Booklet 5 - Exothermic and Endothermic Reactions Flashcards
What are exothermic reactions?
Reactions that transfer energy to the surroundings so the temperature of the surroundings increases
What are endothermic reactioms?
Reactions that takes in energy from the surroundings so the temperature of the surroundings decreases
What other reactions are there of exothermic reactions?
Combustions neutralisations and some oxidations
Where can endothermic reactions be used in everyday life?
Instant ice packs, thermal decompositionz and reaction of citic acid and sodium hydrogencarbonate
What happens in the hand warmers and ice packs?
They are kept apart by a membrane, which when broken allows chemicals to mix, react and produce a warming/cooling effect
What is the advantage of hand warmers/ice packs?
They’re portable + convenient
What is the disadvantage of the hand warmers and ice packs?
Once the reaction has occured it cannot be reversed easily
For particles to react what needs to happen?
They need to collide with each other, but not only that, they must collide with sufficient energy cause a reaction
What the mimimal amount of energy needed for particles to collide with?
Activation energy, EA
What happens if particles do not posses energy needed?
If the dont posses energy equal or greater than, the activation energy cant occur no matter how many times particles collide
In a chemical reaction, what do exisiting bonds in reactants must do?
They must be broken before the new bonds of the products can be formed
What do you have to supply to a chemical reaction?
Supply energy to the bonds if you want to break them and hence start the reaction -> activation energy
What does a reaction profile show?
Can be used to show the relative energies of reactions and products, the activation energy and the overall energy change of a reaction
What happens when bonds in the product are formed?
Energy is released
What can be calculated from bond energies?
The energy needed to break bonds and the energy released when bonds are formed
What is the overall energy change of a reaction?
The difference between the sum of the energy needed to break bonds in the reactants and the sum of energy released when bonds in the products are formed
What is the energy level like in a exothermic reaction?
The energy released from forming new bonds is greater than the energy needed to break existing bonds
What is the energy level like in a endothermic reaction?
The energy needed to break exisiting bonds is greater than the energy released from forming new bonds
What do cells contain?
Contain chemicals which react to produce electricity
What is the voltage produced by a cell dependent on?
The type of electrode and electrolyte
What can a simple cell be made by?
Connecting two different metals in contact with electrolysis
What are batteries?
Consist of two or more cells connected together in series to provide a greater voltage
What happens in non-rechargable cells and batteries?
The chemical reactions stop when one of the reactants has been used up. Eg alkaline batteries
What happens in rechargeable batteries?
They can be recharged because the chemical reactions are reversed when an external electrical current is supplied
What are fuel cells?
They are supplied by an external source of fuel eg hydrogen and oxygen in air. the fuel is oxidised electromagnetically within the fuel cell to produce a potential difference
What does the overall reaction in a hydrogen fuel cell involve?
Involves the oxidation of hydrogen to produce water
What do hydrogen fuel cells do?
Offer a potential difference alternative to rechargeable cells and batteries
What are advantages of hydrogen fuel cells?
- dont need to be electrically recharged
- no pollutant gases are produced
- can be a range of different sizes for different uses
What are dis-advantages of hydrogen fuel cells?
- hydrogen is highly flammable
- hydrogen is produced for the cells by non renewable sources
- hydrogen is difficult to store
What type of reaction is it when energy released from forming new bonds is greater than energy needed to break existing bonds?
Its an exothermic reaction
Why type of reaction is it when energy needed to break exisiting bonds is greater than energy released from forming new bonds?
An endothermic reaction
What is an electrolyte?
A liquid that contains ions which react with the electrodes
- the chemical reaction between the electrodes and electrolyte set up a charge difference between the electrodes
What does it mean the bigger the difference in reactivity?
The bigger the voltage of the cell
what is the electrolyte made of in a hydrogen-oxygen fuel cell?
often a solution of potassium hydroxide, the electrodes are often porous carbon with a catalyst
what happens at the positive electrode in a hydrogen-oxygen fuel cell?
hydrogen loses electrons to produce H+ ions, this is oxidation
- the H+ ions in the electrolyte move to the cathode
= H2 -> 2h+ + 2e
what happens at the negative electrode in a hydrogen-oxygen fuel cell?
oxygen gains electrons from the cathode and reacts with H+ ions to make water, this is reduction
- O2 + 4H+ + 4e -> 2H20
what are the pros and cons of batteries?
PROS:
- batteries in electric vehicles are rechargeable but there’s a limit to how many times they can be recharged before replacing
CONS:
- they store less energy so would need to be recharged
- they’re more expensive than fuel cells
- they are polluting to dispose of as they are made from toxic metal compounds
what are the pros and cons of fuel cells?
PROS:
- don’t produce as many pollutant
