C7 Energy changes Flashcards
What are exothermic reactions?
Reactions that transfer energy to the surroundings
What are endothermic reactions?
Reactions that take in energy from the surroundings
Examples of exothermic reactions (3)
Combustion, oxidation reactions - like respiration, neutralisation reactions
What can exothermic reactions be used for?
To heat things up
What can endothermic reactions be used for?
To cool things down
How do cold packs work?
Water and ammonium nitrate are kept separate, when they mix the ammonium nitrate dissolves and takes in energy from the surroundings
Explain the reaction profile for an exothermic reaction
The products are at a lower energy level than the reactants. This means when the reactants form the products, energy is released into the surroundings - heating them up
Explain the reaction profile for an endothermic reaction
The products are at a higher energy level than the reactants. This means when the reactants form the products, energy is taken in from the surroundings - making them colder
What is activation energy?
The minimum energy needed for the reaction to happen
Is breaking bonds exothermic or endothermic?
Endothermic
Is making bonds exothermic or endothermic?
Exothermic
How do you calculate the energy changes in a reaction?
- calculate the total energy needed to break all of the bonds in the reactants
- calculate the total energy released in making all of the bonds in the products
- calculate the difference between the two totals
If the energy needed to break the bonds is more than the energy released when making the bonds, is the reaction exothermic or endothermic?
Endothermic
If the energy released when making the bonds is more than the energy needed to break the bonds, is the reaction exothermic or endothermic?
Exothermic
How do you make a cell?
Join two metals together using a wire and dip it in salt solution
What effect does the difference is reactivity between the two metals used to create a cell have on the voltage?
The greater the difference, the higher the voltage produced
What is the half equation at the negative electrode in a fuel cell?
2H2 + 4OH- > 4H2O +4e-
What is the half equation at the positive electrode in a fuel cell?
O2 +2H2O + 4e- > 4OH-
What are the advantages of hydrogen fuel cells? (3)
Do not need to be electrically recharged, no pollutants produced, can be a range of sizes for different uses
What are the disadvantages of hydrogen fuel cells? (3)
Highly flammable, difficult to store, can be produced by non-renewable sources
