Energy Changes Flashcards
Name 3 reactions which are exothermic.
Combustion, oxidation, neutralisation.
In an exothermic reaction, do the products have more or less energy content than the reactants?
Less.
In an endothermic reaction, do the products have more or less energy content than the reactants?
More.
Give an example of an endothermic reaction.
Thermal decomposition.
How can you tell whether a reaction is exothermic or endothermic, by observation?
Exothermic:
-Increase in temperature
Endothermic:
- Decrease in temperature
- More energy is required to keep the reaction going
What happens to the heat in endothermic and exothermic reactions?
In exothermic, the heat is given off. In endothermic, the heat is absorbed.
Which reaction involves making bonds, and which involves breaking bonds? What happens to the energy?
Exothermic = Making bonds - more energy is required to break bonds.
Endothermic = Breaking bonds - energy is released.
What does the reaction profile for an exothermic reaction look like?
The energy starts high (in the reactants), and goes up momentarily, before going steeply down, having low energy (in the products).
What does the reaction profile for an endothermic reaction look like?
The energy starts low (in the reactants), and goes up steeply, before going momentarily down, having high energy (in the products).
Where is the activation energy in a reaction profile?
Where the line goes up/the energy increases.
What is the energy needed to break the bond between to atoms called? What is this amount of energy equal two?
Bond energy. It is equal to the amount of energy required to also make new bonds.
How do you calculate the overall energy change of a reaction?
Using a balanced symbol equation:
- Calculate the total amount of energy needed to break all of the bonds in the reactants.
- Calculate the total amount of energy needed to make all of the bonds in the products.
- Find the difference between the two totals.
What is the overall energy change for this reaction:
CH4 + 2O2 = CO2 + 2H2O
C-H = 413 O=O = 498 C=O = 805 H-O = 464
Bonds broken: (4 x C-H)+(2 x O=O)
Energy needed: (4 x 413)+(2 x 498) = 2648 KJ/mol
Bonds formed: (2 x C=O)+(4 x H-O)
Energy needed: (2 x 805)+(4 x 464) = 3466 KJ/mol
Difference: 3466 - 2648 = 818 KJ/mol
Overall energy change = 818 KJ/mol
What difference can be used to make electrical cells and batteries? What solution is needed?
The difference in reactivity between two metals (the greater the difference, the higher the voltage produced). A salt solution or dilute acid is needed.
Which way do the electrons flow in this cell?
From the more reactive metal to the less reactive metal.
What also effects voltage produced in a cell?
The electrolyte used.
What is an advantage of using hydrogen as a fuel?
The reaction does not produce carbon dioxide.
What do hydrogen fuel cells require? What is most of the energy released transferred by?
Hydrogen and oxygen. Most of the energy released is transferred by electricity.
What happens at the negative electrode with a hydrogen fuel cell?
Hydrogen gas is supplied as a fuel to the negative electrode. Hydrogen diffuses through the electrode and reacts with hydroxide ions to form water. It provides a source of electrons to an external circuit.
What happens at the positive electrode with a hydrogen fuel cell?
Oxygen gas is supplied to the positive electrode. Oxygen diffuses through the electrode and reacts with water to form hydroxide ions, accepting electrons from the external circuit.
What are the advantages and disadvantages of hydrogen fuel cells?
Advantages:
- Do not need to be electrically recharged
- No pollutants are produced
Disadvantages:
- Hydrogen is highly flammable
- Hydrogen is sometimes produced for the cell by non-renewable sources.
What is the only waste product of hydrogen fuel cells?
Water.
