Exercises for Rate of Combustion ( Solutions)

1. Write a balanced equation for the combustion of pentane. Pentane is C₅ H₁₂. Identify the fuel and the oxide, and include heat on the appropriate side of the equation.

C₅ H₁₂+ 8 O₂ à5 CO₂ + 6 H₂O + heat

Fuel: C₅ H₁₂ oxides: CO₂ + H₂O

2. Draw a reaction profile in which H _reactants = 100 kJ, the activation energy is 50 kJ, and DH = -25 kJ.

150

E (kJ)

100

75

Note: Ae = H_max – H_reactants =150 - 100 = 50 kJ.

DH = H_products – H_reactants = 75-100 = -25 kJ

3. Which of the following is not absolutely necessary for combustion to take place?

A. a fuel B. oxygen

C. match D. a sufficiently high temperature

Why? A lighter, spark or surrounding air(as in spontaneous combustion) can provide a sufficiently high temp. It does not have to be a match.

4. Carbon burns in air to produce CO₂ and H₂O. Under what conditions would you expect the fastest rate of combustion?

A. Chunks of carbon are heated and allowed to burn naturally in air.

B. Forcing hot air over the carbon as it burns.

C. Powdering the carbon and allowing it to burn naturally.

D. Powdering the carbon and forcing hot air over it.

(the answer is D because you’re increasing surface area through powdering, increasing oxygen by forcing air, and since it’s hot air, you’re raising the temperature.)

5. How does the foam from a fire extinguisher help put out the fire?

It prevents oxygen from reaching the fuel, thus smothering the fire. The foam itself is not flammable.

6. A cook started a fire by forgetting about the oil he was warming. He then pulled the pan off the stove and threw baking soda in the pan.

Explain why the cook acted the way he did. Why did he not use water?

Oil floats on water, and does not mix with it. This will prevent the water from cooling the oil effectively, and above the water the oil will continue to encounter oxygen. Since it stays hot and keeps getting oxygen, the oil keeps burning.

Baking soda, on the other hand, acts like the fire extinguisher in question 5.

1. Olive oil has a lower kindling point than corn oil. Which one is the more practical cooking oil?

Corn oil is more practical because it does not burn as easily (higher kindling point). By reaching a higher temperature before burning, corn oil helps food cook faster.

8. According to the manufacturer, the average rate of combustion of a certain

type of candle made of paraffin, C₂₅H₅₂ , is 8.33 x10^-4 mol/min. This type of

candle is sold in four sizes: 25, 50, 75 or 100 g. You wish to use only one

candle of this type to provide 4 continuous hours of light. What is the smallest

one you can buy for this purpose ?

In 4 hours = 240 minutes,

[ 8.33 x10^-4 mol/min][240 minutes] = 0.1999200000 moles will burn.

0.1999200000 moles(25*12+52*1 g/mole) = 70.4 g (with sig figs = 7 X 10¹ g) will burn.

So you need to buy at least a 75 g candle to last you four hours. That’s one long bath by candle light!

9. From a molecular point of view, why does a combustion reaction need to reach a kindling point before a fire starts?

Only at that temperature will fuel molecules and oxygen molecules be able to collide with enough energy to create oxides.

10. From a molecular point of view, why does surface area increase the rate of a reaction?

Molecules are only exposed at the surface. Those below the surface do not immediately collide with the other reactant. Having more surface exposes more molecules to a reactant at a given instant, increasing the rate. That’s why if you’re in a rush to make mashed potatoes, you should cut open the potatoes before tossing them into the water. Of course, practically speaking, one cut will suffice since too many cuts will create a mess in boiling water.

11. Ae =133 –83 kJ =50. kJ