C5 - Energy Changes Flashcards
What is an exothermic reaction?
- Release energy to surroundings
- Indicated by a rise in temp
What are some examples of exothermic reactions?
- Combustion
- Neutralisation
- Most oxidation reactions
What is an endothermic reaction?
- Absorbs heat energy from surroundings
- Indicated by a fall in temp
What is activation energy?
- Minimum amount of energy
- particles require to collide with each other
- and react
If reactant molecules in a reaction have more energy in their chemical bonds than the product molecules, will energy be absorbed or released overall?
Released
What three things happen during a chemical reaction?
- Bonds of reactant molecules are broken
- Atoms rearrange themselves
- Bonds of product molecules are formed
What do bond energies refer to?
Amount of energy required to break one mole of a particular covalent bond.
What kind of process is bond-breaking?
Endothermic - requires energy
What kind of process is bond-making?
Exothermic - releases energy
How do you calculate the overall energy change of a reaction?
Energy required to break the bonds of the reactants
-
Energy released by forming bonds of the product.
What is a cell?
Use chemical reactions to produce electricity
How is a cell made?
- Connecting two different electrodes with a wire
- Placing them in contact with an electrolyte.
What are the factors that affect the voltage of a cell?
- Greater the difference in reactivity of the two electrode metals, greater the voltage will be.
- Type + concentration of the electrolyte used.
- Conditions like temperature
What are rechargeable cells?
- Can be recharged because the chemical reactions are reversed when external electrical current is supplied.
- Batteries used in laptops and mobile phones
What are non-rechargeable cells?
- Chemical reactions stop when one of the reactants has been used up.
- Alkaline batteries are non-rechargeable.
- Usually used in smoke alarms and TV remotes.
Fuel cells are electrochemical cells. What does this mean?
Converts energy between electrical and chemical forms
What does a hydrogen-oxygen fuel cell do?
Uses hydrogen and oxygen to form water and generates a lot of electrical energy.
Describe the structure of a fuel cell:
- Centre: electrolyte (potassium hydroxide)
- On either side of centre: electrodes
- Positive cathode (right) + negative anode (left)
- Wire connects electrodes
- Anode compartment (left): hydrogen enters inlet
- Cathode compartment (right): oxygen enters inlet + water and heat leave outlet
In fuel cells what are the electrodes made of ?
- Porous carbon (tiny holes)
- Catalysts to speed up reaction
How does a fuel cell actually work?
- Hydrogen enters anode compartment through inlet
- Hydrogen oxidised by anode
- Hydrogen loses electrons
- Electrons move up through wire to cathode whilst H+ move through electrolyte to cathode
- Electrons and H+ react with oxygen which enters cathode compartment through inlet
- All combine to make water
- Water leaves cell
What are the half equations for the reaction which occurs in fuel cells?
H2 –> 2H+ + 2e-
O2 + 4H+ + 4e –> 2H2O
Key points to remember about fuel cells for exams?
- As fuel (hydrogen) enters the cell
- It becomes oxidised (loses electrons)
- Sets up a potential difference across the cell (diff in charge of electrodes)
- Drives electrons around circuit
- Generates electricity
What are the advantages of using hydrogen-oxygen fuel cells?
- Only require hydrogen and oxygen
- Don’t produce waste
- Don’t produce carbon dioxide or other pollutants
- Last longer than batteries
- Less polluting to dispose of
What are the disadvantages of using hydrogen-oxygen fuel cells?
- Hydrogen takes up too much space to store
- Hydrogen is explosive when mixed with air
- Making hydrogen requires energy
