9.1 Enthalpy Changes Flashcards
What is enthalpy?
A measure of the heat energy in a chemical system
What is a system in enthalpy?
Chemicals (reactants and products)
What are the surroundings in enthalpy?
Apparatus, laboratory, everything not in the chemical system
How do you calculate enthalpy change?
Enthalpy of products - Enthalpy of reactants
What is the symbol for enthalpy change?
ΔH
Enthalpy can’t be measured, but what can?
Enthalpy change (ΔH)
What is the rule for conservation of energy?
Energy cannot be created or destroyed, only transferred
Describe what happens to energy during an exothermic ΔH.
Energy is transferred from the system to the surroundings
What happens to the temperature during an exothermic reaction?
Temperature of surroundings increases as it gains energy
What is the enthalpy change in an exothermic reaction?
Negative
Describe what happens to energy during an endothermic ΔH.
Energy is transferred from the surroundings to the system
What happens to the temperature during an endothermic reaction?
Decreases
What is the enthalpy change in an endothermic reaction?
Positive
What is activation energy?
The energy input required to break bonds / the minimum energy required for a reaction to take place
What is the symbol meaning standard?
⦵
What does ⦵ mean?
Standard
What is the symbol for standard change in enthalpy?
ΔH⦵
What is standard pressure?
100kPa
What is standard temperature?
298K / 25C
What is standard concentration?
1 mol dm^-1
What is standard state?
The physical state of a substance under standard conditions
What is the symbol for standard enthalpy change of a reaction?
ΔH⦵r
What is standard enthalpy change of a reaction?
The enthalpy change that accompanies a reaction in molar quantities shown in a chemical equation under standard conditions
What is the symbol for standard enthalpy change of formation?
ΔH⦵f
What is standard enthalpy change of formation?
The enthalpy change that takes place when one mole of a compound is formed from its elements under standard conditions
What is the ΔH⦵f for one element? Why?
0, because no energy is required to form one mole of an element from one mole of an element
What is the symbol for standard enthalpy change of combustion?
ΔH⦵c
What is standard enthalpy change of combustion?
The enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standard conditions
What is standard enthalpy change of neutralisation?
The energy change that accompanies the reaction of an acid by a base to form ONE MOLE of H2O under standard conditions
What is the symbol for standard energy change of neutralisation?
ΔH⦵neut
The experimental value for the enthalpy change of combustion for a chemical is different to the data-book value. Suggest some reasons as to why this might be the case.
-Incomplete combustion occurred
-Heat losses to the atmosphere (so temperature rise is lower)
