Chemistry 5: Chemical Energetics

Front 1

In all chemical reactions, bonds are broken and formed all the time. What does it mean when bonds are broken?

Back 1

The breaking of bonds require energy which makes the reaction endothermic

Front 2

In all chemical reactions, bonds are broken and formed all the time. What does it mean when bonds are formed?

Back 2

The formation of new bonds releases energy making this process exothermic.

Front 3

Define Heat change

Back 3

It is the difference between the quantity of heat needed to break the bonds of the reactants and that evolved during the formation of new bonds in the products.

Front 4

What is meant by the enthalpy change of a reaction?

Back 4

It is the heat change at constant pressure

Front 5

what is the formula for the enthalpy change?

Back 5

(Total enthalpy change in the products) - (total enthalpy change in the reactant)

Front 6

What is the enthalpy change for exothermic reaction?

Back 6

Negative

Front 7

What is the enthalpy change for endothermic reactions?

Back 7

Positive

Front 8

What happens in an exothermic reaction?

Back 8

It gives out heat to the surrounding. The surrounding heat rises as the heat content of the system falls

Front 9

What happens in an endothermic reaction?

Back 9

It absorbs heat from the surrounding. The surrounding temperature falls as the heat content of the system rises

Front 10

Give an example of a exothermic reaction

Back 10

NaOH(aq) + HCl(aq) ----> NaCl(aq) + H2O(l)

ΔH=-56.7KJ/mol

Front 11

Give an example of an endothermic reaction

Back 11

CaCO3(s) ----> CaO(s) + CO2(g)

ΔH=+572KJ/mol

Front 12

Define what is meant by specific heat capacity of a substance

Back 12

It is the quantity of heat required to raise its temperature by 1Kelvin

Front 13

What is the formula for heat change?

Back 13

ΔQ=mcΔT

ΔQ=CΔT

ΔQ= hear change/J
ΔT= change in temp/k
C= specific heat capacity

Front 14

Define Standard enthalpy change of reaction

Back 14

It is the enthalpy change when amounts of reactions, as shown in the reaction equation, react together under standard conditions

2H2(g) + O2 ---> 2H2O(l) ΔH= -575KJ/mol

N2(g) + 3H2(g) ---> 2NH3(l) ΔH= -100KJ/mol

N2(g) + O2(g) ---> 2NO(g) ΔH= +180KJ/mol

Front 15

Define Standard enthalpy change of Formation

Back 15

It is the Enthalpy Change when one mole of substance is formed from its constituent elements under standard conditions

Na(s) + O2(g) ---> NaCl(s) It is the enthalpy change when amounts of reactions, as shown in the reaction equation, react together under standard conditions

2H2(g) + O2 ---> 2H2O(l) ΔH= -575KJ/mol

N2(g) + 3H2(g) ---> 2NH3(l)

Front 16

Define Standard enthalpy change of Combustion

Back 16

It is the enthalpy change when one mole of substance is completely burnt in oxygen under standard conditions

C2H5OH(l) + 3O2(g) ---> 2CO2(g) + 3H2O(l) ΔH= -1368KJ/mol

Front 17

Define Standard enthalpy change of Hydration

Back 17

Standard enthalpy change of an ion is the energy released when one mole of gaseous ion is hydrated under standard conditions

X+(g) + aq --->X+(aq)
Y-(g) + aq ---> Y-(aq)

Front 18

Define Standard enthalpy change of Solution

Back 18

It is the enthalpy change when one mole of substance is completely dissolved in a sufficiently large volume of solvent under standard conditions such as the addition of more solvent does not cause any enthalpy change.

NaCl(s) + aq ---> Na+(aq) + Cl-(aq)

Front 19

Define Standard enthalpy change of Atomisation

Back 19

It is the enthalpy change when a substance decompose to form one mole of gaseous atoms. This reaction is always exothermic

Na(s) ---> Na(g)
1/2Cl2(g) ---> Cl(g)

atomisation energy is half of the bond energy. Bond energy can be obtained from the data booklet

Front 20

Define Standard enthalpy change of Neutralisation

Back 20

It is the enthalpy change when an aqueous acid and aqueous base react together to form one mole of water under standard conditions.

NaOH(aq) + HCl(aq) ---> NaCl(aq) + H2O(l) ΔH= -56.7KJ/mol

Front 21

What is the relationship between standard enthalpy change of solution, hydration and lattice dissociation energy?

Back 21

ΔHsol = ΔHhyd + ΔHLDE

If ΔHhyd>ΔHLDE then ΔHsol = negative (soluble)

If ΔHhyd<ΔHLDE then ΔHsol= positive (insoluble)

Front 22

State Hess's Law

Back 22

Hess's law states that the enthalpy change in a reaction is the same regardless of the route taken by which a chemical change occurs provided that the initial and final conditions are the same.

Front 23

What is the Born-Haber cycle?

Back 23

It is a technique for applying Hess's law to the standard enthalpy changes which occur when ionic compounds are formed.

Front 24

Vaporisation of sodium

Back 24

Na(s) ---> Na(g) ΔHatm

Front 25

Ionisation of Sodium

Back 25

Na(g) ---> Na+(g) + e- ΔHIE

Front 26

Dissociation of chlorine molecule

Back 26

1/2Cl2(g) ---> Cl(g) ΔHatm

Front 27

Electron affinity of Cl atom

Back 27

Cl(g) + e- ---> Cl-(g) ΔHea

Front 28

Reaction between Na+ and Cl- ions

Back 28

Na+(g) + Cl-(g) ---> NaCl(s) ΔHLDE