Unusual Reactions

L.O: To understand what happens in some unusual chemical reactions

• Mini white boards

http://www.bbc.co.uk/apps/ifl/schools/gcsebitesize/science/quizengine?quiz=add_aqa_quantitative_chemistryftest&templateStyle=science

You will be shown some reactions. You need to explain what you see and what you think is happening on your worksheet

Heating ammonium chlorideHeating ammonium chloride

Ammonium salts are made by reacting ammonia with an acid but some of these salts will decompose back into reactants when heated.

Hea

t

Heat makes the solid disappear as it changes into gases. Solid reappears as it

changes back again in the cool part of the tube.

ammonium chloride

ammonia hydrogen chloride

NH4Cl(s) NH3 (g) + HCl(g)

Which direction is giving out heat?And which is taking it in?

• Now look at the other reactions on your sheet

ALL the reactions you have seen are reversible

A reversible reaction is where products can, under appropriate conditions, turn back into reactants.

• If the reaction releases energy in one direction (exothermic), it will take in same amount of energy in the other (endothermic).

A B A B

these combine these decompose

Reversible Reactions

Mg + 2HCl MgCl2 + H2

Irreversible reactions

• Most Chemical reactions are considered irreversible in that products are not readily changed back into reactants.

• When wood burns it is pretty difficult to un-burn it back into wood again!

• When magnesium reacts with acid it is not easy to unreact it and get back the magnesium.

Wood reacting with oxygen

Simple reversible reactions

Heating copper sulphate

• Explain what hydrated means• Explain what anhydrous means• Explain how copper sulphate can be used to test for

water.

hydrated copper sulphate

Heat

anhydrous copper sulphate

steam

CuSO4.5H20 ⇌ CuSO4 + 5H2O

Blue Cobalt crystals turn pink when they become damp. The formula for the two forms can be written as CoCl2 .2H2O and CoCl2.6H2 O

1. How many moles of water will combine with 1 mole of CoCl2 .2H2O

2. Write a reversible balanced equation

3. You have some pink cobalt chloride crystals. Suggest how you could turn them into pink cobalt crystals

The rate at which reactants change into products and products change into reactants will eventually become the same.

So… the overall amount of reactants and products does not change.

A B A B

these combine these decompose

Equilibrium

If we start with only reactants how will the rates change over time of reactants forming products, and products forming reactants?

A B

Lots of reactants forming products

A B A B

Rate of reactants to products is highReversible rate is low

A B A B

Rate of reactants to products slows down

Reversible rate increases

A B A B

Rate of reactants to products eventually equals the rate of the reversible reaction

A closed system• In a closed system no reactants or

products can get in or out• Eventually a balance between the

amounts of reactants and products is reached. (not necessarily the same amounts of each).

• Temperature and Concentration (Pressure) can change the amount of products and reactants.

1. What is a reversible reaction?

2. If heat is lost in the forward reaction will the reverse be endothermic or exothermic?

3. What happens to the rate of a forward reaction over time if you start with reactants only.

4. What will happen to the rate of reaction in both directions when equilibrium is reached?

5. What two things could change the amount of reactants and products in a reversible reaction?

6. What is a closed system?

Why do you think reversible reactions are a problem to industries?