Chemistry 534

Rates of Reactions (continued)

1. Combustion and Activation Energy

· Combustion, as you recall, involves a reaction between a fuel and oxygen.

· The products of a combustion reaction always include an oxide that binds to a victimized atom within the fuel (we’re dramatizing here: the victim is simply an atom that lost electrons to the electron-hungry oxygen).

· If there is hydrogen within the molecules of the fuel, water will also be produced.

Example 1: Predict what will form from the combustion of the following:

Fuel	Products of combustion	Equation
Aluminum ( Al)	Al₂O₃	2 Al + 1.5 O₂--> Al₂O₃
Methane (CH₄)	CO₂ + H₂O	CH₄+ 2O₂--> CO₂ + 2H₂O
Gasoline ( C₈H₁₈)	CO₂ + H₂O	C₈H₁₈+ 12.5O₂--> 8CO₂ + 9H₂O
coal (mixture of S + C)	SO₂ + CO₂	S+ O₂--> SO₂ ; C+ O₂--> CO₂
wood (mixture of [C₆H₁₀O₅]_n and [C₁₀H₁₂O₃]_n )	CO₂ + H₂O	-----------------------------------

· Because combustion forms very stable products, combustion reactions are exothermic.

Example 2: Draw a reaction profile (enthalpy versus reaction-progress) for a combustion reaction.

Enthalpy(kJ)

Progress of reaction

· Although combustion reactions eventually release heat, they also need heat to get started. The temperature at which oxygen molecules smash into fuel molecules with enough energy so that electrons are stolen and products are formed is known as the kindling point.

Substance	Kindling Point (^oC)
Paper	232
Wood (varies with type)	190-266
Cotton	266
Methyl alcohol	464
Natural gas(depends on composition)	482-632

· The little bump in the graph on the previous page represents the amount of energy needed to reach the kindling point. Many non-combustion reactions also need a jump-start , and in general it is known as the activation energy, A_e. The activation energy can be calculated by :

A_e = H _maximum– H _reactants

Example 3. Calculate the activation energy for the following:

350 kJ

300 kJ

200 kJ

Activation energy = A_e = 350-300 kJ = 50 kJ

2. Rates of Combustion

The first three factors that control the rate of combustion are components of the fire triangle. They are heat, oxygen and fuel.

If any of the three components is missing, the rate will be zero; in other words, the fire will stop. For obvious reasons, firemen are taught the fire triangle:

Water puts out most fires because it absorbs a great deal of heat with its high specific heat capacity, so it cools the fire.

CO₂ extinguishers which release carbon dioxide by reacting Na₂CO₃ with H₂SO₄,( or by having liquid carbon dioxide under pressure)

smother the fire and deprive it of oxygen.

In fighting fires it is essential to prevent a fire from reaching areas rich in fuels such as oil furnaces. Any gas lines leading to the burning area must be automatically shut off as the alarm is triggered. For more firefighting details see this link.

The following affect the rate of combustion, or simply put, how fast something burns. Note that 3 components of the fire triangle are present among the five factors.

· The nature of the fuel used.

· The concentration of oxygen

· The surface area of the fuel

· Temperature

· Presence of catalysts

Example 1 Relate how each of the first four factors listed above affect the rate at which wood is burned in a fireplace.

a. The nature of the fuel used:

· The amount of moisture still present in wood slows the rate of combustion.

· Certain types of wood contain more resin or oils, and these increase the combustion rate.

· Structurally, different trees synthesize different fibers and these don't all burn at the same rate.

b. The concentration of oxygen:

· The higher the concentration of oxygen, the faster wood will burn. This applies to all other combustible materials.

c. The surface area of the fuel:

· If you take two identical logs and convert one into wood chips, the intact one will burn a lot slower. Because of its lower surface area, the intact log has less of its molecules exposed to oxygen. This means that oxygen molecules will only collide with a tiny proportion of the total available wood molecules. Without collisions, no reaction will take place, and so low surface area dramatically slows down the rate of combustion.

d. Temperature:

· Raising the temperature increases the kinetic energy of both oxygen and fuel molecules. This will lead to more overall collisions and to more effective collisions between the fuel and O₂. Since more molecules are reacting in the same period of time this increases the rate of combustion.

e. Catalysts:

· A catalyst is a chemical that speeds up a reaction. (The catalyst, as we'll explore later, is itself not consumed in the reaction.) So any chemical that makes it easier for oxygen to take electrons away from wood will lower the activation energy and increase the rate at which wood burns.

Exercises for Rate of Combustion

1. Write a balanced equation for the combustion of pentane. Pentane is C₅H₁₂. Identify the fuel and the oxide, and include heat on the appropriate side of the equation.

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

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.

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

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?

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

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

type of candle made of paraffin, C₂₅H₅₂ , is 8.33 x10^-4mol/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 ?

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

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