1.4 - Energetics

Question 1

What energy change is breaking bonds associated with?

Answer 1

Energy is taken in to break bonds - endothermic reactions

Question 2

What energy change is making bonds associated with?

Answer 2

Energy released to make bonds - exothermic

Question 3

What are some uses of thermochemistry?

Answer 3

Measuring and comparing the energy values of fuels.
Calculating energy requirements for industrial processes.
Working out the theoretical amount of energy released/taken in a reactions.
Predicting if a reaction will take place

Question 4

What is an endothermic reaction?

Answer 4

One with an overall positive enthalpy change -
Energy and breaking bonds > energy out making bonds

Question 5

What is an exothermic reaction?

Answer 5

One with an overall negative enthalpy change – energy and breaking bonds less than energy out making bonds

Question 6

If a reversible reaction is endothermic, one way what type of reaction is the other way?

Answer 6

Exothermic

Question 7

Give two examples of exothermic reactions

Answer 7

Combustion of fuels
Neutralisation

Question 8

Give an example of an endothermic reaction

Answer 8

Thermal decomposition

Question 9

Define enthalpy change; what symbol is used to represent it?

Answer 9

Energy change of a system at a constant pressure
Represented by 🔺H

Question 10

What are the standard conditions?

Answer 10

100kPa
298K

Question 11

What does ‘in standard state’ mean?

Answer 11

The state an element/compound exists at in standard conditions

Question 12

How would Antheil be change diagrams look for an endothermic reaction and exothermic reaction?

Answer 12

Question 13

Define standard enthalpy of formation

Answer 13

The envelope change when one mole of a compound is formed from its constituent elements in standard conditions with reactants and products in their standard states

Question 14

Define standard enthalpy of combustion

Answer 14

The enthalpy change when one mole of a substance is burnt completely in oxygen and standard conditions with reactants and products in their standard states

Question 15

What is the difference between heat and temperature?

Answer 15

Heat is the sum of all particles energy therefore it is affected by the amount of substance temperature is related to the main kinetic energy of the particle in the system so is independent of the number of particles present

Question 16

How can you calculate an enthalpy change from experimental data?

Answer 16

Use the equation Q =MCAT where M is the mass C is specific heat capacity in Delta T is change in temperature

Question 17

How can you make a calorimeter more accurate?

Answer 17

Add a lid on top of the beaker
Add mineral around the beaker

Question 18

What is a flame calorimeter? How does it differ to a simple calorimeter?

Answer 18

Reduces heat loss to the surroundings to give more accurate results has a spiral chimney made of copper and enclosed flame and the fuel is burned in pure oxygen not air

Question 19

What is Hess’s law?

Answer 19

States that the anthropology change for a reaction is the same regardless of the route taken

Question 20

What is the enthalpy of an element?

Answer 20

Zero

Question 21

Define bond dissociation enthalpy

Answer 21

The enthalpy change required to break a covalent bond with all species in the gaseous state differs for the same type of bond in different molecules

Question 22

Define mean bond enthalpy

Answer 22

Average value for the bond dissociation enthalpy of a given bond

Question 23

Why may experimental methods for an enthalpy determination not be very accurate?

Answer 23

Heat is lost to the surroundings
Not in standard conditions
Reaction may not go to completion

Question 24

Why will using bond dental peers not be as accurate as using standard anthropology of combustion/formation?

Answer 24

Bond enthalpy are mean for the same bond across different molecules standard enthalpy of combustion and formation applied to that molecule therefore they are more accurate