Topic 5 energy changes Flashcards
What are the two types of energy changes in a chemical reaction
Exothermic and Endothermic reactions
What is an exothermic reaction
A reaction that releases energy (EXO)
What is an Endothermic reaction
A reaction that absorbs energy
Give an example of an Exothermic reaction
Combustion burning fuels releases heats
Give an example of an Endothermic reaction
Photosynthesis as plants absorb sunlight to make glucose
How does temp change during an exothermic reaction
The surrounding temp increases
How does temp change during an endothermic reaction
The surrounding temp decreases
What happens to bonds during an Exothermic reaction
New bonds are formed as energy released by forming bonds is greater than breaking bonds
What happens to bonds during an Endothermic reaction
Bonds are broken as energy is absorbed to break bonds
What is activation energy
The minimum energy required to start a reaction
How does a catalyst affect activation energy
It lowers activation energy making the reaction happen faster
What is the equation for bond energy calculations
Energy change= Energy required to breaking bond - energy released forming bonds
What is the difference between non-rechargeable and rechargeable batteries?
Non-rechargeable alkaline
The reaction is irreversible; once reactants are used, the battery stops working.
Rechargeable lithium-ion
The reaction can be reversed using an external electrical current.
What is a fuel cell?
A device that generates electricity from a fuel (e.g., hydrogen) and oxygen without combustion, producing only water.
What is the overall equation for a hydrogen fuel cell?
2H₂ + O₂ → 2H₂O (exothermic reaction)
What happens at the anode in a hydrogen fuel cell?
Hydrogen is oxidised:
H₂ → 2H⁺ + 2e⁻
What happens at the cathode in a hydrogen fuel cell?
Oxygen is reduced and reacts with hydrogen ions to form water:
O₂ + 4H⁺ + 4e⁻ → 2H₂O
What are the advantages of hydrogen fuel cells?
No CO₂ emissions (only water produced).
Lightweight and efficient.
No need for recharging like batteries.
What are the disadvantages of hydrogen fuel cells?
Hydrogen storage is difficult (flammable & needs high pressure).
Hydrogen production often involves fossil fuels.
Expensive technology.
