Module 3 Chapter 9 Flashcards
What is enthalpy defined as
The total heat content within a system
What is a chemical system
All the atoms, ions, and molecules that make up all the chemicals in a region of observable space
What is enthalpy measured by
The heat absorbed from or released into the surroundings.
what does the universe consist of
The systems and surroundings
What is the law of conservation of energy
Energy is not created or destroyed. It is only transferred during the process.
What is the heat energy released by a chemical reaction equal to
It is equal to the heat energy gained by the surroundings.
What is the formula for enthalpy change
^H = H (products) - H (reactants)
Because of the conservation of energy what happens to the energy
The change in energy has to be transferred and this transfer takes place between the system and the surroundings.
What are examples of exothermic reactions
Combustion
neutralisation
Oxidation reactions
Displacement
What is an exothermic reaction
A reaction that transfers energy to their surroundings and can be identified by an increase in the temperature of the surroundings.
What is the energy transfer in an exothermic reaction
Energy transfer from systems to surroundings
What are the enthalpies of the products and reactants relative to each other in an exothermic reaction and what is ^H
H products < H reactants
^H is negative
What is activation energy
The minimum amount of energy required for a reaction to take place
What does the enthalpy profile diagram and reaction profile diagram of an exothermic reaction look like
DRAW IT
What can an exothermic reaction be identified by
An increase in the temperature of the surroundings
What is an endothermic reaction in terms of energy and temperature
A reaction that takes in energy from its surroundings and can be identified by a decrease in the temperature of the surroundings.
What is the relative energy of the products and reactants in an endothermic reaction and what is ^H
H products > H reactants
^H is positive
What does the enthalpy profile diagram and reaction profile diagram look like for an endothermic change
DRAW IT
If the reaction conditions change what happens
The reactants and products enthalpy can change which means ^H can change as well.
What are standard conditions
Standard pressure - 100kPA
Standard temperature - 298K (25 C)
Standard concentration - 1mol/dm3 (only with solutions)
Standard state - the physical state of a substance under standard conditions
What is the symbol for when an experiment is done at standard conditions
DRAW IT
What is 1 atmosphere in kPA
101 kPA =101,000 Pa = 1 atm
What is enthalpy change of formation
The enthalpy change when 1 mole of a compound is formed from its elements under standard conditions with all reactants and products in their standard state.
What is the enthalpy change of formation of an element always
0 KJ/mol
What is the enthalpy change of combustion
When 1 mol of a substance reacts with oxygen under standard conditions with all reactants and products in their standard state.
What is enthalpy change of neutralisation
The enthalpy change that accompanies a reaction of an acid and base to form 1 mole of water under standard conditions with all reactants and products in their standard state.
What is the enthalpy change for neutralisation always
- 57 Kj/mol
What is the equation to find the heat energy in a temperature change
^q = m x c x t
Heat energy (j) = mass (g) x specific heat capacity (J/gk) x temperature change (K)
What can you use to dirt mine the enthalpy change of combustion
A calorimeter
What does a calorimeter experiment look like
DRAW IT
How can you use a calorimeter to work out the enthalpy change of combustion
Record the volume of water
Record the mass of the fuel before and after heating
Record the temperature change in water
Use q = m x c x ^t
Find the moles of fuel burnt
Why will the calculated change be less than the actual value
. Non standard conditions
. Droughts affect the flame
. Heat transferred to beaker. Surroundings instead of H2O/ Heat loss
. Soot formed on beaker - incomplete combustion
. Evaporation of fuel - weigh as soon as possible after extinguishing flame
What are average bond enthalpies used for
For bonds that occur in different molecules
What does the bond enthalpy depend on
The value depends on the bond environment
What are the limitations of average bond enthalpy
Using average bond enthalpies rather than the actual energy involved
The energy of individual bonds would be slightly different due to the different environments
What is Hess’ law
The enthalpy change of a reaction is independent of the route taken if initial and final conditions are the same.
What is the definition of average bond enthalpy
The average energy required to break one mole of a specified type of bond in gaseous molecules, averaged over a range of compounds
Enthalpy change of a reaction definition
Enthalpy change that accompanies a reaction in the molar quantities stated in the equation under standard conditions with all reactants and products in standard state.
