Enthalpy Flashcards
What is enthalpy?
Describes all of the heat energy in a single chemical system
Enthalpy symbol
H
What can we use to measure enthalpy with instead (because it is too difficult to find)?
Enthalpy change
Enthalpy change symbol
△H
Unit of enthalpy/enthalpy change
Joules (J)
Enthalpy change on a reaction profile diagram
The difference between the enthalpy of reactants to the enthalpy of the products
reaction profile diagram of exothermic reaction
The reactants are higher than products (reactants have higher enthalpy than products)
Because energy was given out
Enthalpy change of an exothermic reaction
The enthalpy of products (lower) take away reactants (higher)
This value is negative to show energy lost to surroundings
Reaction profile diagram of an endothermic reaction
The products have higher enthalpy than reactants to show energy gained by molecules
Enthalpy change of an endothermic reaction
Enthalpy of products (higher) take away reactants (lower)
So value is positive to show energy taken in from surroundings
Under what conditions are enthalpy changes measured in?
Standard conditions
Standard conditions
25°c temp
100 kPa
1molddm-3
All chemicals in their standard states (solid, liquid gas etc)
Standard enthalpy change symbol
△H⦵
Units for standard enthalpy change
KJmol-1
The enthalpy change (in energy, kilojoules) per MOL
Standard enthalpy change of combustion
The enthalpy change in a reaction when 1 mol of a substance reacts completely with oxygen (in complete combustion) under standard conditions where all species are in standard states
Standard enthalpy change of combustion symbol
△cH⦵
Important about standard enthalpy change of combustion?
BALANCED AROUND 1 MOL OF SUBSTANCE COMBUSTING
Balancing combustion questions?
Balance the C in co2 first so equal to the carbon amount in 1 mol of the substance being burnt
Then hydrogen in water to be equal to hydrogen in 1 mol of substance
Then oxygen
Standard enthalpy change of neutralisation symbol
△neutH⦵
Standard enthalpy change of neutralisation definition
The enthalpy change in a reaction when 1 mole of water is formed in a neutralisation reaction between an acid and an alkali under standard conditions when all species are in standard states
What is important about standard enthalpy change of neutralisation ?
Equation
The equation is balanced around 1 mole of water always
Standard enthalpy change of neutralisation values are always…
Always negative: neutralisation reactions are exothermic
Standard enthalpy change of combustion values are always…
Always negative
Combustion reactions are exothermic
Standard enthalpy change of neutralisation between a strong acid and strong alkali
Usually around 57kjmol-1 : string acids and alkalis fully ionise to H+ + OH- ——> H2O
Standard enthalpy change of formation symbol
△fH⦵
Standard enthalpy change of formation definition
The enthalpy change in a reaction when 1 mole of a COMPOUND is formed from its elements under standard conditions (where all species are in its standard states)
What’s important about standard enthalpy change of formation equations?
Balanced around 1 mole of a compound
The elements are in its standard states: eg oxygen is O2, hydrogen is H2, halogens etc
Standard enthalpy change of reaction symbol
△rH⦵
Standard enthalpy change of reaction definition
The enthalpy change in a reaction when a reaction takes place in the molar quantities shown in the BALANCED chemical equation under standard conditions in standard states
When is standard enthalpy change of reaction used?
If the reaction is not neutralisation, combustion or formation
So for any other reaction
When defining any enthalpy change of x, what must you specific?
The standard conditions such as:
298K
100 KPa
1 molddm-3
Standard states
System
The atoms and bonds involved in a chemical reaction
Law of conservation
Energy in a closed system will always remain the same, none can be created or destroyed
Only transferred from one form to another
