TOPIC

EI Multiple Choice Questions

During the combustion of coal, a buming matchstick or an ignition source is usually used. What is a

possible reason for doing so?

A. The reaction is endothermic and heat energy must be supplied for buming to occur.

B. Heat energy is used to overcome the energy barrier before the reaction can proceed.

C. It is to ensure that there are enough oxygen molecules for a complete reaction.

D. It is to reduce the enthalpy change so that the reaction can take place faster.

Solid candle wax is a hydrocarbon and will produce carbon dioxide and water vapour upon complete

combustion. The reaction scheme below shows four stages in the conversion of solid candle wax into itscombustion products.

CroFlur(s) -+ Crol{ur(l) -+ CroHur(g) -+ 3OCOr(g) + 3lHrO(g) + 30COr(g) + 3lHrO(l)PaRS

3. When l.fi) g of hydrogen reacts with fluorine, 539 kJ of heat is absorbed. How much energy is absorbed

when two moles of hydrogen reacts with excess fluorine?

A. 539 kJ

B. 1078 kJ

c. 2t56kJD. 43t2kI

4. A reaction is endothermic when

I the enthalpy change ofheat ofreaction is positive.

II the temperature of the surroundings increases.

m the energy ofthe products is greater than the energy ofthe reactants.

A. I and II only

B. II and III only

C. I and III only

D. AII of the above

1.

2.

102Topic 15

Energy Chonges

Which of the following statements is correct?

A. All four stages are endothermic.

B. All four stages are exothermic.

C. Stages P, Q and R are exothermic whereas stage S is endothermic.

D. Stages P, Q and R are endothermic whereas stage S is exothermic.

II

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9. Refer to the following chemical reaction:

NH,(g) + HC/(g) -+NHoC/(s)

Which statement about the reaction is correct?A. The volume decreases and the reaction is endothermic.B. The volume decreases and the reaction is exothermic.C. The volume increases and the reaction is endothermic.D. The volume increases and the reaction is exothermic.

When a strong acid is neutralised by a strong base, the change in heat content has a constant value ofLH = -57.6 kJ. What is the reason?

A. Strong acids and strong bases produce salt and water only.B. Strong acids and strong bases have equal strength.C. The reaction only involves hydrogen and hydroxide ions to produce water.D. The reaction is carried out at room temperature and pressure.

Ammonia is made from the reaction between nitrogen and hydrogen in the Haber process, which is an

exothermic reaction' N, + 3H, + 2NH3

Which of the following statements is correct?A. The total energy change for bond formation is greater than that for bond breaking.B. The products possess more energy than the reactants.C. The number of bonds broken is less than the number of bonds formed.D. The breaking of the N=N bond is an exothermic process.

When one mole of carbon is bumt in a sufficient amount of oxygen, 40.0 kJ of heat is liberated based on

the equation:C(s) + Or(g) -+ COr(g) AH = -40.0 kJ/mol

Calculate the mass of carbon required to provide 12.0 H of heat when bumt in a sufficient amount ofoxygen.A. 3.60 g

B. 4.80 gC. 7.20 g

D. 9.60 s

When steam is passed through white-hot coke, areaction occurs and the temperature of the coke decreases.

What does this indicate?A. Coke is an oxidising agent.B. The reaction is endothermic.C. The reaction is exothermic.D. The reaction is reversible.

AH = -ve

10. In reaction 1, solid cobalt (II) chloride turns from pink to blue when it is heated while in reaction 2, blue

cobalt (ID chloride paper tums pink and becomes warm when water is added to it.Which of the following correctly shows the energy changes involved in the two reactions?

endothermic endothermicA.

exothermicB. endothermic

exothermicC exothermic

endothermicD. exothermic

Energy Changesr03

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11. Which of the following involves an endothermic reaction?

A. reaction of sodium with waterB. change of water to ice at melting pointC. dissolving ammonium chloride in waterD. addition of sodium hydroxide to sulfuric acid

12. Which of the following reactions is endothermic?

A. burning of paper

B. reduction of silver ions to silver atoms in photography

C. oxidation of glucose in the process of respirationD. formation of iron sulf,de from iron filings and sulfur powder

E Stn chrred Ques{ions

(a) The table below shows the energy released by the complete combustion of some compounds which

are used as fuels.

Compound Formula M, AH / kJ mol-r Heat released / kJ g-t

methane CH, 16 -880ethanol C,H.OH 46 -l 380

propane C,Ht 44 -2200heptane CrH,u 100 -4800

(i) Complete the table by calculating the amount of heat released when 1.00 g of each compound

is completely combusted.(ii) By referring to the table, which compound will produce the most heat per gram of

compound?(iii) Which fuel should be used in order to reduce air pollution? Give a reason for your answer.

(b) The reaction between the solutions of potassium carbonate and magnesium sulfate is endothermic.Sketch a graph to show how the temperature of potassium carbonate solution will change with timewhen magnesium sulfate solution is added to it.(Assume that the solutions are initially at room temperature and continue the graph until there is no

further change in the temperature.)(c) Explain how the catalyst affects the

(i) activation energy,

(ii) rate ofreaction, and

(iii) enthalpy change.

2. The table below shows some bond energies, measured in kJ/mol.

Bond

H-H 436 CI_CI 242

H_C/ 431 C-H 412

C=C 6t2 C-C 348

N=N 409 N-N 163

r04Topic 15

Bond Bond Energy / kJ rnol'tDanya

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(a) What is the sign of bond energy? Explain your answer.

(b) Which of the bonds listed is the strongest?

(c) Is a double bond between two carbon atoms twice as strong as a single bond? Use the informationgiven to explain your answer.

(d) Use the information given to calculate the total energy required to break one mole of methane into

atoms.

(e) Bond-making is an exothermic process (AH = -ve). Calculate the energy change expected from

the reaction of one mole of hydrogen with one mole of chlorine to form two moles of hydrogen

chloride.

3. An increasing amount of potassium nitrate was added to 15.0 cm3 of water at room temperature. Afterthorough stirring, the lowest temperatures were recorded and plotted in the graph as shown below.

temperature

l'c

mass / g

(a) Is the reaction endothermic or exothermic? Give a reason for your answer.

(b) What was the room temperature on the day of the experiment?

(c) What was the decrease in temperature when 2.00 g of potassium nitrate was dissolved in water?

(d) Suggest a reason why the temperature did not decrease any further after 6.00 g ofpotassium nitrate

had been added.

(e) Briefly explain in terms of bond-breaking and bond-forming why a decrease in temperature can

occur.

Energy Changest05

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Hydrazine, NrHo, burns in oxygen according to the following equation:

NrHlg) + Or(g) +Nr(g) + 2HrO(g)

Bond Bond energy i kJ mol-r

N_H 391

N-N 945

O=O 498

N-N 158

o-H 464

Using the information provided, answer the following questions.

(a) Deduce the structural formula of hydrazine, which is a covalently bonded molecule.(b) Calculate the change of heat of reaction, AH for the combustion of hydrazine.(c) Is the reaction exothermic or endothermic?

(d) Draw the energy profile diagram for the reaction between hydrazine and oxygen. Include in the

diagram, the value of AH as calculated in (b) and indicate the activation energy for the reaction.

Many scientists and engineers are exploring the use of hydrogen-oxygen fuel cells to run vehicles on the

road in future. Such fuel cells have in fact been used to supply electrical energy in space vehicles.

negative terminal(anode)

positive terminal(cathode)

+

porous carbonelectrode

porous carbonelectrode

hydrogen <-- oxygen

electrolyte

water <_ ----> water

Reaction at the cathode: Or(g) + 2HrO(l) + 4e -+ 4OH-(aq)

Reaction at the anode: 2Hr(9 + 4OH-(aq) -+ 4HrO(l) + 4e-(a) Which of these reactions is an oxidation reaction? Explain your answer.

(b) Construct the overall equation, with state symbols, for the reaction that takes place in the fuel cell.(c) How is electricity being generated in the fuel cell?(d) State two advantages of using hydrogen-oxygen cells to power vehicles.(e) If a certain fuel cell uses 240 dm3 of oxygen to generate electricity, determine the mass of water that

is formed at room temperature and pressure.

(f) State two possible methods to obtain hydrogen with the use of current technology.

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