Chapter 9 Enthalpy Flashcards
What is enthalpy?
A measure of the heat energy in a chemical system
Cannot be measured, but the change can
Enthaly change= H (products)- H (reactants)
What is a chemical system?
The atoms, molecules or ions making up the chemicals
What is a system?
The chemicals, surroundings that is everything except the chemical system
What are exothermic reactions?
Energy loss by the chemical system to the surroundings, enthalpy change is negative , temperature rises
What are endothermic reactions?
Chemical gains energy loss by the surroundings, enthalpy change is positive, temperature decreases
What is the activation energy?
Energy input required to break bonds that acts as an energy barrier to the reaction
Is the minimum energy required for a reaction to take place
What is the standard enthalpy change?
Enthalpy change under standard conditions.
Temperature- 298K
Pressure- 100kPa
Concentration- 1moldm^-3
What is the standard enthalpy change of reaction?
Enthalpy change that accompanies a reaction in the molar quantities shown in a chemical equation under standard conditions
What is the standard enthaply change of formation?
Enthalpy change when one mole of a compound is formed from its elements under standard conditions
What is the standard enthalpy change of formation for elements?
0kJmol^-1
What is the standard enthalpy change of combustion?
Enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standard conditions
What is the standard enthalpy change of neutralisation?
Energy change that accompanies the reactionof an acid by a base to form one mole of water
All -57kJmol^-1
What is specific heat capacity?
Energy required to raise the temperature by 1 degrees celcius each gram
What is the equation of specific heat capacity?
q=mc T
m - g
c - Jg^-1K^-1
T - final-initial temperature
Why is experimental value different from data value?
Heat loss to surroundings
Incomplete combustion
Non- standard conditions
What is the average bond enthalpy?
Energy required to break one mole of a specified type of bond in gaseous
Calculated from the actual bond enthalpies in different chemical environments
Bond enthalpies are always endothermic, which always have positive enthalpy value
How to calculate bond change in enthalpy?
H=bond enthalpies in reactants- bond enthalpies in products
What is Hess’ law?
Allows enthalpy change determined indirectly by 2 routes
