R1.3 Energy from Fuels

R1.3 Energy from Fuels

​Combustion

​To write a combustion reaction, the reactants are a fuel and diatomic oxygen (O₂) and the products are CO₂ and H₂O.

• ​The IB expects you to be able to write combustion equations for hydrocarbons (alkanes, alkenes, alkynes) and alcohols. 

• Combustion reactions are always exothermic.

​Complete vs Incomplete Combustion

​Incomplete combustion occurs when there is insufficient oxygen to convert all of the fuel to carbon dioxide and water. 

• When we see smoke, that is evidence of incomplete combustion.

• The smoke is black because of soot, which is pure carbon.

​Experiment: Start a candle and use tongs to hold a metal plate near the flame. When it is near and blocking some oxygen flow, black soot is produced.

​Writing Incomplete Combustion Equations

​Instead of carbon dioxide being produced, you produce carbon (soot) or carbon monoxide. Usually either is accepted, or both - but read the question to see if they are leading you to write a specific one.

​Consequences of Incomplete Combustion

• ​Less heat released: ΔH of complete combustion > ΔH of incomplete combustion

• Harmful by-products enter the body, including carbon monoxide and carbon (soot) particulates.

​Video Link

​Fossil Fuels

​There are 3 main examples of fossil fuels to know:

​Comparing CO₂ Emissions Per Unit Energy

​Different fuels have different carbon emissions (carbon footprint) per unit of energy produced.

• To calculate this, we can use our data booklet and this equation.

• We often multiply by 1000 to convert kJ to J so that the numbers are larger, but this is optional.

Natural Gas < Crude Oil < Coal

​Comparing CO₂ Emissions Per Unit Mass

​Different fuels have different carbon emissions (carbon footprint) per unit of mass burned.

STEP 1: Choose a mass (ie, 1 g or 1 kg is common) and convert each fuel to moles using n = m/M

STEP 2: Write out a balanced chemical equation.

STEP 3: Multiply by the mole ratio to get moles of CO2 produced & compare results. 

STEP 4: If they ask for it in grams, volume, etc use mole map to convert accordingly.

STEP 5: Divide by the mass of fuel to get correct units.

​You Try: Compare the CO₂ emissions of burning 1.000 g masses of coal (C), and octane (from crude oil) and methane (from natural gas)

​Greenhouse Effect

​The greenhouse effect causes the average temperature of Earth to increase. As it is an average, climates in different areas can change differently.

• ​Greenhouse gases like CO₂ allow ultraviolet radiation from the sun to pass through to the ground.

• The ground absorbs the UV and re-emits some of it as longer wave infrared radiation.

• Some of the infrared radiation is absorbed by greenhouse gases.

• The greenhouse gases re-emit some of the infrared radiation back to Earth, warming it.

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​Connections

Recall that we can also calculate heat released by fuels in kJ using Q = nΔHc 

You Try: Using the data booklet calculate the heat released by burning 2.15 mol of CH₄.

​Connecting to Incomplete Combustion

​Incomplete combustion releases less energy.
Overall natural gas is the least likely fossil fuel to undergo incomplete combustion compared to coal/crude oil

​Factor 1: Impurities - Coal has more incomplete combustion due to sulfur impurities. The sulfur reacts with oxygen, leaving less oxygen for the carbon in coal to react with.

​Connecting to Incomplete Combustion

Factor 2: Length of Carbon Chain - Longer hydrocarbon chains (like in crude oil) require more oxygen and are therefore more likely to undergo incomplete combustion than shorter hydrocarbon chains (like in natural gas).

​Non-Renewable vs Renewable Energy

​Non-renewable energies are not replenished over short time spans. Renewable energies can be replenished over shorter timespans.

Biofuels like ethanol are renewable because they are made from biomass.

• Biomass is produced via photosynthesis, which removes CO₂ from the atmosphere.

• When we burn biofuels, we emit the CO₂ back that was recently removed by photosynthesis.

• The CO₂ from burning the biofuel can then be reabsorbed in another plant via photosynthesis.

• This cycle means theoretically there can no net change in CO₂ concentration by burning biofuels, meaning they do not contribute to climate change.

​Biofuels vs Fossil Fuels

• Fossil fuels instead take carbon that has been removed from the atmosphere for hundreds of millions of years and re-emits it back.

• This causes a net increase in CO₂ concentration in the atmosphere as there is no fast process to take it out of the atmosphere.

​Biofuels vs Fossil Fuels

​Pros and Cons of Biofuels

​From previous exam papers, these are what they are looking for: