Chapter 6 Enthalpy changes Flashcards
enthalpy (or heat content)
The total chemical energy inside a substance
enthalpy changes
-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
Exothermic reactions
- 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
Endothermic reactions
- 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
The transition state
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
activation energy
‘the minimum amount of energy needed for reactant molecules to have a successful collision and start the reaction’
An energy level diagram
is a diagram that shows the energies of the reactants, the transition state(s) and the products of the reaction with time
the reactants In an exothermic reaction
- 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
the reactants In an endothermic reaction
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
Enthalpy Changes at Standard Conditions
- –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
Standard Enthalpy Change of: reaction
The enthalpy change when the reactants in the stoichiometric equation react to give the products under standard conditions
symbol:ΔHr⦵
Exo/Endo: both
Standard Enthalpy Change of: Formation
The enthalpy change when one mole of a compound is formed from its elements under standard conditions
Symbol:ΔH f.
Exo/endo: both
Standard Enthalpy Change of: Combustion
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
Standard Enthalpy Change of: Neutralization
The enthalpy change when one mole of water is formed by reacting an acid an alkali under standard conditions
symbol: ΔHn⊖
exo/endo: exothermic
Energy (in the form of heat) is needed to
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
the reaction is endothermic
If more energy is required to break bonds than energy is released when new bonds are formed
the reaction is exothermic
If more energy is released when new bonds are formed than energy is required to break bonds,
Exact bond energy
- 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
Average bond energy
- 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
Calculating enthalpy change from bond energies
-Bond energies are used to find the of a reaction when this cannot be done experimentally
Calculate the standard enthalpy of reaction
ΔHr^⦵= enthalpy change for bonds broken + Enthalpy change for bonds formed
ΔHr^⦵= -mcΔT
Calorimetry
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
specific heat capacity (c) of the liquid
The energy needed to increase the temperature of 1 g of a substance by 1 degree
q=m x c x ΔT
Hess’s Law
- “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
Calculating ΔHr from ΔHf using Hess’s Law energy cycles
-The products can be directly formed from the elements = ΔH2
OR
-
The products can be indirectly formed from the elements
ΔH2 = ΔH1 + ΔHr
Hess’ Law is used to calculate enthalpy changes
which can’t be found experimentally using calorimetry
The enthalpy change from elements to products (direct route)
is equal to the enthalpy change of elements forming reactants and then products (indirect route)
Calculating ΔHf from ΔHc using Hess’s Law energy cycles
-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
Calculating average bond energies using Hess’s cycles
- 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.
Assumptions when calculating specific heat
- 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
