Chapter 9 Enthalpy Flashcards
What is enthalpy?
it is a measure of the heat energy in a chemical system. It is sometimes thought of as the energy stored within bonds
What is the enthalpy change?
In a chemical reaction , the reactants and products have different enthalpies. The difference in the enthalpys is the enthalpy change
Products - Reactants
It can be negative or positive depending on whether the reactants contain more or less energy than the products.
What is the law of conservation of energy
States that energy can not be created or destroyed.
When a chemical reaction takes place enrgy is transferred between the system and the surroundings.
What are the two directions of energy transfer
endothermic - from the surroundings to the system
exothermic - from the system to the surroundings
What is the activation energy
the bond in the reactants need to be broken for a reaction to occur. Then energy input is known as the activation energy. Activation energy is the minimum energy required for a reaction to take place.
What are the standard conditions
Stand pressure is 100kPa
Stand temp - 25*
Stand conc - 1 mol/dm3
standard state - physical state of the substances under standard conditions
What is the standard enthalpy change of reaction?
The enthalpy change that accompanies a reaction in molar
quantities expressed in a chemical equation under standard
conditions, all reactants and products being in their standard
states.
What is the standard enthalpy change of combustion
The enthalpy change that takes place when one mole of a
substance reacts completely with oxygen under standard
conditions, all reactants and products being in their standard
states.
What is the standard enthalpy change of formation
The enthalpy change that takes place when one mole of a
compound is formed from its constituent elements in their
standard states under standard conditions.
The enthalpy of formation of an element is 0 kJ mol-1
What is the standard enthalpy change of neutralisation
It is the energy change that accompanies the reaction of an acid with a base to form one mole of H2O, under standard conditions.
What is the specific heat capacity
4.18 in water and aqueous substances.
It is the amount of energy required to raise the temperature of 1g of a substance by 1K.
How do you calculate the enthalpy change
Q=MC T
M - mass of surroundings
C - 4.18
T - Change in temperature.
Why are our entahlpy change values not accurate
Heat loss to the surroundings not to the water.
Incomplete combustions
Sometimes their may be evaporation from the wick.
Sometimes it won’t be carried out in standard conditions
What is average bond enthalpies
the energy required to break one mole of a specific type of bond in a gaseous molecule
Bond enthalpies are always endothermic
and they are always positive
What are the limitations of average bond enthalpies
The actual bond enthalpies can vary depending on the different chemical environments.
How to calculate average bond enthalpy changes
Total enthalpy of reactants - total enthalpy of products
What is hess’s law
If a reaction can take place by two routes then the enthalpy change of the start and end products is the same.
How to calculate enthalpy change of combustion
Enthalpy of reactants - enthalpy of products
How to calculate enthalpy change of formation
enthalpy of products - enthalpy of reactants.
