5.3

• Natuurkunde / Scheikunde
• Middelbare school
• havo
• Leerjaar 3

What have we discussed up to now?

• Combustion reactions
• Decomposition reactions

        Electrolysis

        Thermolysis  

What substance always takes part in a combustion reaction? (full name)

Which of these equations is characteristic for a decomposition reaction?

1. A + B → AB
2. AB → A + B
3. AB → C + D
4. A + B → C

Look at the image. How many grams of oxygen are used in this reaction? one decimal number.

8.0 g

Finding the limiting reactant

When there is not enough of one reactant in a chemical reaction, the reaction stops abruptly. To figure out the amount of product produced, it must be determined which reactant will limit the chemical reaction (the limiting reagent) and which reactant is in excess (the excess reagent).

For example:

Substance A and substance B respectively react in a mass ratio of 5g:10g.

10 grams of both substance A and substance B are mixed in a jar and they react with eachother.

How many grams of which substance will remain after the reaction?

5 grams of substance B

Steps to find the limiting/excess reactant

1. Balance equation
2. Find mass ratio with periodic table
3. Mark with X
4. Use Cross product
5. If needed, calculate mass of the product

Now, it's your turn.

Calculate the mass of magnesium oxide (MgO)that is created when 2.4 g Mg reacts with 10.0 g  O2 

Remember the steps:

Find the limiting reagent by looking at the number of grams of each reactant.

1. Determine the balanced chemical equation for the chemical reaction.
   
2. Use your periodic table and balanced reaction equation to find the mass ratio.
   
3. Find the limiting/excess reactant by marking it with an X.
   
4. Use cross products to calculate the mass of the other reactant.
   
5. If needed, calculate the mass of the product