Limiting Reactants (pg 127)

Question 1

What is Limiting?

Answer 1

Reactions don’t go on forever - you need stuff in the reaction flask that can react. If one reactant gets completely used up in a reaction before the rest, then the reaction will stop. That reactans’s call Limiting.

Question 2

When do a reaction stop?

Answer 2

when one reactant is used up.

Question 3

How can you tell if Magnesium carbonate (mgCO3) is reacting when placed into a beaker of hydrochloric acid?

Answer 3

You can tell a reaction is taking place because you see lots of bubbles of gas being given off.

After a while the amount of fizzing slows down and the reaction eventually stops.

Question 4

How can you tell if a reactant has stopped?

Answer 4

The reaction stops when all of one of the reactants is used up. Any other reactants are in excess. They’re usually added in excess to make sure that the other reactant is used up.

Question 5

What is limiting reactant?

Answer 5

The reactant that’s used up in a reaction is called the limiting reaction, because it limits the amount of product that’s formed.

Question 6

The amount of product formed is directly proportional to the amount of a limiting reactant.

so for example what will happen if you halve the amount of limiting reactant, what will happen to the product formed?

Answer 6

The product formed will also halve.

also if you double the amount of limiting reactant the amount of product will double (as long as it is still the limiting reactant).

This is because if you add more reactant there will be more reactant particles to take part in the reacton, which means more product particles.

Question 7

How do you calculate the mass of a product formed in a reaction?

Answer 7

by using the mass of the limiting reactant and the balanced reaction equation.

1. Write out the balanced equation.
2. Work out relative formula masses (Mr) of the reactant and product you want.
3. Find out how many moles there are of the substance you know the mass of.
4. Use the balanced equation to work out how many moles there’ll be of the other substance. In this case, that’s how many moles of product will be made of this many moles of reactant.
5. Use the number of moles to caculate mass.

Question 8

Calculate the mass of aluminium oxide formed when 35g of aluminium is burned in air?

Answer 8

1. Write out the balanced equation

4Al + 30²&raquo_space;> 2Al²O³

2. Calculate the relative formula masses

Al: 27 Al²O³: (2 x 27 + (3x16) = 102

3. Calculate the number of moles of aluminium in 135g

         Mass          135 Moles = \_\_\_\_    =    \_\_\_\_    =  5
             Mr             27

4. Look at the ratio of moles in the equation.

4 mole of Al react to product 2 moles of Al²O³ - half the number of moles are produced. So 5 moles of Al will react to produce 2.5 moles of Al²O³

(if the question asked for the number of moles of aluminium oxide formed you’d stop at 2.5 moles of Al²O³)

5. Calculate the mass of 2.5 moles of aluminium oxide

Mass = moles x Mr = 2.5 x 102 = 255g

(look at page 127 for calculation example)

Question 9

The balanced equation for the reaction between chlorine and potassium bromide is:

Cl² + 2KBr &raquo_space;» Br² + 2KCl

Calculate the mass of potassium chloride produced when 2.8g of potassium bromide reacts in an excess of chlorine Ar(K) = 39, Ar(Br) = 80, Ar(Cl)= 35.5

(4 marks)

Answer 9

Mr(KBr) = 110, Mr(KCl) = 74.5 (1 mark)

No. of moles of KBr = 23.8 + 119 = 0.200 mol (1 mark)

From the reaction equation, 2 moles of KBr react to form 2 moles of KCl. So 0.200 moles of KBr reacts to form 0.200 moles of KCl ( 1 mark)

Mass KCl = 74.5 x 0.200 = 14.9g (1 mark)