Understanding Combustion

What is Combustion?

• Combustion requires a fuel to ignite.
• It involves the rapid reaction between a substance with an oxidant, usually oxygen, to produce heat and light.
• A fuel's flash point is the lowest temperature at which enough vapor is present to ignite.
• Petrol vaporizes easily at room temperature and ignites with a match.
• A spark plug provides the energy needed to start this reaction in a car engine.
• Paraffin (kerosene) requires higher temperatures or a spirit burner and wick.

Burning Solid Fuels

• Burning solid fuels requires two changes of state.
• Long chain hydrocarbons like candle wax (C30H62) melt.
• The molten wax moves up the wick, where it vaporizes.
• Combustion involves wax vapor around the wick reacting to form carbon dioxide and steam.

Complete and Incomplete Combustion

• Complete combustion occurs when fuels, mainly containing carbon and hydrogen, form carbon dioxide and water.
  • An example of complete combustion is: CH₄ (g) + 2O₂ (g) → CO₂ (g) + 2H₂O (l).
• Incomplete combustion happens when a limiting reactant is present, such as oxygen.
• Carbon atoms may be released as black soot particles when oxygen is limited which can be seen in 'smoking' candles, and on glassware heated by a Bunsen burner's 'safety' flame
• Incomplete combustion can produce carbon monoxide.
  • An example involves methane and oxygen: 2CH₄ (g) + 3O₂ (g) → 2CO (g) + 4H₂O (l).
• Carbon monoxide (CO) is a dangerous alternative product.
• Carbon monoxide is odorless and colorless.
• Carbon monoxide binds with haemoglobin in red blood cells, preventing oxygen transport.

Fire Safety and Combustion Products

• Fire safety equipment should be identified.
• Various metals will burn to produce metal oxides.
  • An example is: 2Fe (s) + 1½ O₂ (g) → Fe₂O₃ (s)
• A systems diagram of a candle involves the fuel, oxygen, and heat.
• The fire triangle consists of oxygen, fuel, and heat.

Chemical Reactions and Energy

• Chemical reactions can be exothermic or endothermic.
• Exothermic reactions release energy.
• Endothermic reactions can be represented with copper(II) sulfate (aq), magnesium, citric acid, and NaHCO3 (aq).

Activation Energy

• Activation energy is the energy needed to start a reaction.
• Energy profile diagrams can show the energy as an 'energy barrier'.
• Energy level diagrams illustrate the enthalpy of combustion, using methane as an example.
• ΔH represents the enthalpy change.

Particle Diagrams for Understanding Combustion

• Particle diagrams help explain how substances combust.
• Particle models illustrate solids, liquids, and gases.
• Particle models explain how a solid conducts heat and how fluids convect heat.

What Makes a Suitable Fuel?

• Key properties include: how much smoke it produces, ease of ignition, emission of pollutants, amount of ash left behind, the cost, thermal energy released per unit, steady burning, safety, and ease of storage.
• People may change their fuel choices based on various factors.
• 4 billion people lack access to modern energy cooking services.
• Clean cooking initiatives are improving access to better resources in places such as Rwanda, Africa.

Fair Access to Clean Energy

• It is important to suggest an alternative to burning fuel.
• One such alternative is the hydrogen fuel cell.
• Hydrogen fuel cells can convert water into hydrogen and oxygen by using energy from a renewable source such as solar cells.
• Once the hydrogen and oxygen recombine to form water, the energy produced is harnesses as electrical energy.
• Electrical energy is used to split water into hydrogen and oxygen.
  • A word equation to show this is: Water → Hydrogen + Oxygen
  • A balanced equation to show this is: 2H₂O (I) → 2H₂(g) + O2(g)
• The hydrogen gas needs to be stored to be used.
• Step 2 involves hydrogen and oxygen combining to make water.
  • A word equation is: Hydrogen + Oxygen → Water
  • A balanced equation is: 2H2(g) + O2(g) → 2H₂O (1)
• The hydrogen comes from a 'fuel tank' and the oxygen from the air.
• Hydrogen atoms enter the fuel cell at the anode, while oxygen is fed to the cathode.
• The fuel cell contains a membrane coated with a catalyst.
• The hydrogen splits into hydrogen ions and electrons when hitting the catalyst.
• Positively charged hydrogen ions pass through the membrane; negatively charged electrons flow through an electric circuit to generate current.
• Hydrogen ions, electrons, and oxygen then combine to produce heat and water at the cathode side.
• Complete fuel cell systems have several hundred membranes stacked together.

Voltaic Cells vs. Fuel Cells

• A voltaic cell splits lithium metal into lithium cations and electrons at the anode.

  • This can be shown as: Li → Li+ + e-

• Lithium cations pass through the membrane towards the cathode via the external circuit.

• At the cathode lithium ions regain electrons as they react with cobalt oxide to form lithium cobalt oxide.

  • The formula: Lit + e + CoO2→ LiCoO2

• Voltaic cells cannot make more electrons once all the Li has turned into Lit (and LiCoO2).

• The cell must be recharged to reverse the reaction.

• A fuel cell splits hydrogen gas into hydrogen ions and electrons at the anode.

  • The formula: H₂→ 2H+ + 2e-

• Hydrogen cations pass through the membrane.

• Electrons pass through the external circuit.

• Electrons from the air gain electrons to make oxide ions at the cathode.

  • The formula: O2 + 4e- 2O2-

• Oxide ions react with hydrogen ions to make water.

  • The formula: 2H+ + O2-→ H₂O

Life Cycle Assessment of Fuels

• A life cycle assessment involves: raw material extraction, manufacturing, transportation, use, and end-of-life management.
• Raw material is extracted through oil fields: Fig. 10.3.4.1 ("List of oil fields") and oil rigs: Fig. 10.3.4.2 (“The 5 Different Types of Oil Rigs").
• Transportation includes tankers and trucks.
• Manufacturing is done in oil refineries: Fig. 10.3.4.4 (Five Ways US Oil Refineries Can Reduce Emissions Today).
• Use occurs in vehicles.
• End-of-life can involve vehicle piles or oil spills into the sea.

Energy From Bonds

• Calculate the molar mass of a compound (M) using the data from the periodic table.
• Determining the moles of a substance from mass uses the formula: n = m/M, where n = number of moles (mol), m = mass (g), and M = formula mass.
• Ionic bonding is the electrostatic attraction between oppositely charged ions.
• Covalent bonding is the attraction of atoms to shared pairs of electrons.
• Finding a reaction's enthalpy change uses bond enthalpy data and the equation: ΔΗ = ΣΗ bonds broken – ΣΗ bonds formed
• Bonds break taking in energy and can be shown as endothermic.
• New bonds give out energy and can be shown as exothermic.
• Some common chemical bonds and their bond energy in kJ mol-1 are:
  • C-H: 414
  • O=O: 498
  • C=O: 804
  • H-O: 463
  • C-C: 346
  • C-O: 358
  • H-H: 436
  • I-I: 151
  • H-I: 298

Enthalpy Change

• It's important to calculate the amount of thermal energy involved in a reaction.
  • One equation is: Q = mcΔT
• To find the enthalpy change of reaction, use the equation: ∆H = -Q/n
• Formulas used to calculate heat energy change involves enthalpy (H):
  • q=mcΔT where:
    • q is the heat energy change in Joules (J)
    • m is the mass of the substance heated in grams
    • c is the specific heat capacity of the substance that is heated
      • for water the valve is 4.18 Jg" c-"
    • ΔT is the temperature change of heated substance in °C
    • shows if the reaction is exothermic (-ve) or endothermic (+ve)
    • number of moles of substance that causes the enthalpy change (mol)
    • The reaction in kJ, remembering to give it the correct sign.
    • The number of moles using: Use a copy of 10.3 Enthalpy change calculator to find the number.
    • Change in kJ: ΔΗ for the reaction in kJ mol¹, remembering to use the correct sign for q.
    • One needs to draw an energy level diagram, including the scale.
      • A thermochemical equation needs to be created including ΔΗ
      • A simple calorimeter needs to be made, including suggestions.
    • The validity of the method should be evaluated and suggestions to reduce error.

Energy from Electron Transfer

• Metals should be placed in order of increasing reactivity.
• Oxidation is defined as the loss of electrons.
• Reduction is defined as the gain of electrons.
• Oxidation reactions can be shown as exothermic with heat.
• The activity of a metal is how readily it is oxidised.
• An example includes displacement of copper(II) ions.
  • A chemical equation is: zinc + copper sulfate → zinc sulfate + copper
  • A balanced equation is: Zn (s) + CuSO₄ (aq) → ZnSO₄ (aq) + Cu (s)
  • An ionic equation is: Zn (s) + Cu²⁺ (aq) → Zn²⁺ (aq) + Cu (s)
  • A half equation to show this is: Zn (s) → Zn²⁺ (aq) + 2e⁻(oxidation)
  • A half equation to show this is: Cu²⁺ (aq) + 2e⁻ → Cu (s) (reduction)
• Transfer reactions happen in batteries like the Daniell Cell - A Voltaic Cell
• The electrons transfer from one half of the cell to the other through electrical wires.
• Voltage show the potential difference between the two half-cells.