Chapter 6 Enthalpy changes

Question 1

enthalpy (or heat content)

Answer 1

The total chemical energy inside a substance

Question 2

enthalpy changes

Answer 2

-When chemical reactions take place, changes in chemical energy take place

• An enthalpy change is represented by the symbol ΔH (Δ= change; H = enthalpy)
• An enthalpy change can be positive or negative

Question 3

Exothermic reactions

Answer 3

• A reaction is exothermic when the products have less enthalpy than the reactants
• Heat energy is given off by the reaction to the surroundings
• —The temperature of the environment increases
• —The temperature of the system decreases
• There is an enthalpy decrease during the reaction so ΔH is negative
• Exothermic reactions are thermodynamically possible (because the enthalpy of the reactants is higher than that of the products)
• However, if the rate is too slow, the reaction may not occur. In this case the reaction is kinetically controlled

Question 4

Endothermic reactions

Answer 4

• A reaction is endothermic when the products have more enthalpy than the reactants
• Heat energy is absorbed by the reaction from the surroundings
• —The temperature of the environment decreases
• —The temperature of the system increases

-There is an enthalpy increase during the reaction so ΔH is positive

Question 5

The transition state

Answer 5

is a stage during the reaction at which chemical bonds are partially broken and formed

• The transition state is very unstable – it cannot be isolated and is higher in energy than the reactants and products
• The activation energy (Ea) is the energy needed to reach the transition state

Question 6

activation energy

Answer 6

‘the minimum amount of energy needed for reactant molecules to have a successful collision and start the reaction’

Question 7

An energy level diagram

Answer 7

is a diagram that shows the energies of the reactants, the transition state(s) and the products of the reaction with time

Question 8

the reactants In an exothermic reaction

Answer 8

• The reactants are therefore closer in energy to the transition state
• This means that exothermic reactions have a lower activation energy compared to endothermic reactions

Question 9

the reactants In an endothermic reaction

Answer 9

are lower in energy than the products

• The reactants are therefore further away in energy to the transition state
• This means that endothermic reactions have a higher activation energy compared to exothermic reactions

Question 10

Enthalpy Changes at Standard Conditions

Answer 10

• –A pressure of 101 kPa
• –A temperature of 298 K (25 oC)
• –Each substance involved in the reaction is in its normal physical state (solid, gas or liquid)
• To show that a reaction has been carried out under standard conditions, the symbol ⦵is used
• —–ΔH⦵ = the standard enthalpy change

Question 11

Standard Enthalpy Change of: reaction

Answer 11

The enthalpy change when the reactants in the stoichiometric equation react to give the products under standard conditions

symbol:ΔHr⦵
Exo/Endo: both

Question 12

Standard Enthalpy Change of: Formation

Answer 12

The enthalpy change when one mole of a compound is formed from its elements under standard conditions

Symbol:ΔH f.
Exo/endo: both

Question 13

Standard Enthalpy Change of: Combustion

Answer 13

The enthalpy change is when one mole of a substance is burnt in excess oxygen under standard conditions

symbol:ΔH c^⊖[B]
exo/endo: exothermic
Negative

Question 14

Standard Enthalpy Change of: Neutralization

Answer 14

The enthalpy change when one mole of water is formed by reacting an acid an alkali under standard conditions
symbol: ΔHn⊖
exo/endo: exothermic

Question 15

Energy (in the form of heat) is needed to

Answer 15

overcome attractive forces between atoms

• -Bond breaking is therefore endothermic
• Energy is released from the reaction to the surroundings (in the form of heat) when new bonds are formed
• –Bond forming is therefore exothermic

Question 16

the reaction is endothermic

Answer 16

If more energy is required to break bonds than energy is released when new bonds are formed

Question 17

the reaction is exothermic

Answer 17

If more energy is released when new bonds are formed than energy is required to break bonds,

Question 18

Exact bond energy

Answer 18

• The amount of energy required to break one mole of a specific covalent bond in the gas phase is called the bond dissociation energy
• Bond dissociation energy (E) is also known as exact bond energy or bond enthalpy
• The type of bond broken is put in brackets after E
• —Eg. EE(H-H) is the bond energy of a mole of single bonds between two hydrogen atoms

Question 19

Average bond energy

Answer 19

• Bond energies are affected by other atoms in the molecule (the environment)
• Therefore, an average of a number of the same type of bond but in different environments is calculated
• This bond energy is known as the average bond energy
• Since bond energies cannot be determined directly, enthalpy cycles are used to calculate the average bond energy

Question 20

Calculating enthalpy change from bond energies

Answer 20

-Bond energies are used to find the of a reaction when this cannot be done experimentally

Question 21

Calculate the standard enthalpy of reaction

Answer 21

ΔHr^⦵= enthalpy change for bonds broken + Enthalpy change for bonds formed

ΔHr^⦵= -mcΔT

Question 22

Calorimetry

Answer 22

is a technique used to measure changes in enthalpy of chemical reactions
-A calorimeter can be made up of a polystyrene drinking cup, a vacuum flask or metal can

Question 23

specific heat capacity (c) of the liquid

Answer 23

The energy needed to increase the temperature of 1 g of a substance by 1 degree

q=m x c x ΔT

Question 24

Hess’s Law

Answer 24

• “The total enthalpy change in a chemical reaction is independent of the route by which the chemical reaction takes place as long as the initial and final conditions are the same.”
• This means that whether the reaction takes place in one or two steps, the total enthalpy change of the reaction will still be the same

Question 25

Calculating ΔHr from ΔHf using Hess’s Law energy cycles

Answer 25

-The products can be directly formed from the elements = ΔH2
OR
-
The products can be indirectly formed from the elements
ΔH2 = ΔH1 + ΔHr

Question 26

Hess’ Law is used to calculate enthalpy changes

Answer 26

which can’t be found experimentally using calorimetry

Question 27

The enthalpy change from elements to products (direct route)

Answer 27

is equal to the enthalpy change of elements forming reactants and then products (indirect route)

Question 28

Calculating ΔHf from ΔHc using Hess’s Law energy cycles

Answer 28

-The combustion products can be formed directly from elements to combustion products = ΔH1
OR

-The combustion products can be formed indirectly from elements to compound to combustion products = ΔHf+ ΔH2

*combustion products are at the bottom of the cycle.
so element + oxygen –> ΔHf –> compound formed oxygen

come together to combustion

Question 29

Calculating average bond energies using Hess’s cycles

Answer 29

• Bond energies cannot be found directly so enthalpy cycles are used to find the average bond energy
• This can be done using enthalpy changes of atomisation and combustion or formation
• The enthalpy change of atomisation (ΔHat⦵ ) is the enthalpy change when one mole of gaseous atoms is formed from its elements under standard conditions.

Question 30

Assumptions when calculating specific heat

Answer 30

• pure water density =1 and always use no matter if any others are present
• Specific heat capacity of water only, even if there is other substances present