Energetics - Physical 1

Question 1

What energy change is breaking bonds associated with?

Answer 1

Energy is taken in to break bonds → endothermic reaction

Question 2

What energy change is making bonds associated with?

Answer 2

Energy is released to make bonds → exothermic reaction

Question 3

What are some uses of thermochemistry?

Answer 3

Measuring and comparing the energy values of fuels Calculating the energy requirements for industrial processes Working out the theoretical amount of energy released/taken in in a reaction
Predicting if a reaction will take place or not

Question 4

What is an endothermic reaction?

Answer 4

One with an overall positive enthalpy change (+ΔH) → energy in breaking bonds > energy out making bonds

Question 5

What is an exothermic reaction?

Answer 5

One with an overall negative enthalpy change (-ΔH) → energy in breaking bonds < energy out making bonds

Question 6

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

Answer 6

exothermic

Question 7

Give 2 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 ΔH

Question 10

Draw an enthalpy change diagram for an endothermic reaction, and one for an exothermic reaction

Answer 10

Draw an enthalpy change diagram for an endothermic reaction, and one for an exothermic reaction

PLEASE CHECK ON PAPER/THE INTERNET THE ENTHALPY CHANGE DIAGRAMS!!!!!

Question 10

What are the standard conditions?

Answer 10

100kPa / 1atm pressure 298K / 250C temperature

Question 11

What does “in standard state” mean?

Answer 11

The state an element/compound exists at in standard conditions (100kPa, 298K)

Question 12

Define standard enthalpy of formation

Answer 12

The enthalpy change when one mole of a compound is formed from it constituent elements in standard conditions (100kPa, 298K), with reactants and products in their standard states.

Question 13

Give an example of an equation which represents standard enthalpy of formation

Answer 13

There are many e.g.H2 (g)+1⁄2O2 (g)→H2O(l)

Question 14

Define standard enthalpy of combustion

Answer 14

The enthalpy change when one mole of a substance is burnt completely in oxygen in standard conditions (100kPa, 298K), with reactants and products in their standard states.

Question 15

Give an example of an equation which represents standard enthalpy of combustion

Answer 15

E.g.C(s)+O2 (g)→CO2 (g)

Question 16

What is the difference between heat and temperature?

Answer 16

Heat is the sum of all particles’ energy, therefore it is affected by the amount of substance; temperature is related to the mean kinetic energy of the particles in a system, so is independent of the number of particles present.

Question 17

How can you calculate enthalpy change from experimental data?

Answer 17

Use the equation Q = mcΔT, where m is the mass of the substance being heated (usually water), c is the specific heat capacity of that substance (water’s SHC = 4.18gJ-1K-1) and ΔT is the change in temperature

Question 18

Draw a simple calorimeter

Answer 18

CHECK THE INTERNET ETC!

Question 19

How could this calorimeter be made more accurate?

Answer 19

Add draught screens at the sides, add a lid on top of the beaker, add mineral wool around the beaker → all to insulate and reduce heat lost to the surroundings

CHECK THE INTERNET ECT FOR IMAGES!!!!!!

Question 20

What is a flame calorimeter; how does it differ to a simple calorimeter?

Answer 20

Reduces heat lost to the surrounding to give more accurate results: has a spiral chimney made of copper, an enclosed flame and the fuel is burnt in pure oxygen, not air.

Question 21

How would you measure the enthalpy change for a reaction occurring in (aq)?

Answer 21

Use an expanded polystyrene cup as a calorimeter (good insulator → reduce heat loss). Heat is generated in the solution; measure this temperature change. Take heat capacity of solution to be 4.18 and density of solution = 1gcm-3.

Question 22

What can you use to make experimental determination of enthalpy change of reaction more accurate?

Answer 22

Cooling curves

Question 23

What is Hess’s Law?

Answer 23

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

Question 24

What is the enthalpy of an element?

Answer 24

The enthalpy of all elements in their standard states (the states in which they exist at 100kPa and 298K) is defined as 0

Question 25

Define bond dissociation enthalpy.

Answer 25

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

Question 26

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

Answer 26

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

Question 27

Define mean bond enthalpy.

Answer 27

Average value (across different chemical environments) for the bond dissociation enthalpy of a given bond.

Question 28

Why will using bond enthalpies not be as accurate as using standard enthalpy of combustion/formation?

Answer 28

Bond enthalpies are a mean for the same bond across different molecules; standard enthalpy of combustion and formation apply just to that molecule, therefore they are more accurate.