Energy changes C5

Fuel Cells definition

• Fuel Cells: are a type of electrochemical cell. • Converts energy from chemical energy into electrical energy (electricity) used to power things.

Hydrogen-Oxygen Fuel Cell components

• Uses two components: Hydrogen and Oxygen to produce water (H₂O). • It works through an exchange of electrons.

Fuel Cell Structure basics

• Contains two electrodes: a positive anode and a negative cathode. • Note: Charges are opposite to electrolysis!

Fuel Cell Electrode materials

• Made with thin platinum foils. • Contains porous carbon electrodes matching the outside catalyst to speed up reactions.

Fuel Cell Electrolyte definition

• A solution that ions can move through. • Can be Potassium Hydroxide (KOH) for an alkaline electrolyte, or Phosphoric acid for an acidic electrolyte.

Fuel Cell Mechanism: Step 1

• As fuel enters the cell, hydrogen molecules lose electrons. • Equation: 2H₂ → 4H⁺ + 4e⁻.

Fuel Cell Mechanism: Step 2

• This process happens as the hydrogen ion's electrons are passed to the positive anode. • This sets up a potential difference across the cell due to a difference in charge.

Fuel Cell Mechanism: Step 3

• The potential difference drives the electrons around the circuit, generating electricity. • The electrons then reach the negative cathode and are given to the oxygen ion.

Fuel Cell Mechanism: Step 4

• Electrons and hydrogen combine inside oxygen (O₂) that enters in the other side to produce pure water gas. • It is vital for 4 molecules of hydrogen to react with 1 molecule of oxygen (1:4 ratio).

Hydrogen-Oxygen Fuel Cell: Anode half equation

• Hydrogen is oxidised at the positive anode. Half equation: 2H₂ → 4H⁺ + 4e⁻.

Hydrogen-Oxygen Fuel Cell: Cathode half equation

• Oxygen is reduced at the negative cathode. Half equation: O₂ + 4H⁺ + 4e⁻ → 2H₂O.

Hydrogen-Oxygen Fuel Cell: Overall equation

• The overall balanced chemical equation is: 2H₂ + O₂ → 2H₂O.

Hydrogen-Oxygen Fuel Cell replacement uses

• Could be used to replace fossil fuel engines and batteries. • Main idea: Replace fossil fuel engines in cars to reduce pollution.

Hydrogen-Oxygen Fuel Cell: Pros

• Only requires hydrogen and oxygen which are abundant. • Don't produce any carbon dioxide or pollutants as waste (only produce water). • Simple devices that last longer than batteries. • Less polluting to dispose of than toxic battery chemicals.

Hydrogen-Oxygen Fuel Cell: Cons

• Hydrogen is a gas, so it takes more space to store than fossil fuels or batteries. • Explosive when stored, which is dangerous. • Making hydrogen fuel requires energy from fossil fuels.

Exothermic Reaction definition

• A chemical reaction that transfers energy to the surroundings, usually leading to a rise in the temperature of the surroundings. • Examples: Combustion, neutralisation, and many oxidation reactions.

Endothermic Reaction definition

• A chemical reaction that takes in energy from the surroundings, usually leading to a decrease in the temperature of the surroundings. • Examples: Thermal decomposition, citric acid and sodium hydrogencarbonate reaction.

Activation Energy definition

• The minimum amount of energy that particles must have when they collide in order to react successfully.

Exothermic Reaction profile features

• The energy level of the products is lower than the energy level of the reactants. • The overall energy change is negative, showing that energy has been released to the surroundings.

Endothermic Reaction profile features

• The energy level of the products is higher than the energy level of the reactants. • The overall energy change is positive, showing that energy has been absorbed from the surroundings.

Bond Breaking energy change

• Energy must be supplied and taken in to break existing bonds in the reactants. • This is entirely an endothermic process.

Bond Making energy change

• Energy is released and given out when new bonds are formed to create the products. • This is entirely an exothermic process.

Calculating Energy Changes rule

• Overall Energy Change = Total energy needed to break bonds − Total energy released when bonds are made. • A negative final answer means the reaction is exothermic; a positive answer means it is endothermic.

Chemical Cells basics

• A simple chemical cell contains two different metals dipped in an electrolyte solution. • The chemical reactions between the metals and the electrolyte set up a charge difference, producing a potential difference.

Factors affecting Cell Voltage

• The type of electrodes used: a bigger difference in reactivity between the two metals produces a larger potential difference. • The type and concentration of the electrolyte solution used.

Rechargeable vs Non-Rechargeable cells

• Non-rechargeable cells: The chemical reactions are irreversible. Once one of the reactant chemicals is completely used up, the cell stops producing a potential difference. • Rechargeable cells: The chemical reactions can be reversed by connecting the cell to an external electrical current, resetting the reactants.