Energetics

Question 1

What are reactions called with give out heat energy?

Answer 1

Exothermic reactions

• Combustion
• Neutralisation
• Displacement reactions

Question 2

What are reactions called with take in heat energy?

Answer 2

Endothermic reactions

• Melting
• Cracking of alkanes
• Thermal decomposition of carbonates
• Photosynthesis

Question 3

What are calometric reactions?

Answer 3

• A reaction where the heat energy change is measured
• You use a fixed volume of water/solution, and add an excess of the other reactant to ensure the reaction is complete
• You must make sure as little heat escapes as possible by using insulating materials, and covering up as many gaps as possible

These experiments can also be done with combustion, just like measuring the energy content of food in Biology

Question 4

How do you calculate heat energy change (enthalpy) in a caloimetry experiment?

Answer 4

Specific heat capacity (4.2) x Mass of Water or Solution x Temperature Change

• All solutions are assumed to have the same density and heat capacity of water
• If a reaction is exothermic, the heat energy change is negative

Question 5

How do you calculate molar enthalpy change in a caloimetry experiment?

Answer 5

• Find the heat energy change by recalling the equation
• Divide the heat energy change by the number of moles of the substance in question to find the amount of heat energy change per mole (molar enthalpy change)

Question 6

What would the energy level diagram for an exothermic reaction look like?

Answer 6

• The reactants will have more energy than the products
• There will be an initial rise in energy which will be the activation energy, before the amount of energy falls lower than it was to start as the energy is released
• The difference between the initial energy of the reactants and the final energy of the products is the energy released

Question 7

What would the energy level diagram for an endothermic reaction look like?

Answer 7

• The products will have more energy than the reactants
• There will be an initial rise in energy which will be the activation energy, and then it will drop but still be higher than it was at the start due to energy being absorbed
• The difference between the initial energy of the reactants and the final energy of the products is the energy absorbed

Question 8

What type of reaction is bond-making?

Answer 8

Exothermic as it releases energy

Question 9

What type of reaction is bond-breaking?

Answer 9

Endothermic as energy must be taken in to break the bonds

Question 10

In terms of bonds, what makes a reaction endothermic?

Answer 10

The energy taken in to break the bonds of the reactants being higher than the energy released to make the bonds of the products

Question 11

In terms of bonds, what makes a reaction exothermic?

Answer 11

The energy taken in to break the bonds of the reactants being lower than the energy released to make the bonds of the products

Question 12

How do you use bond energies to calculate the heat energy change of a reaction?

Answer 12

• Add up the all bond energies between the reactants
• Add up all the bond energies between the products
• Find the difference between these two to find the heat energy change

If the energy of the products are higher, it is a positive value and is endothermic, if the energy of the reactants are higher, it is a negative value and is an exothermic reaction