mass} \\ = \frac{6}{24} \\ \scriptsize = 0.25mol\)

1 mol Mg will produce 1 mol H2 gas

24 g Mg will produce 22.4 dm3 H2 gas

6.0 g Mg will produce: \( \frac{6 \: \times \: 22.4}{24} \\ = \frac{134.4}{24} \\ = \scriptsize 5.6 dm^3\)

Question 4

Draw an energy profile diagram to illustrate a catalysed exothermic reaction and label parts of the curve representing the following.
i. Activated complex [without catalyst]
ii. Activation energy [with catalyst]
iii. Enthalpy change

Answer:

exothermic reaction

Exothermic Reaction: 

where ∆H = Enthalpy change

Ea1 – activation energy of uncatalysed reaction

Ea2 – actuation energy of catalyzed reaction


(a) State two conditions that can lead to ineffective collisions during chemical reactions.

Answer:

– When the reactants are not properly aligned (orientated)
– When the energy available isn’t up to the activation energy required.


(b) The photocatalytic reaction between hydrogen and chlorine, when carried out in a dimly lit room, is represented by the following equation.
H2[g] + Cl2[g]  → 2HCl[g]     △H = -XKJmol-1
Explain what happens when the reaction is carried out in sunlight.

Answer: The rate of reaction increases


(c) i. Define the term catalyst

Answer: A catalyst is a substance that alters the rate of a chemical reaction without being used up in the reaction.

ii. Give two characteristics of a catalyst

Answer:

– It is not used up at the end of the reaction
– It lowers the activation energy required

iii. Using energy profile diagrams, show catalysed and uncatalyzed reactions.

Answer:

exothermic reaction

endothermic reaction

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