Enthalpy

Question 1

Reaction when WATER is added to SOLID CALCIUM CHLORIDE

Answer 1

CaCl2(s) + aq —— CaCl2(aq)

Observations ~ test tube feels HOT

EXOTHERMIC:
- heat is given out from the system to the surroundings during the reaction.

Question 2

Reaction when WATER is added to SOLID AMMONIUM NITRATE

Answer 2

NH4NO3(s) + aq —- NH4NO3(aq)

Observations ~ test tube feels COLD

ENDOTHERMIC:
- heat is taken from the surroundings into the system during the reaction

Question 3

what is meant by :
- CHEMICAL SYSTEM
- SURROUNDINGS
- UNIVERSE

Answer 3

CHEMICAL SYSTEM ~ includes the reactants reacting with each other to form products.

SURROUNDINGS ~ everything outside the chemical system.

UNIVERSE ~ system + surroundings

Question 4

Law of conservation of energy

Answer 4

• During chemical reactions , no heat is LOST
• It is only TRANSFERRED from one place to another.

Question 5

Enthalpy , H

Answer 5

Describes the HEAT ENERGY stored in a chemical system

i.e in the the BONDS of a substance

Question 6

Enthalpy diagram for an EXOTHERMIC reaction

Answer 6

• Heat is GIVEN OUT from the system to the surroundings
• The enthalpy of products is LOWER than that of the reactants

Change in enthalpy:
H products - H reactants
- Has a NEGATIVE value

• An exothermic enthalpy change is indicated by a DOWNWARD arrow drawn from the reactants to the products enthalpy line

Question 7

Enthalpy diagram for an ENDOTHERMIC reaction

Answer 7

• Heat is TAKEN from the surroundings into the system
• The enthalpy of products is HIGHER than that of the reactants

Change in enthalpy :
H products - H reactants
- Has a POSITIVE value

• An endothermic enthalpy change is indicated by an UPWARD arrow drawn from the reactants to the products enthalpy line.

Question 8

Activation energy

Answer 8

• Reactions are NOT SPONTANEOUS
• They need a MINIMUM amount of energy
• This energy is required to start BREAK THE BONDS in the reactants
• Once the reaction has started , the heat released by the reaction continues to supply the activation energy ~ reaction becomes SELF-SUSTAINING
• The activation energy prevents substances from SPONTANEOUSLY COMBUSTING

Question 9

Enthalpy change

Answer 9

• The heat energy at a constant pressure
• By performing the reactions under the same conditions, called STANDARD CONDITIONS , the enthalpy changes for different reactions can be tabulated and compared.

Question 10

Standard conditions of :
- pressure
- concentration of a solution
- temperature

Answer 10

pressure ~ 100KPa

Concentration of a solution ~ 1mol dm-3

Temperature ~ 298 K (25 C)
T(K) = T(C) + 273

Question 11

Standard states

Answer 11

• For a standard enthalpy change , all reactants and products must be in their standard states
• In other words, their physical state under standard conditions.

Examples:
H2(g)
C(s)
H2O(l)

Question 12

Standard enthalpy change of REACTION

Answer 12

The enthalpy change that accompanies a reaction in MOLAR QUANTITIES shown in a chemical equation under STANDARD CONDITIONS ,

with all reactants and products int heir STANDARD STATES

Question 13

Standard enthalpy change of FORMATION

Answer 13

• This is the enthalpy change when ONE MOLE of a substance is formed from its CONSTITUENT ELEMENTS ,
• under STANDARD CONDITIONS
• with all reactants and products being in their STANDARD STATES

Question 14

The standard enthalpy change of FORMATION of an ELEMENT

Answer 14

• The formation of ONE MOLE of an elements from its element.
• There is NO ENTHALPY CHANGE
• So all elements by definition have an enthalpy change of formation of ZERO (0kJ mol-1)

Question 15

Standard enthalpy change of COMBUSTION

Answer 15

• This is the enthalpy change when ONE MOLE of substance reacts COMPLETELY WITH OXYGEN
• under STANDARD CONDITIONS
• with all reactants and products being in their STANDARD STATES
• The PRODUCTS are the oxides of all the elements in the substance