chemical energetics Flashcards
(43 cards)
What is the total chemical energy inside a substance called?
Enthalpy
Enthalpy is also referred to as heat content.
What symbol represents an enthalpy change?
ΔH
Δ represents change and H represents enthalpy.
What type of reaction is characterized by products having less energy than the reactants?
Exothermic reaction
Heat energy is released to the surroundings.
In an exothermic reaction, what happens to the temperature of the environment?
Increases
This can be measured with a thermometer.
What is the sign of ΔH during an exothermic reaction?
Negative
This indicates an enthalpy decrease.
What characterizes an endothermic reaction?
Products have more energy than reactants
Heat energy is absorbed from the surroundings.
What happens to the temperature of the environment during an endothermic reaction?
Decreases
This can be measured with a thermometer.
What is the sign of ΔH during an endothermic reaction?
Positive
This indicates an enthalpy increase.
What is an important consideration when specifying enthalpy changes?
Specify the physical states of each species
Changes in state can cause large changes in enthalpy.
What is a reaction pathway diagram used for?
Shows the energies of reactants, transition states, and products
Also known as energy profile diagrams.
What is the transition state in a reaction pathway diagram?
A stage at which chemical bonds are partially broken and formed
It is very unstable and higher in energy than reactants and products.
What does the activation energy (Ea) represent?
The minimum amount of energy needed for reactant molecules to have a successful collision
It is necessary to reach the transition state.
In an exothermic reaction, how do the energies of reactants compare to products?
Reactants are higher in energy than products
This leads to a lower activation energy.
In an endothermic reaction, how do the energies of reactants compare to products?
Reactants are lower in energy than products
This leads to a higher activation energy.
What are standard conditions for enthalpy changes?
Pressure of 101 kPa and temperature of 298 K
Each substance must be in its normal physical state.
What does ΔHθ represent?
Standard enthalpy change
It indicates that a reaction has been carried out under standard conditions.
Define the standard enthalpy change of formation.
Enthalpy change when one mole of a compound is formed from its elements under standard conditions
Symbol: ΔHθf.
What is the standard enthalpy change of combustion?
Enthalpy change when one mole of a substance is burnt in excess oxygen under standard conditions
Symbol: ΔHθc.
What is bond dissociation energy?
Energy required to break one mole of a specific covalent bond in the gas phase
Also known as exact bond energy or bond enthalpy.
What is the average bond energy?
Average energy of a number of the same type of bond in different environments
It accounts for variations in molecular structures.
What is the equation to calculate the standard enthalpy change of reaction using bond energies?
ΔHθr = enthalpy change for bonds broken + enthalpy change for bonds formed
Positive values for bonds broken and negative for bonds formed.
What happens during bond breaking and forming?
Bond breaking is endothermic and bond forming is exothermic
Energy is absorbed when bonds break and released when bonds form.
What is the enthalpy change for the combustion of methane?
ΔHθ = –890 kJ mol-1
This indicates that energy is released during the reaction.
What is the ΔHθf of O2 in its standard state?
Zero
ΔHθf of an element in its standard state is always zero.
