4.2 Enthalpy Flashcards
The amount of energy/heat taken in or given out in a reaction depends on many factors
What does this mean for doing reactions
Temperature, concentration of solutions, pressure
This means that you must state the conditions under which measurements are made, eg use normal atmospheric pressure
What is enthalpy change, how is it written
When you measure heat change at a constant pressure
enthalpy is written as H, and change is written as delta (the triangle)
So enthalpy change is written as AH
(A is supposed to be a triangle)
What are the standard conditions for measuring enthalpy change
Atmospheric pressure is 100KPa
Temperature is 298K (room temp 25 degrees)
What is the standard state of an element
The state in which it exists at these standard conditions
How would you measure the heat change of a reaction, at a constant temperature
In exothermic
. Standard conditions is 298 kelvin
Although heat changes will cause the temperature change.
. Imagine both reactants at 298K when they haven’t been reacted yet
. When they react, heat is produced (in an exothermic reaction)
. This heat is given out to the surroundings
The reaction isn’t over until the products have cooled back to 298K, and the heat given off while it cools is the enthalpy change.
So measure the highest temperature they reach, and when it starts decreasing, they are cooling down.
In exothermic, the products have less heat energy than the reactants because they gave it off to the surroundings, so the enthalpy change is written with a negative symbol.
How does enthalpy change occur in endothermic reactions
Some endothermic reactions that occur in aqueous solution take heat from the water and cool it down
Eg when dissolving ammonium nitrate in water
So the reactants have to take in heat from the surroundings for this, so are initially cold
. However the product eventually ends up with more energy than before so the enthalpy change is positive, so is written with a positive symbol
How does pressure affect the heat energy given out by reactants that involve gases,
If a gas is given out, pressure is required to push away the atmosphere
The greater the atmospheric pressure, the higher the amount of energy is used for this.
As a result, less heat energy remains to be given out in the reaction
So it is important to have standard pressure for these reactions
How do the physical states of the reactants and products affect the enthalpy change of a reaction
Eg heat must be put in to change a liquid to a gas, and heat is given out to change a gas to a liquid.
This is why you always include state symbols in equations
When hydrogen burns in oxygen to produce water, it can either produce liquid or gasous H2O
This is exothermic
When gas H2O is formed, the reactants were already gaseous so there was no change in state required so no energy was lost or gained from doing that
However when liquid H2O is formed, changing from gas reactants to liquid, energy is released from doing this so the overall enthalpy change for doing this will be more negative than the previous because energy is lost as heat.
So the difference in energy between them is the amount of energy needed to turn 1mol of water into steam
What are enthalpy level diagrams
Represent the relative enthalpy changes of the reactants to the products
Vertical axis represents enthalpy change, and x axis represents the extent of the reaction
