Exothermic & Endothermic Reactions (Cambridge O Level Chemistry)

Propane reacts with chlorine to form a mixture of chloropropanes. This is a photochemical reaction.

Bond breaking is an endothermic change and bond forming is an exothermic change.

Bond energy is the amount of energy in kJ / mol needed to break one mole of the specified bond.

Use the following bond energies to determine whether this reaction is exothermic or endothermic. You must show your reasoning.

Propanol reacts with methanoic acid to form the ester propyl methanoate.

4.0 g of methanoic acid was reacted with 6.0 g of propanol.

Iodine reacts with chlorine to form dark brown iodine monochloride.

I₂ + Cl₂ → 2ICl

This reacts with more chlorine to give yellow iodine trichloride.

An equilibrium forms between these iodine chlorides.

What do you understand by the term equilibrium?

When the equilibrium mixture is heated, it becomes a darker brown colour.

Suggest if the reverse reaction is endothermic or exothermic. Give a reason for your choice.

The pressure on the equilibrium mixture is decreased.

Calculate the overall energy change for the reaction between iodine and chlorine using the bond energy values shown.

Show your working.

Draw a labelled energy level diagram for the reaction between iodine and chlorine using the information in (d).

The first three elements in Period 6 of the Periodic Table of the Elements are caesium, barium and lanthanum.

How many more protons, electrons and neutrons are there in one atom of lanthanum than in one atom of caesium. Use your copy of the Periodic Table of the Elements to help you.

number of protons ................................................

number of electrons ................................................

number of neutrons ................................................

All three metals can be obtained by the electrolysis of a molten halide. The electrolysis of the aqueous halides does not produce the metal.

All three metals react with cold water. Complete the word equation for these reactions.

Barium chloride is an ionic compound. Draw a diagram that shows the formula of the compound, the charges on the ions and gives the arrangement of the valency electrons around the negative ion.

The electron distribution of a barium atom is 2.8.18.18.8.2

Use x to represent an electron from a barium atom.
Use o to represent an electron from a chlorine atom.

Describe, by means of a simple diagram, the lattice structure of an ionic compound, such as caesium chloride.

The reactions of these metals with oxygen are exothermic.

Thermal energy is released to the surroundings when hydrogen is burnt.

State the name of the type of reaction which transfers heat to the surroundings.

The structures of the reactants and products of this reaction are shown.

Some bond energies are shown in the table.

Calculate the energy change for the reaction between propene and chlorine using the following steps.

•  Calculate the energy needed to break the bonds.
•  Calculate the energy released when bonds are formed.
•  Calculate the energy change for the reaction between propene and chlorine.

Ammonia, NH₃, is used to produce nitric acid, HNO₃. This happens in a three-stage process.

The reaction in stage 3 is exothermic.

4NO₂ + 2H₂O + O₂ → 4HNO₃

Complete the energy level diagram for this reaction. Include an arrow that clearly shows the energy change during the reaction.

Iodine reacts with aqueous sodium thiosulfate, Na₂S₂O₃.

Balance the chemical equation for this reaction.

The energy level diagram for this reaction is shown. 

Explain how this diagram shows that the reaction is exothermic.

Heat is given out when iron reacts with sulfuric acid.

What term describes a reaction which gives out heat?

The reaction of iron(II) carbonate with hydrochloric acid is exothermic.

What is meant by the term exothermic?

The reaction between ethanoic acid and ethanol is exothermic.
Draw an energy level diagram for this reaction.
On your diagram label:

• the reactants and products
• the energy change of the reaction, ∆H.

A mixture of ethanoic acid, ethene and oxygen is passed over a catalyst at 200 °C to manufacture ethenyl ethanoate, CH₃COOCH=CH₂.

CH₃COOH + CH₂=CH₂ + ½O₂ → CH₃COOCH=CH₂ + H₂O

The reaction is exothermic.

Draw a reaction pathway diagram for this reaction on Fig. 7.1.

On your diagram label:

• the axes
• the reactants and products
• the enthalpy change of the reaction, ∆H
• the activation energy, Ea.

Fig. 7.1

Explain why the enthalpy change of the reaction is exothermic.

Use ideas about bond breaking and bond making.

Period 3 contains the elements sodium to argon. This question asks about the chemistry of each of the Period 3 elements or their compounds.

Sodium nitrate is a white crystalline solid. When heated it melts and the following reaction occurs.

2NaNO₃ (l) → 2NaNO₂ (l) + O₂ (g)

A 3.40 g sample of sodium nitrate is heated.

Calculate the

• number of moles of NaNO₃ used,

.................................. mol

• number of moles of O₂ formed,

.................................. mol

• volume of O₂ formed, in dm³ (measured at r.t.p.).

.................................. dm³

Magnesium reacts slowly with warm water to form a base, magnesium hydroxide.

Aluminium oxide is amphoteric. It is insoluble in water.

Describe experiments to show that aluminium oxide is amphoteric.

Silicon(IV) oxide has a giant structure.

Calcium phosphate is used in fertilisers. The bonding in calcium phosphate is ionic. Calcium phosphate contains the phosphate ion, PO₄^3–.

Sulfur tetrafluoride, SF₄, can be made by combining gaseous sulfur with fluorine.

The reaction is exothermic.

Chlorine and compounds of chlorine are important in water treatment and in laboratory testing for water.

Argon is an unreactive noble gas.

Explain why argon is unreactive.

Chemical reactions are always accompanied by an energy change.

Aluminium is extracted by the electrolysis of a molten mixture which contains aluminium oxide, Al₂O₃. This decomposes to form aluminium at the negative electrode and oxygen at the positive electrode.

The cell shown below can be used to determine the order of reactivity of metals.

The combustion of propane, C₃H₈, is exothermic.

Give an equation for the complete combustion of propane.

Photosynthesis is an unusual endothermic reaction.

The balanced equation for the combustion of ethene is

C₂H₄ (g) + 3O₂ (g) → 2CO₂ (g) + 2H₂O (l)

Use the information in the table to calculate the following:

Complete the reaction pathway diagram for the reaction in part (a). Include an arrow which clearly shows the energy change during the reaction.

The forward reaction for the production of ethanol from ethene is shown below.

C₂H₄ (g) + H₂O (g) CH₃CH₂OH (g)   H = -45 kJ / mol

In terms of temperature and pressure, explain which conditions would give an economically viable yield. 

Separate: Chemistry and Extended Only

The table below has some enthalpy data for a different chemical reaction. Hydrazine, N₂H₄ can react with hydrogen peroxide in an exothermic reaction, as shown below. 

N₂H₄ + 2H₂O₂ → N₂ + 4H₂O   ΔH = -789 kJ / mol 

Bond	Bond Energy kJ / mol
N-N	+158
NN	+945
O-H	+463
O-O	+144

The structures of the reactants and products are shown. 

Using the reaction equation and the data in the table above, calculate the value of the N-H bond in hydrazine.

Bond energy of N-H bond = .......... kJ / mol

Draw a dot cross diagram for hydrazine. 

Use x to represent electrons from nitrogen atoms

Use o to represent electrons from hydrogen atoms

When nitrogen and oxygen are heated to very high temperatures, they combine to form nitrogen monoxide and a lot of heat is absorbed in this reaction. 

Complete the reaction pathway diagram to show the formation of nitrogen monoxide from nitrogen and oxygen.