Fuels
Define a biofuel and provide two examples.
A biofuel is a fuel derived from biological sources (recently living organisms). Examples: bioethanol and biodiesel.
What does a negative ΔH indicate about a chemical reaction?
It indicates the reaction is exothermic—heat is released.
What is the equation used to calculate heat energy in calorimetry?
q = mcΔT
Define oxidation and reduction in terms of electron transfer.
Oxidation: loss of electrons. Reduction: gain of electrons.
What is the main function of the salt bridge in a galvanic cell?
To maintain electrical neutrality by allowing ion flow between half-cells.
Compare the renewability and CO₂ emissions of coal and biogas.
Coal is non-renewable and releases significant net CO₂. Biogas is renewable and considered carbon-neutral as the CO₂ released was recently absorbed by plants.
Sketch a basic energy profile diagram for an exothermic reaction. Label ΔH and Ea.
Diagram shows reactants higher than products; ΔH is downward; Ea is the peak above reactants.
Calculate the energy released when 100 g of water is heated from 25°C to 40°C. (c = 4.18 J/g°C)
q = 100 × 4.18 × 15 = 6270 J
Identify the oxidising and reducing agents in the reaction: Zn + Cu²⁺ → Zn²⁺ + Cu.
Oxidising agent: Cu²⁺
Reducing agent: Zn.
In a galvanic cell, where does oxidation occur?
At the anode.
Write the balanced chemical equation for the complete combustion of propane (C₃H₈).
C₃H₈ + 5O₂ → 3CO₂ + 4H₂O
Describe the role of activation energy in a chemical reaction.
Activation energy is the minimum energy required for reactants to form products by overcoming the energy barrier.
In a calorimetry experiment, 0.500 g of ethanol is burned to heat 200.0 g of water by 10.0°C. Calculate the energy released.
q = 200 × 4.18 × 10 = 8360 J
Balance the redox reaction in acidic solution: Fe²⁺ + MnO₄⁻ → Fe³⁺ + Mn²⁺
5Fe²⁺ + MnO₄⁻ + 8H⁺ → 5Fe³⁺ + Mn²⁺ + 4H₂O
Describe one similarity and one difference between primary and fuel cells.
Similarity: both produce electrical energy via redox reactions. Difference: primary cells are single-use; fuel cells require continuous fuel input.
Explain one advantage and one disadvantage of using biodiesel instead of petrodiesel.
Advantage: Biodiesel is biodegradable and produces fewer particulate emissions. Disadvantage: It may absorb water, affecting engine performance.
Explain how a catalyst affects an energy profile diagram.
A catalyst lowers the activation energy, making the reaction pathway shorter but doesn’t affect ΔH.
Determine the energy per gram of ethanol in the previous question.
8360 J / 0.500 g = 16,720 J/g
Using the electrochemical series, determine whether a spontaneous reaction will occur between Al(s) and Cu²⁺(aq).
Yes. Al is higher on the electrochemical series and will donate electrons to Cu²⁺, forming Al³⁺ and Cu(s).
Compare a hydrogen-oxygen fuel cell and a lithium-ion battery in terms of recharging and energy source.
Hydrogen fuel cell cannot be recharged—fuel must be replaced. Lithium-ion is rechargeable and stores energy chemically.
Compare the energy contents and environmental impacts of natural gas, coal, and ethanol.
Natural gas has a high energy content and cleaner combustion than coal. Coal has high CO₂ and particulate emissions. Ethanol has lower energy content but is renewable and carbon-neutral in theory.
Compare the energy profiles of combustion of methane and ethanol, assuming complete combustion. Which releases more energy per mole and why?
Ethanol releases more energy per mole due to more bonds breaking/forming. Energy profile would show deeper ΔH for ethanol.
Discuss two sources of error in a simple calorimetry experiment and how they affect the result.
Heat loss to surroundings: causes energy calculated to be lower than actual.
Incomplete combustion: less heat released than expected, lowering calculated energy per gram.
Explain how redox reactions underpin the operation of a galvanic cell.
In a galvanic cell, a spontaneous redox reaction occurs where electrons flow from the anode (oxidation) to the cathode (reduction), generating electrical energy.
Design a galvanic cell using Zn and Ag⁺. Include half-equations, identify anode/cathode, direction of electron flow, and overall equation.
Anode (oxidation): Zn(s) → Zn²⁺ + 2e⁻
Cathode (reduction): Ag⁺ + e⁻ → Ag(s)
Electron flow: Zn → Ag
Overall: Zn(s) + 2Ag⁺ → Zn²⁺ + 2Ag(s)