Definitions to know...
Enthalpy - energy
These definitions are concise and they cover the content needed to get the marks needed.
Standard State ; The standard state of a substance is its most stable state under standard conditions ( 1 atmosphere pressure and 298K).
Standard enthalpy change of reaction - is the enthalpy change when the molar quantites of the reactants as stated in the equation react together under standard conditions.
Standard enthalpy change of combustion - is the enthalphy change when 1 mole of the substance is burned under standard conditions.
Standard enthalpy change of formation - is the enthalpy change when 1 mole of a compound is formed from its elements - again with both the compound and its elements being in theor standard states.
Exothermic and endothermic
In a reaction, both endothermic and exothermic processes happen, BUT, the overall description of the reaction depends on which is greater.
In the reactants... bonds are broken = EXOTHERMIC
In the products ... bonds are made = ENDOTHERMIC
Exothermic, releases energy, and is -H
Endothermic, takes in energy, and is +H
Combustion is always exothermic is heat is always released, so there is more energy in the reactants than in the products. Energy has been lost.
Overall enthalpy change in reaction =
Reactants - products
*in an exam, they usually give you the standard enthalpy of the reactants and products. If it tells you in the question that it is combustion then you should get a negative answer.
To understand this, you must understand that energy is neither created or destroyed but is transferred in different ways (eg. heat).
How do you calculate the enthalpy change of a reaction?
It is almost impossible to measure the enthalpy change directly from a reactant to a product. BUT if the conditions are the same for both routes, you can measure the enthalpy change of an indirect route. This is the same as the direct route.
indirect route = direct route
This is Hess's Law
That the enthalpy change of any chemical reaction is independant of its immediate stages, so long as the intial and final conditions are the same for each route.
A Catalyst is a substance that speeds up the rate of a reaction but is not chemically changed itself.
Catalysis is the process of speeding up a chemical reaction using a catalyst.
A heterogeneous catalyst is a catalyst which is in a different physical state to the reactants They work by...
- Reactacts are adsorbed onto catalyst surface
- the bonds in the reactants break
- the bond in products form
- The products are released from catalyst surface
solid zeolite is used in cracking processes, the reactants are liquids. Zeolite has a large surface area so more reactants can be aDsorbed onto the surface.
Entropy is a measue of the 'number of ways' a substance can be disorded. It is lowest in solids and highest in gases. As when gases mix they can be more disordered than if solids mix.
Entropy increases in a reaction when ...
- solids turn into liquids or liquids turn to as in the reaction
- there are more moles of gaseous products than reactants
eg. in catalytic converters in cars, carbon monoxide and nitrogen monoxide are converted into carbon dioxide and nitrogen;
2CO + 2NO -------> 2CO(2) + N(2)
*(2) means small 2
In this reaction there are 4 moles of gases reactants ans 3 moles of gaseous products, therefore entropy decreases.