A3 Energy Chnages In Industry

Question 1

Definition of enthalpy change 🔼H

Answer 1

The change in heat energy at constant pressure per mole of substances

Question 2

The set standard condition have been defined for enthalpy measurement

Answer 2

Standard cond - 100Kpa or 1 atmosphere
Pressure and temp - 298k or 25*C
Standard conc - 1.00 mol dm-3

Question 3

Standard conditions also imply

Answer 3

• reactant and product must be in physical states example solid, liquid and gas they are normal under these conditions
• any solution have concentration of 1.0 mol dm-3

Question 4

Calculating enthalpy change

Answer 4

🔼H = 🔼U + P🔼V

🔼H = enthalpy (KJ)
🔼U = internal energy (KJ)
P = pressure of system (KPa)
🔼V = volume of system (m3)

Question 5

Measuring enthalpy change experimentally

Answer 5

The enthalpy change of chemical reaction can be determined by a process called calorimetery -
• an experiment is carried out and change in temp (🔼T) is measured
• this is used to calculate the heheh change (Q) using formula:
Q=Mc🔼T

Energy = mass of water x specific heat capacity(4.18) x temp change

Question 6

Enthalpy change of combustion value obtained from calorimeter may differ from those values obtained by calc due to some common fac

Answer 6

• heat loss
• incomplete combustion of fuel
• heat energy transferred into beaker has not been taken into account
• some fuel evaporate

Question 7

Enthalpy change value is obtained from calorimeter may differ from values obtained from book due

Answer 7

The data book are based on calculations made under standard conditions

Question 8

Some improvement of calorimeter include

Answer 8

•add a lid
• reduce distances between flame and beaker
• put a sleeve around flame to protect it from draft

Question 9

Enthalpy level diagram

Answer 9

• Enthalpy change can be illustrated using enthalpy profile diagram
Compare the enthalpy of reaction • with ent happy or product and show exo and endo nature of reaction
• Chemical reaction have energy barrier that prevent many reaction from happening spontaneous.
• activation energy Ea.
Activation energy Ea the minimum needed energy to start a reaction

Question 10

Exothermic reaction ( - )

Answer 10

• release of energy example heat loss to the surrounding from system
• the value of 🔼H has negative sign

Question 11

Endothermic reaction (+)

Answer 11

• gain of energy example heat is gained by system from surroundings
• the value of 🔼H has positive sign

Question 12

Enthalpy change definition

Answer 12

Can be used to describe any chemical reactions under standard conditions

Question 13

Standard enthalpy of formation

Answer 13

• enthalpy enthalpy when one mole of a compound is formed
• from its constituents elements under standard conditions
• all reactant and products in their standard state

Formation of water -285.8 kJmol-1

Question 14

Standard enthalpy of formation

Answer 14

All elements in their standard state is zero
Example Na is an element
So Na is not standard state

Question 15

Standard enthalpy of combustion

Answer 15

🔼cH0
• the enthalpy change when one mole of a substance is completely burned in excess oxygen
• under standard conditions all reactions and product being used in their standard state
•

Question 16

Why must the enthalpyof combustion of a compound be difficult to measure

Answer 16

Incomplete combustion may occur

Question 17

Standard enthalpy of hydration

Answer 17

🔼Hhyd-

When one mole of an ion is in their gaseous state are dissolved in water to infinite dilution under standard conditions

Question 18

Hydration of water

Answer 18

The ions with water molecules positive ions will be attracted to slightly negative oxygen in water molecules whilst negative ions will attract the slightly positive hydrogen of water molecules

Question 19

What is hydration enthalpy

Answer 19

Enthalpy hydration is always exothermix (bonds made with water molecules)

Ben + mex

Mex = making exothermic
Ben = bending endothermic

Question 20

Hess law

Answer 20

Not always possible to measure enthalpy change of reaction directly from experiments
• reaction may have a very high activation energy
• reaction rate may be to slow
• possibility of competing reaction

If that’s the case then we have to use Hess law to calculate enthalpy change indirectly
The enthalpy change for a chemical reaction is the same whatever route is taken from reaction to product