Energy changes C5

Fuel Cells definition

• Type of electrochemical cell that converts chemical energy into electrical energy (electricity) used to power things e.g hydrogen- oxygen fuel cell.

Whilst electrolysis is the opposite as it converts electrical energy into chemical energy.

Hydrogen-Oxygen Fuel Cell components

• Uses two components: Hydrogen and Oxygen to produce water (H₂O), it works through an exchange of electrons through a wire and the ions move through the electrolyte e.g Phosphoric acid.

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

The electrodes are made of porous carbon with tiny holes that contain a catalyst to speed up the reaction.

Hydrogen-oxygen Fuel Cell Mechanism: Step 1

• Hydrogen will enter from the inlet, with each hydrogen atom loses an electron at the negative anode through oxidation to become an hydrogen ion

• Half equation: 2H₂ → 4H⁺ + 4e⁻ (or can simplify but this matches the oxygen half equation)

Hydrogen- oxygen Fuel Cell Mechanism: Step 2

• The electrons are then passed along the wire to the cathode, while the hydrogen ions move through the electrolyte to the cathode.

• This sets up a potential difference across the cell due to a difference in charge between electrodes that drive electrons around circuit to generate electricity.

Hydrogen-oxygen Fuel Cell Mechanism: Step 3

• When the electrons and hydrogen ions reach the positive cathode they can react with the oxygen atoms that comes in from the inlet to form water through reduction, leaving the fuel cells. Two molecules of hydrogen (4 hydrogen atoms) react with one molecule of oxygen (2 oxygen atoms).

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: Pros

• No greenhouse gases produced as only water is made as waste, Simple devices that last longer without losing charging capacity than batteries, Faster to refuel with gas than waiting hours for a battery to charge.

Hydrogen-Oxygen Fuel Cell: Cons

• Highly explosive and flammable gas which makes it dangerous to store, Producing the hydrogen gas relies on electricity from burning fossil fuels, Very few hydrogen refueling stations currently exist compared to battery charging points.

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 and Overall energy change for Reaction Profile

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

On reaction profile activation energy is between the reactant energy level to the highest point on the curve with the symbol Ea (arrow pointing upwards always).

On reaction profile overall energy change is between the reactant energy level and the product energy level with the symbol + or - Δ𝐻 (arrow pointing from reactants to products- up for endo, down for exo)

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 (acronym BENDO)

• Energy must be supplied and taken in to break existing bonds in the reactants.

• This is an endothermic process.

Bond Making energy change (acronym MEXO)

• Energy is released and given out when new bonds are formed to create the products.

• This is an exothermic process.

Overall energy change equation

• Overall Energy Change (kJ/mol) = Total energy needed to break bonds − Total energy released when bonds are made.

Total energy needed is reactants, whilst total energy released is products.

• 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 (which are the 2 electrodes) dipped in an electrolyte solution.

• The chemical reactions between the different metals and the electrolyte set up a charge difference, producing a potential difference (voltage). If the metals were the same, they react with the electrolyte identically so no charge difference and 0V.

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 (voltage), so must be replaced (e.g alkaline batteries).

• Rechargeable cells: The chemical reactions can be reversed by connecting the cell to an external electrical current, which turns products back into reactants, the cells produce a voltage again.