Energy Changes Flashcards
Exothermic Reaction
Energy is released into the surroundings, bonds are formed
Endothermic Reaction
Energy is taken in from the surroundings, bonds are broken
Activation Energy
The minimum amount of energy needed for a reaction to take place
Bond Energies
The amount of energy required to break one mole of a particular covalent bond
Bond Forming
Exothermic or Endothermic?
Why?
Bond forming is exothermic as it releases energy to the surroundings
Bond Breaking
Exdothermic or Endothermic?
Why?
Bond breaking is endothermic as it requires energy from the surroundings
In bond energies calculations, exothermic reactions give what?
A negative number
In bond energies calculations, endothermic reactions give what?
A positive number
Electrolyte
A substance that can conduct electric current when molten or in solution
Electrolysis
The decomposition of a compound using an electric current
How do cells produce energy?
Through chemical reactions
Battery
2 or more cells connected in series to produce a greater voltage
What factors affect voltage in a cell/battery?
- Metals used for electrodes - the greater the difference in reactivity of the 2 metals, the greater the voltage
- The concentration of the electrolyte
- Conditions, e.g. temperature
Rechargeable Cell/Battery
A cell/ battery where the reactions within it are reversible when an external current is applied
Non Rechargeable Cell/Battery
A cell/battery where voltage is produced until one of the reactants is used up
In a fuel cell, is the cathode positive or negative?
Positive
In a fuel cell, is the anode positive or negative?
Negative
How does a hydrogen fuel cell work
- Hydrogen enters the cell, is attracted to the negative electrode and is oxidised (loses electrons) to become e- and H+ ions
- e- ions travel through a wire to the cathode, H+ ions travel through electrolyte
- At the cathode, 4e- ions and 4h+ ions react with O2 to create water
- Water leaves fuel cell
Negative Electrode Fuel Cell Half Equation
H2 –> 2e- + H+
Positive Electrode Fuel Cell Equation
O2 + 4e- + 4H+ –> 2H2O
Overall Fuel Cell Equation
O2 + 2H2 –> 2H2O
How do hydrogen fuel cells generate electricity?
- Fuel (hydrogen) enters cell
- Hydrogen is oxidised (loses electrons)
- This sets up potential difference across the cell that drives the electrons around it generating electrical energy
Pros of Hydrogen Fuel Cells
- Easy to maintain as there are no moving parts
- Small size
- Water is the only chemical product
- Last longer than batteries
- Reactants are abundant
Cons of Hydrogen Fuel Cells
- Very expensive to manufacture
- Need a constant supply of hydrogen fuel, which is a flammable gas
- Hydrogen takes up storage space
Do exothermic reactions create temperature increase or decrease?
Temperature increase
Do endothermic reactions create temperature increase or decrease?
Temperature Decrease
