5 Energetics / Thermochemistry
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What is temperature a measure of?
We say that temperature is a measure of the average kinetic energy of the particles
What is heat a measure of?
heat is a measure of the energy content of a substance
Why do particles have kinetic energy?
The particles have kinetic energy because they are moving The faster they move the more energy they have and the higher the temperature of the substance
What is energy?
Energy is a measure of the ability to do work
What 3 types of systems are there?
Systems come in three types: open, closed and isolated:
What is the rarest type of system?
Isolated systems are very rare; most chemical reactions are open systems
What is the Law fo Conservation of Energy?
Although energy can be exchanged between open and closed systems and the surroundings, the total energy of the process cannot change
What is enthalpy?
The total chemical energy inside a substance is called the enthalpy (or heat content)
Why does enthalpy change in a chemical reaction?
When chemical reactions take place, changes in chemical energy take place and therefore the enthalpy changes
When is a reaction exothermic?
A reaction is exothermic when the products have less enthalpy than the reactants
Where is energy transferred in an exothermic reaction?
Heat energy is given off by the system to the surroundings
How is the temperature of the surroundings and system affected in an exothermic reaction?
The temperature of the surroundings increases The temperature of the system decreases
What is the sign of delta H and why for exothermic?
There is an enthalpy decrease during the reaction so ΔH is negative
Are exothermic reactions thermodynamically possible?
Exothermic reactions are thermodynamically possible (because the enthalpy of the reactants is higher than that of the products)
When is a reaction endothermic?
A reaction is endothermic when the products have more enthalpy than the reactants
Where is heat taken from in an endothermic reaction?
Heat energy is absorbed by the system from the surroundings
How is the temperature of the surroundings and system affected in an endothermic reaction?
The temperature of the surroundings decreases The temperature of the system increases§
What is the sign of delta H for an endothermic reaction and why?
There is an enthalpy increase during the reaction so ΔH is positive
What must be done to compare changes in enthalpy?
To compare the changes in enthalpy between reactions, all thermodynamic measurements are made under standard conditions
What are the standard conditions?
These standard conditions are: A pressure of 100 kPa A concentration of 1 mol dm-3 for all solutions Each substance involved in the reaction is in its standard state (solid, gas or liquid)
What is the temperature for standard conditions?
Temperature is not part of the definition of standard state, but a temperature of 298 K (25 oC) is usually given as the specified temperature
What symbol is used to show that a reaction has been carried out under standard conditions?
To show that a reaction has been carried out under standard conditions, the symbol ⦵ is used Eg. ΔHꝊ = the standard enthalpy change
What is the standard enthalpy change of reaction?
the enthalpy change when the reactants in the stoichiometric equation react to give the products under standard conditions (both exo and endo)
What is the standard enthalpy change of formation?
the enthalpy change when one mole of a compound is formed from its elements under standard conditions (both)