# Chemistry 30 Review Sessions

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### 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

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### 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

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### 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

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### 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

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### 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

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### 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!

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### 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,

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### 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

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### 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

i ii

a 411.2 x

b 411.2 y

c 822.4 x

d 822.4 y

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r

3

2

2

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

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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)

r

f

m

prod

f

m

reac

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r

f

m

prod

f

m

reac

r

f

m

prod

f

m

reac

r

r

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### 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

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### 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.

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### 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

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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?

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### 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.

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