Enthalpy Flashcards
(16 cards)
What is Enthalpy?
Enthalpy (H) is a measure of the total heat content of a system at constant pressure.
It’s not something we can measure directly, but enthalpy changes (ΔH) can be measured.
What is Enthalpy Change?
An enthalpy change is the heat energy transferred in a chemical reaction at constant pressure.
Δ𝐻=𝐻products−𝐻reactants.
If ΔH is negative → Exothermic reaction (gives out heat, temperature increases).
If ΔH is positive → Endothermic reaction (takes in heat, temperature decreases).
Explain the term exothermic.
The conservation of energy means that energy transferred from the system-energy transferred to the surroundings.
So heat out of the system.
Enthalpy of products is smaller than enthalpy of the reactants.
Enthalpy change is negative.
Explain the term endothermic.
The conservation of energy means that energy is transferred from the surroundings-energy transferred to the system.
So heat into the system.
Enthalpy of products is greater than enthalpy of the reactants.
Enthalpy change is positive.
What is enthalpy change of reaction?
Enthalpy change when a reaction occurs in molar quantities under standard conditions, all reactants and products being in their standard states.
What is enthalpy change of formation?
The enthalpy change when one mole of a compound is formed from its elements in their standard states under standard conditions.
What is enthalpy change of combustion?
The enthalpy change of combustion is the enthalpy change when one mole of a substance reacts completely with oxygen in their standard states under standard conditions.
What is enthalpy change of neutralisation?
The enthalpy change of neutralisation is the enthalpy change of a reaction of an acid by a base to form one mole of water under standard condition with reactants and products in their standard states.
What is activation energy?
The minimum energy required to start a reaction by the breaking of bonds.
What is the equation for calculating an energy change?
q =mcΔT
q=Heat Energy
m=mass
c=Specific Heat Capacity of the surroundings
ΔT=Change in temperature. Final-Initial
What is the definition of Specific Heat Capacity?
The energy required to raise the temperature of 1 gram of a substance by 1 degree celsius.
What are the standard conditions?
Standard Pressure is 100 kPa
Standard Temperature is 298 K or 25 degrees celsius
Standard Concentration is 1 mol dm-3
Standard State is the physical state of a substance under standard conditions. Most data tables show it to be 100kPa and 298K.
What is the definition of Average Bond Enthalpy?
The energy required to break one mole of a specified type of bond in a gaseous molecule.
Why is Bond enthalpies always endothermic?
As energy is always required to break bonds.
Breaking bonds is endothermic.
Bond enthalpies always have a positive enthalpy value.
Explain bond making and bond breaking.
Energy is required to break bonds, bond breaking is endothermic.
Energy is released when bonds form, bond making is exothermic.
How can you find the enthalpy change of reaction from bond enthalpies?
Enthalpy change of reaction = Sum of bond enthalpies in reactants minus Sum of bond enthalpies in products.
