Energy Changes

Question 1

Name 3 reactions which are exothermic.

Answer 1

Combustion, oxidation, neutralisation.

Question 2

In an exothermic reaction, do the products have more or less energy content than the reactants?

Answer 2

Less.

Question 3

In an endothermic reaction, do the products have more or less energy content than the reactants?

Answer 3

More.

Question 4

Give an example of an endothermic reaction.

Answer 4

Thermal decomposition.

Question 5

How can you tell whether a reaction is exothermic or endothermic, by observation?

Answer 5

Exothermic:

-Increase in temperature

Endothermic:

• Decrease in temperature
• More energy is required to keep the reaction going

Question 6

What happens to the heat in endothermic and exothermic reactions?

Answer 6

In exothermic, the heat is given off. In endothermic, the heat is absorbed.

Question 7

Which reaction involves making bonds, and which involves breaking bonds? What happens to the energy?

Answer 7

Exothermic = Making bonds - more energy is required to break bonds.

Endothermic = Breaking bonds - energy is released.

Question 8

What does the reaction profile for an exothermic reaction look like?

Answer 8

The energy starts high (in the reactants), and goes up momentarily, before going steeply down, having low energy (in the products).

Question 9

What does the reaction profile for an endothermic reaction look like?

Answer 9

The energy starts low (in the reactants), and goes up steeply, before going momentarily down, having high energy (in the products).

Question 10

Where is the activation energy in a reaction profile?

Answer 10

Where the line goes up/the energy increases.

Question 11

What is the energy needed to break the bond between to atoms called? What is this amount of energy equal two?

Answer 11

Bond energy. It is equal to the amount of energy required to also make new bonds.

Question 12

How do you calculate the overall energy change of a reaction?

Answer 12

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.

Question 13

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

Answer 13

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

Question 14

What difference can be used to make electrical cells and batteries? What solution is needed?

Answer 14

The difference in reactivity between two metals (the greater the difference, the higher the voltage produced). A salt solution or dilute acid is needed.

Question 15

Which way do the electrons flow in this cell?

Answer 15

From the more reactive metal to the less reactive metal.

Question 16

What also effects voltage produced in a cell?

Answer 16

The electrolyte used.

Question 17

What is an advantage of using hydrogen as a fuel?

Answer 17

The reaction does not produce carbon dioxide.

Question 18

What do hydrogen fuel cells require? What is most of the energy released transferred by?

Answer 18

Hydrogen and oxygen. Most of the energy released is transferred by electricity.

Question 19

What happens at the negative electrode with a hydrogen fuel cell?

Answer 19

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.

Question 20

What happens at the positive electrode with a hydrogen fuel cell?

Answer 20

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.

Question 21

What are the advantages and disadvantages of hydrogen fuel cells?

Answer 21

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.

Question 22

What is the only waste product of hydrogen fuel cells?

Answer 22

Water.