Fire Unit Investigation III: Energy for Change Lesson 1: No Going Back Lesson 2: Fire Starter Lesson 3: Formations Lesson 4: Ashes to Ashes

Fire Unit

Investigation III: Energy for ChangeLesson 1: No Going Back

Lesson 2: Fire Starter

Lesson 3: Formations

Lesson 4: Ashes to Ashes

Fire Unit – Investigation III

Lesson 1:

No Going Back

Unit V • Investigation III

© 2004 Key Curriculum Press.

ChemCatalyst

Humans generate energy from burning fuels, such as coal, oil, natural gas, and hydrogen. For example, the combustion of coal can be written as

C(s) + O2(g) CO2(g)

• Do you think you can reverse the reaction to form coal, C(s), and oxygen, O2, from CO2? Explain your thinking.



The Big Question

• How do we keep track of the energy changes in a chemical reaction?



You will be able to:

• Describe the direction of energy changes in a combustion reaction



2 H2 + O2 2 H2 + O2

2 H2O 2 H2O

Reaction 1: Combustion of hydrogen

2 H2 + O2 2 H2O

Reaction 2: Decomposition of water

2 H2O 2 H2 + O2

• Energy diagrams show the difference in energy from the beginning of a reaction to the end of the reaction.

Notes



Activity

Purpose: In this lesson you will use energy diagrams to examine the energies from the beginning of a reaction to the end.

(cont.)



2 H2 + O2 2 H2 + O2

2 H2O 2 H2O

Reaction 1: Combustion of hydrogen

2 H2 + O2 2 H2O

Reaction 2: Decomposition of water

2 H2O 2 H2 + O2

(cont.)

(cont.)



Reaction 1: Combustion of methane

CH4 + 2 O2 CO2 + 2 H2O

Reaction 2: Formation of methane

CO2 + 2 H2O CH4 + 2 O2

(cont.)



Making Sense

• Humans generate energy from burning fuels we dig out of the earth, such as coal, oil, and natural gas. Do you think it will be easy to replenish these fuels? Explain your thinking.



• Heat of reaction is the amount of energy gained or lost during a chemical reaction. If the sign for the heat of reaction is negative, the reaction is exothermic. If the sign is positive, the reaction is endothermic.

• Conservation of energy is a law that states that energy is neither created nor destroyed. Thus, if a chemical process releases energy, then the reverse process must require an input of the exact same amount of energy.

Notes



Check-In

• Sketch an energy diagram for the combustion of carbon (coal) to form carbon dioxide. The heat of reaction is –394 kJ/mol.

• What energy is required to form coal from carbon dioxide?



Wrap-Up

• The heat of reaction is the energy change in going from reactants to products.

• The heat of reaction is positive for an endothermic reaction. It is negative for an exothermic reaction.

• Energy is conserved in a chemical reaction. The reverse reaction requires an equal amount of energy transferred in the opposite direction.


Lesson 2:

Fire Starter



ChemCatalyst

In the previous lesson we showed you an energy diagram for the combustion of hydrogen. In actuality, that diagram was simplified. This new energy diagram is more accurate.



• What is different about this diagram? Explain what you think is going on, and why you think the diagram has the shape it has.

2 H2 + O2

2 H2O–286 kJ/mol H2

(cont.)



The Big Question

• Why do some chemical reactions need a “spark” or some other kind of energy input to get them started?




• Explain the role of the “activation energy” for a chemical reaction.



• Energy of activation (activation energy): The energy that is required to get a reaction started.

reactants

Ea

products

Notes



Activity

Purpose: In this lesson you will have practice interpreting energy diagrams and activation energies.

(cont.)



Reaction 1 Reaction 2Energy change

in kJ/mol

200

100

0

-100

-200

(cont.)

(cont.)



Ea

reactants

products

transition state

bond breaking

bond making

(cont.)

(cont.)



H2 + Cl2 H2 + Br2

Reaction 1:H2 + Cl2 2 HCl

2 HCl

2 HBr

Reaction 2:H2 + Br2 2 HBr

(cont.)

(cont.)



paper + O2paper + KNO3

6 CO2 + 6 H2O 6 CO2 + 6 H2O + 6 KNO2

(cont.)



Making Sense

• Explain the energy of activation and the heat of reaction in terms of bond breaking and bond making.



• Most chemical reactions (not just combustion reactions) require some sort of energy input to get them started. This is called the activation energy.

Notes

(cont.)



• Bond breaking requires an input of energy into a system.

• Bond making, on the other hand, releases a certain amount of energy.

• Bond energy: The energy required to break a bond. Bond breaking is endothermic. Bond making is exothermic.

E N NE

Notes (cont.)

(cont.)



• Reaction rate: The speed at which a reaction proceeds. The reaction rate is effected by temperature, mixing, and surface area. Reactions with high activation energies proceed slowly.

effect of catalyst

• Catalyst: A substance that lowers the activation energy for a reaction. A catalyst is not consumed by the reaction.

(cont.)



Check-In

Use the energy diagram to answer the questions.

a.

b. c.(cont.)



• Which arrow represents the activation energy—heat going into system?

• Which arrow represents the heat of reaction—net energy released by the reaction?

• For the reaction described by the energy diagram, is the energy required to break bonds greater than the energy released upon forming bonds? Explain.

(cont.)



Wrap-Up

• The energy of activation for a chemical reaction is the energy that is required to get a reaction started.

• Breaking bonds requires energy. Making bonds releases energy.

• Energy is required to start a reaction because bonds need to be broken as a first step.

(cont.)



• The heat of reaction is the difference between the energy required to break bonds and the energy released in forming bonds.

(cont.)


Lesson 3:

Formations



ChemCatalyst

• H2 (g) + 1/2 O2 (g) H2O (l) + 68 kcal

• H2 (g) + 1/2 O2 (g) H2O (l)∆H = –68 kcal/mol H2O

These two equations seem to contradict each other, but they both refer to the exact same chemical reaction. What does each equation mean?



The Big Question

• How can we calculate the energy of a reaction without measuring it experimentally?




• Use the concept of “heat of formation” to calculate the energy changes for various chemical reactions.



• You could say that the focus of the first equation is the combustion of hydrogen as a fuel.

• You could say that the focus of the second equation is the formation of liquid water.

Notes

(cont.)



• Sometimes it takes heat to form a certain product and sometimes heat is released in the formation of a certain product.

• Whether the heat is positive or negative, it is referred to as the heat of formation.

• Its symbol is ∆Hf°.

∆Hrxn = (the sum of ∆Hf products) – (the sum of ∆Hf reactants)

Notes (cont.)



Activity

Purpose: This lesson provides you with practice calculating heats of reaction using heats of formation values. Heats of formation:

(cont.)



Substance Heat of formation

∆Hf°

Substance Heat of formation

∆Hf°

CO2 (g) –394 kJ/mol C2H6 (g) –85 kJ/mol

C (s) 0 C6H12O2 (s) –1273.0 kJ/mol

H2O (l) –286 kJ/mol Fe (s) 0

O2 (g) 0 Fe (g) 416 kJ/mol

N2 (g) 0 FeO (s) –272 kJ/mol

N (g) 473 kJ/mol Fe2O3 (s) –822 kJ/mol

NO (g) 90 kJ/mol CaO (s) –636 kJ/mol

NO2 (g) 34 kJ/mol HCl (aq) –167 kJ/mol

N2O4 (g) 9.7 kJ/mol CaCO3 (s) –1207 kJ/mol

CH4 (g) –75 kJ/mol MgO (s) –602 kJ/mol

O (g) 248 kJ/mol Mg (s) 0



∆Hrxn = (∆Hf products) - (∆Hf reactants)

–∑∆Hf°(reactants)

= –[∆Hf° (CaO) + ∆Hf° (CO2)]

∆Hf° = 0 (elements)

∑∆Hf°(products)

= ∆Hf° CaCO3

(cont.)



Making Sense

• Explain how you use heats of formation to determine the heat of a reaction.



• Hess's Law, also known as the Law of Heat Summation, states that the sum of the heats of formation of the various steps of a reaction will be equal to the heat of the overall reaction.

(cont.)

Notes



• Calculate the heat of reaction for the reaction of NO2 with itself to form N2O4:

2 NO2 N2O4

∆Hrxn = (∆Hf° products) – (∆Hf° reactants)

∆Hrxn = (∆Hf° N2O4) – 2∆Hf° (NO2)

• Now solve for ∆Hf°rxn:

heat of reaction= (+9.7 kJ/mol) – 2(34 kJ/mol)

= (9.7 kJ/mol) – (68 kJ/mol)

= –58 kJ/mol(cont.)

Notes (cont.)



• Enthalpy of reaction: Enthalpy is simply the energy of the reaction adjusted to take into account atmospheric pressure.

∆Hrxn = ∑ ∆H(products) – ∑ ∆H(reactants)

(cont.)

Notes (cont.)



• Heat of reaction - energy input or output of a reaction

• Molar heat of reaction - energy input or output of a reaction per mole of reactant (or product) used

• Enthalpy - the heat (or energy) content of a system at constant pressure

• Heat of formation - the heat released or required (the change in enthalpy) during the formation of a pure substance from its elements

Notes (cont.)



Check-In

• Explain how you can you calculate the heat of reaction (or the enthalpy of reaction) for the following reaction, from the heats of formation of the reactants and products.

2Mg (s) + O2 (g) 2 MgO(s)

• Write out the formula for this calculation, using the compounds in the above reaction.



Wrap-Up

• The heat of formation of a substance is the energy required to create a mole of the substance from its constituent elements in their standard states.

• We can calculate the "energy" of a reaction by measuring the difference in energy between the reactants and products. ∆H = ∆H(products) – ∆H(reactants).

(cont.)



• Enthalpy is a more accurate value to use when talking about the energy content of a reaction.

• Enthalpy is similar to heat of reaction except that it takes into account atmospheric pressure and the work that gases do when they are produced or removed by a reaction.

(cont.)


Lesson 4:

Ashes to Ashes



ChemCatalyst

Many reactions are easily reversible. However, when a tree burns down, it is essentially impossible to recover the tree by reversing the combustion reaction. Examine the two chemical equations and explain why only one is easily reversible.

• 2 NO2 N2O4 ∆H = 9.7 kJ/mol

• 2 C8H18 + 25 O2 16 CO2 + 18 H2O∆H = –5439 kJ/mol



The Big Question

• How are the concepts in the Fire unit useful in describing the energy related to chemical changes?




• Identify the essential concepts of the Fire unit and explain how they can be used to describe energy changes in chemical reactions.



Activity

Purpose: This lesson provides you with practice problems that will allow you to review the concepts you've learned in this unit.

(cont.)



(cont.)

(cont.)



2 H2O (g)

2 H2O (l)

–58 kcal/mol

–10 kcal/mol

2 H2 + O2

(cont.)

(cont.)



+ 68 kcal/mol

2 H2O (l)

2 H2 + O2

(cont.)



Making Sense

• What information would you need to tell if a chemical reaction might result in a fire?



Notes



Check-In

• No Check-In.



Wrap-Up

• No Wrap-Up.

Documents

Fire Unit Investigation III: Energy for Change Lesson 1: No Going Back Lesson 2: Fire Starter Lesson 3: Formations Lesson 4: Ashes to Ashes