Key points to be noted:
* If you see [ ] = concentration of an element
*You only use Kc values in an equilibrium question.
* The units are always different for every reaction so don't memorize the units. You will need to work them out in an exam.
General formula:
You must learn this formula as it will not be provided in the exam.
Rules to remember when calculating Kc value:
You must raise them to the power of their moles.
You must remember to put square brackets around your reactants and products to show the examiner you are talking about concentration.
If you change one variable, you must change the other reactant/product.
Kc, we cannot change in a chemical reaction.
Every Kc value has its own unique units.
For example:
Calculate the Kc for this reaction below using the information provided.
= 1 moldm³ at equilibrium
= 3 moldm³ at equilibrium
Method:
Substitute the provided information into the Kc formula.
Raise the powers according to the number of moles.
Solve the equation to find Kc.
Write the expression:
Full working out:
Kc = (3)² / 1 = 9.0 moldm³
Unit manipulation for Kc:
Every Kc value has its own unique units which are different for every reaction.
To find the units of Kc, start by listing out the units like so...
**Remember, the best way of getting better at calculating Kc values is to do some past paper questions. **
Value and Kc meaning:
If Kc is very large (about 1000), we will have mostly product species present at equilibrium which will give a forward direction to make the product.
If Kc is very small ( about 0.001), we will have mostly reactant species present at equilibrium which will give a backward direction to make the reactant.
If Kc is between 0.001 and 1000, we will have a significant concentration of both reactant and product species present at equilibrium.
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