Chemical energetics Flashcards
what is the first law of thermodynamics?
Energy cannot be created or destroyed only converted from one store to another
what is an exothermic reaction and what effect does this have on the system and surrounding ?
A reaction where heat energy is released to the surroundings. This causes the systems temperature to reduce and the surroundings temp to increase
what is an endothermic reaction and what effect does this have on the system and surrounding?
A reaction where heat energy is transferred from the surroundings to the system, this results in the surroundings temperature reducing and the systems temperature increasing
What are the conditions for standard condition?
100kpa
298K
What is chemical energy made up of, give definitions of them
kinetic energy: measure of motion of particles in a substance
potential energy: measure of how strongly the particles interact with one another
define enthalpy
measure of the total energy of a system
How can the enthalpy of a system be detemined
by measuring the enthalpy change that occurs during a physical or chemical change
What is enthalpy change?
The heat energy transferred between the system and surroundings at a constant pressure
What is meant by ‘system’
The substances that are reacting (the reaction mixture)
what is meany by ‘surroundings’
Everything outside the system- that aren’t actually a part of the reaction mixture. (For example the air in the room where a reaction is taking place)
What things must we include in enthalpy level diagrams?
-Formulae and state symbol for both reactants & products
- Value for enthalpy change including correct sign (+ or -)
- Arrow pointing from reactant to products
-Labelled axis
x= progress of reaction
y= enthalpy
How do we calculate the enthalpy change of a reaction
enthalpy of products - enthalpy of reactants
What is meant by standard enthalpy change of combustion
The enthalpy change measured at 100kpa and 298K when ONE MOLE of a substance is completely burned in oxygen
How do we calculate the enthalpy change of combustion
Use Q=mc^T to calculate the heat energy of the entire system
Then divide Q by the number of moles of reactant to calculate the enthalpy change per mole of reactant.
state what the letters represent and the units for the equation: Q=mc^T
Q=heat energy (J or KJ)
m= mass (g)
c= specific heat capacity (J g-1 K-1)
^T= change in temperature (k same value as in C)
How to experimentally determine the enthalpy of combustion of a liquid (calorimetry)
- weigh liquid in a spirit burner
- add known volume of water to copper can
- measure original temp of water
- burn the burner
- stir mixture with thermometer until changes are no longer seen
- measure final temp
- use q=mc^T to get enthalpy change
states a few sources of errors and assumptions made in calorimetry experiment
- some of heat energy may be transferred to air or copper can and not water
-The conditions were not standard resulting in production of water vapour instead of liquid water
-Experiment takes a long time, not all of heat energy transferred from water to surroundings is compensated for
-
What is meant by standard enthalpy change of neutralisation
The enthalpy change measured at 100kpa and 298k when ONE mole of water is produced by the neutralisation of an acid with an alkali
Why are some standard enthalpy change of neutralisation values constant for some acids and alkalis
As strong acids and alkali’s fully disociate in aqueous solutions, so the reaction is essentially the same in each of them, so makes sense that enthalpy changes are similar.
(H+ + OH- –> H2O
How to experimentally determine the enthalpy change of neutralisation
- pipette acid into polysterene cup
- measure temp of acid
- pipette alkali with a slighty stronger concentration into beaker(this is to ensure all of acid is neutralised
- measure temp of alkali
- add alkali to acid and stir with thermometer, record maximum temperature reached
How do we calculate the enthalpy change of neutralisation
Use Q=mc^T to calculate the heat energy of the entire solution
Then divide Q by the number of moles of reactant (acid or alkali) to calculate the enthalpy change per mole of reactant.
state any sources of error and assumptions made in the experiment (enthalpy change of neutralisation)
- measurement uncertainties with pipette and thermometer
- some of heat energy will be transferred to cup and thermometer
what is meant by standard enthalpy change of formation
The enthalpy change measured at 100kpa and 298K when ONE mole of a substance is formed from its elements in their standard states
What does Hess law state?
The enthalpy change of a reaction is independent of the path taken in converting reactants to products as well as conditions are the same in each case
