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Chemistry 30 Review Sessions

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(1)

THERMOCHEMISTRY

Chemistry 30 Review

Sessions

(2)

Curriculum bullets

Use Q = mcΔt to analyze heat transfer.

Use calorimetry data to determine the enthalpy changes in chemical reactions.

(3)

A Bomb Calorimeter

Enthalpy Change in Bomb shows up as Temperature Change in Calorimeter

Calorimeters rely on the principle of

energy conservation:

Heat Gained = Heat Lost

Qcal = ΔrH

mcΔt = nΔrHm

(4)

Typical Heat Lost=Heat Gained Problem

Wikipedia gives the molar enthalpy of

combustion for methane as –890 kJ/mol.

What minimum mass of methane must be burned to warm 4.00 L of water from 22.4ºC to 87.6ºC, assuming no heat

losses?

(5)

Heat Lost = Heat Gained

Water increased in temperature – heat gained – Q Burning Methane (CH4) – enthalpy change - ΔH

Q = ΔH

mcΔt= nΔrHm n = mcΔt/ ΔrHm

n = (4.00 kg)(4.19kJ/kg°C)(87.6-22.4°C)/(890kJ/mol) n = 1.2278 mol

m = nM

m = (1.2278 mol)(12.01 +4.04 g/mol) = 19.7 g

(6)

Curriculum Bullet

design a method to compare the molar

enthalpy change when burning two or more fuels

evaluate the economic and environmental impacts of different fuels by relating carbon dioxide emissions and the heat content of a fuel

provide examples of personal reliance on the chemical potential energy of matter, such as the use of fossil fuels

(7)

Comparing Fuels

Compare propane with methane in terms of

a.energy per gram of fuel

b.energy per mole of CO2(g) produced

Molar Enthalpies of Combustion

propane ΔcHm= -2220 kJ/mol methane ΔcHm=  -890.4 kJ/mol

(8)

Energy per mass of Fuel

Calculation: molar enthalpy /molar mass

= ΔcHm /M

Propane: -2220 kJ/mol/44.11g/mol = - 50.33 kJ/g

Methane: -890.4 kJ/mol/16.05g/mol = -55.47 kJ/g

Methane is a little better, i.e. more energy/gram

(9)

Comparing Fuels: Energy/CO2 produced

CO2(g) contributes to global warming.

Write the balanced equation for 1 mole of fuel C3H8(g) + 5 O2(g) 3 CO2(g) + 4 H2O(g) CH4(g) + 2 O2(g) CO2(g) + 2 H2O(g)

Calculate : ΔcHm /n CO2

propane ΔcHm /n CO2 = - 2220 kJ/mol/3 = -740 kJ/mol methane ΔcHm /n CO2 = -890.4 kJ/mol/1 = -890.4 kJ/mol Methane gets you more energy per CO2 - a good

thing!

(10)

Curriculum Bullet

Explain, in a general way, how stored energy in the chemical bonds of hydrocarbons

originated from the sun.

but Hydrocarbons are decomposed living things!

and all living things rely upon photosynthesis…

which just converts light to chemical energy and

Hydrocarbon combustion releases energy, so HCs are storing energy,

light comes from the sun

so… light from the sun provides the

energy which builds organic molecules

(11)

Curriculum bullet

Define enthalpy and molar enthalpy

Write balanced equations for chemical reactions that include energy changes.

Classify endothermic or exothermic, including : photosynthesis, cellular respiration and hydrocarbon

combustion.

Use and interpret ΔH notation

(12)

One of these ain’t like the other ones, one of these just ain’t the same…

State the molar enthalpy

2 Na(s) + Cl2(g) 2 NaCl(s) ΔfHm=  -411.2 kJ/molNaCl

Include the absolute value of the energy in the balanced equation.

2 Na(s) + Cl2(g) 2 NaCl(s) +

 822.4 kJ

State the enthalpy change at the end of the reaction.

2 Na(s) + Cl2(g) 2 NaCl(s) ΔH=  

- 822.4 kJ

(13)

What’s the energy change here?

The molar enthalpy of formation of sodium chloride solid is -411.2 kJ/mol. This reaction is balanced if __i__ kJ of energy is included at position __ii__ .

2 NaCl(s) + x 2 Na(s) + Cl

2

(s) + y

i ii

a 411.2 x

b 411.2 y

c 822.4 x

d 822.4 y

(14)

Hess’ Law

Predict the enthalpy change for

chemical equations using standard enthalpies of formation.

Explain and use Hess’ law to calculate energy changes for a net reaction

from a series of reactions.

(15)

Solve for Δ

r

H by summation

SO

3

(g) + H

2

O(g) H

2

SO

4

(l) ΔH = ? Calculate the enthalpy of the above reaction using:

H2S(g) + 2O2(g) H2SO4(l) ΔH= -78.5 KJ H2S(g) + 2O2(g) SO3(g) + H2O(l) ΔH= -69 KJ

H2O(l) H2O(g) ΔH= +11 KJ

(16)

Solve for Δ

r

H with formation reactions

George Bush has made corn very expensive because he plans to make automobile fuel out of ethanol – which is made from corn.

1.Write the balanced chemical equation for the combustion of liquid ethanol.

 

 2. Calculate the enthalpy change for the balanced reaction.

C2H5OH(l) + 3 O2(g) 3 H2O(g) + 2 CO2(g)

Δ

r

H = [n Δ

f

H

m

]

prod

– [n Δ

f

H

m

]

reac

(17)

Δ

r

H = [n Δ

f

H

m

]

prod

– [n Δ

f

H

m

]

reac

Δ

r

H = [n Δ

f

H

m

]

prod

– [n Δ

f

H

m

]

reac

Δ

r

H =[ 3 x h20+ 2 x co2 ] – [1 x ethanol]

Δ

r

H =

(18)

Curriculum Bullet

Identify that liquid water and carbon

dioxide gas are reactants in photosynthesis and products of cellular respiration

gaseous water and carbon dioxide gas are

the products of hydrocarbon combustion in an open system.

6 H2O(l) + 6 CO2(g) + 2802.5 kJ 6 O2(g) + C6H12O6(s) CH4(g) + 2 O2(g) 2 H2O(g) + CO2(g) + 890 kJ

(19)

Curriculum Bullet

Define activation energy as the energy barrier that must be overcome for a

chemical reaction to occur.

draw and interpret enthalpy diagrams

for chemical reactions

(20)

Curriculum Bullet

Explain the energy changes that occur during chemical reactions, referring to bonds

breaking and forming and changes in potential and kinetic energy.

Analyze and label energy diagrams of a chemical reaction, including reactants, products, enthalpy change and activation energy.

(21)

Enthalpy Diagrams

Use the enthalpy diagram to determine the molar enthalpy of reaction for: S

8(s) and Cl2 (g) SCl2(g)

SCl4(l)

SCl4(l) SCl2(g) + Cl2(g)

56.1

ΔH (kJ/mol)

19.7

(22)

The Effect of a Catalyst

1 How does the graph show the enthalpy change is not affected by the catalyst?

2. What is the benefit of the catalyst?

3. Where is the

activation energy of the forward reaction.

4. Where is the

activation energy of the reverse reaction?

5. Is the forward reaction endo or exo?

(23)

Curriculum Bullet

Explain that catalysts increase reaction rates by providing alternate pathways for changes, without affecting the net amount of energy involved; e.g., enzymes in living systems.

References

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