End of Section 15-2 Section 15-3 Section 15.3 Thermochemical Equations Write thermochemical equations for chemical reactions and other processes. combustion.

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Presentation transcript:

End of Section 15-2

Section 15-3 Section 15.3 Thermochemical Equations Write thermochemical equations for chemical reactions and other processes. combustion reaction: a chemical reaction that occurs when a substance reacts with oxygen, releasing energy in the form of heat and light Describe how energy is lost or gained during changes of state. Calculate the heat absorbed or released in a chemical reaction or phase change.

Section 15-3 Section 15.3 Thermochemical Equations (cont.) thermochemical equation enthalpy (heat) of combustion molar enthalpy (heat) of vaporization molar enthalpy (heat) of fusion Thermochemical equations express the amount of heat released or absorbed by chemical reactions.

Section 15-3 Writing Thermochemical Equations A thermochemical equation is a balanced chemical equation that includes the physical states of all reactants and products, and energy change.thermochemical equation The enthalpy (heat) of combustion of a substance is the enthalpy change for the complete burning of one mole of the substance.enthalpy (heat) of combustion

Section 15-3 Writing Thermochemical Equations (cont.)

Burning natural gas is an exothermic reaction. What is the equation for this reaction? CH 4 (g) + 2 O 2 (g)  CO 2 (g) + 2 H 2 O (g) We can add the energy component to this reaction in a couple different ways. 1) We can show the ΔH comb as a separate quantity. CH 4 (g) + 2 O 2 (g)  CO 2 (g) + 2 H 2 O (g) ΔH comb = -891 kJ/mol The fact that is negative tells us that energy is released during the process.

2) We can show the energy in the equation. CH 4 (g) + 2 O 2 (g)  CO 2 (g) + 2 H 2 O (g) kJ The fact that is on the product side tells us that energy is released during the process.

When you heat a substance, the kinetic energy of the particles increases. You can calculate this energy change using q = cmΔt. When goes through a phase change, like melting or boiling, the potential energy of the particles changes. There is no temperature change. To calculate the energy change we use q = mΔH.

Section 15-3 Changes of State Molar enthalpy (heat) of vaporization ΔH vap refers to the heat required to vaporize one mole of a liquid substance.Molar enthalpy (heat) of vaporization Molar enthalpy (heat) of fusion ΔH fus is the amount of heat required to melt one mole of a solid substance.Molar enthalpy (heat) of fusion

Section 15-3 Changes of State (cont.) Each substance has unique values for ΔH vap and ΔH fus

Section 15-3 Changes of State (cont.)

1. If the H vap of water is 40.7 kJ/mol, how much energy does it take to convert 1.00 g of water at 100. o C into steam at 100. o C? Water has a molar mass of 18.0 g/mol 40.7 kJ mol x 1 mol H 2 O 18.0 g =2.26 kJ/g (or 2260 J/g)

2. If the H fus of water is 6.01 kJ/mol, how much energy does it take to convert 1.00 g of ice at o C into water at 0.00 o C? Water has a molar mass of 18.0 g/mol 6.01 kJ mol x 1 mol H 2 O 18.0 g =0.334 kJ/g (or 334 J/g)

3. How much energy will it take to change 15.0 g of ice at o C into water at 30.0 o C? You need to find 3 answers and add them together: a)Energy to heat the ice from o C to 0.0 o C b)Energy to melt the ice c)Energy to heat the water from 0.0 o C to 30.0 o C heat melt heat q=cmΔt q=mΔH q=(2.01J/g o C)(15.0g)(20.0 o C)=603 J q=(15.0g)(334 J/g)=5010J q=(4.184J/g o C)(15.0g)(30.0 o C)=1880J Total = 7490 J

4. How much energy will it take to change 25.0 g of water at 50.0 o C into steam at o C? a)Energy to heat the water from 50.0 o C to o C b)Energy to vaporize the water c)Energy to heat the steam from o C to o C Heat steam q=(2.01J/g o C)(25.0g)(35.0 o C)= 1760 J Vaporize water q=(25.0g)(2260 J/g)= J Heat water q=(4.184J/g o C)(25.0g)(50.0 o C)= 5230 J Total = J

A.A B.B C.C D.D Section 15-3 Section 15.3 Assessment The amount of energy required to melt one mole of a solid is called ____. A.molar enthalpy of vaporization B.molar enthalpy of melting C.molar enthalpy of fusion D.molar enthalpy of condensation

A.A B.B C.C D.D Section 15-3 Section 15.3 Assessment A thermochemical equation specifies about changes in ____. A.temperature B.pressure C.enthalpy D.molar mass