CIE IGCSE Chemistry

Topic Questions

IGCSEChemistryCIETopic Questions5. Chemical Energetics5.1 Exothermic & Endothermic Reactions

5.1 Exothermic & Endothermic Reactions

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1a2 marks

Separate: Chemistry and Extended Only

The following are two examples of substitution reactions. Only the reaction involving chlorine is a photochemical reaction.

 
CH4 + Cl2 → CH3Cl + HCl
 
CH4 + Br2 → CH3Br + HBr
 
i)
Explain the phrase substitution reaction.
 
[1]
 
ii)
How do photochemical reactions differ from other reactions?
 
[1]

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1b2 marks

Extended Only

Bond forming is exothermic, bond breaking is endothermic.

Explain the difference between an exothermic reaction and an endothermic reaction. 

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1c
Sme Calculator4 marks

Separate: Chemistry and Extended Only

Use the bond energies to show that the following reaction is exothermic.

 

Bond energy is the amount of energy (kJ / mol) which must be supplied to break one mole of the bond.

 
7-1-displayed-chlorination-of-methane-m5c-
 

Bond energies in kJ /mol
Cl–Cl   +242
C–Cl    +338
C–H     +412
H–Cl    +431
 

bonds broken   energy in kJ /mol
 

......................        .............................
 

......................        .............................
 

total energy = .............................
 

bonds formed    energy in kJ /mol
 

......................        .............................
 

......................        .............................
 

total energy = .............................

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2a3 marks

Extended Only

Fuel cells are used in spacecraft to produce electrical energy.

7-2-fuel-cell-m8a-

Hydrogen and oxygen react to form water.

 
2H2 + O2 → 2H2O

 

i)
Give an example of bond breaking in the above reaction.
 
[1]
 
ii)
Give an example of bond forming in the above reaction.
 
[1]
 
iii)
Is the change given in (i) exothermic or endothermic?
 
[1]

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2b3 marks

Extended Only

i)
Give two reasons why hydrogen may be considered to be the ideal fuel for the future.
 
[2]

 

ii)
Suggest a reason why hydrogen is not widely used at the moment.
 
[1]

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3a4 marks

Acids have characteristic properties.

Hydrochloric acid reacts with magnesium.

Name the products of this reaction and give the observations.

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3b
Sme Calculator3 marks

Separate: Chemistry and Extended Only

Hydrogen and chlorine react to form hydrogen chloride gas, as shown in the equation.

H2 + Cl2 → 2HCl 

This equation can be represented as shown.

straight H minus straight H space plus space Cl minus Cl space rightwards arrow 2 straight H minus Cl

Some bond energies are shown in the table.

bond bond energy
in kJ/mol
H–H 436
Cl–Cl 243
H–Cl 432

Calculate the energy change for the reaction between hydrogen 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.

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4a2 marks

Bromine reacts with hydrogen sulfide, H2S.

Complete the chemical equation for this reaction.

....... + H2S →  ......HBr  +  S

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4b1 mark

Extended Only

The energy level diagram for this reaction is shown.

q4dii-0620_s19_qp_32

Explain how this diagram shows that the reaction is exothermic.

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5a1 mark

The reaction of iron(II) carbonate with hydrochloric acid is exothermic.
What is meant by the term exothermic?

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5b2 marks

Rust contains compounds of iron.
State two conditions needed for iron to rust.

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1a
Sme Calculator3 marks

Extended Only

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

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1b3 marks

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

 
i)
Explain what is meant by the term base.
 
[1]
 
ii)
Write a chemical equation for the reaction between magnesium and warm water.
 
[2]

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1c3 marks

Extended Only

Aluminium oxide is amphoteric. It is insoluble in water.

Describe experiments to show that aluminium oxide is amphoteric.

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1d3 marks

Separate: Chemistry and Extended Only

Silicon(IV) oxide has a giant structure.

 
i)
Name the type of bonding in silicon(IV) oxide.
 
[1]
 
ii)
Give two physical properties of silicon(IV) oxide.
 
[2]

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1e3 marks

Extended Only

Calcium phosphate is used in fertilisers. The bonding in calcium phosphate is ionic. Calcium phosphate contains the phosphate ion, PO43–.

 
i)
What is ionic bonding?
 
[2]
 
ii)
Deduce the formula of calcium phosphate.
 
[1]

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1f
Sme Calculator6 marks

Separate: Chemistry and Extended Only

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

 
S (g) + 2F2 (g) → SF4 (g)
 

The reaction is exothermic.

 
i)
Complete the energy level diagram for this reaction. Include an arrow which clearly shows the energy change during the reaction.
 
7-1-chemical-energetics-m1f-energy-profile
 
[3]
 
ii)
During the reaction the amount of energy given out is 780 kJ / mol.
 
The F–F bond energy is 160 kJ / mol.
 
Use this information to determine the bond energy, in kJ / mol, of one S–F bond in SF4.
 
7-1-chemical-energetics-m1f-bonds
 
.................................. kJ / mol [3]

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1g4 marks

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

 
i)
Chlorine is added to water to make the water safe to drink.
 
Explain why adding chlorine makes water safe to drink.
 
[1]
 
ii)
A compound of chlorine is used in the laboratory to test for the presence of water.
 
Name the compound of chlorine used in this test and describe the colour change seen in a positive result of this test.
 
name of compound ..................................................
 
colour change from .............................. to ..............................
 
[3]

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1h1 mark

Argon is an unreactive noble gas.

Explain why argon is unreactive.

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2a3 marks

Extended Only

Three of the factors that can influence the rate of a chemical reaction are:

  • physical state of the reactants
  • light
  • the presence of a catalyst

The first recorded dust explosion was in a flour mill in Italy in 1785. Flour contains carbohydrates. Explosions are very fast exothermic reactions.

 
i)
Use the collision theory to explain why the reaction between the particles of flour and the oxygen in the air is very fast.
 
[2]
 
ii)
Write a word equation for this exothermic reaction.
 
[1] 

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2b3 marks

The decomposition of silver(I) bromide is the basis of film photography. The equation for this decomposition is:

2AgBr → 2Ag + Br2
white  black

This reaction is photochemical.

A piece of white paper was coated with silver(I) bromide and the following experiment was carried out.

7-2-m10b

Explain the results.

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2c8 marks

The fermentation of glucose is catalysed by enzymes from yeast. Yeast is added to aqueous glucose, the solution starts to bubble and becomes cloudy as more yeast cells are formed.

C6H12O6 (aq) → 2C2H5OH (aq) + 2CO2 (g)

The reaction is exothermic.

Eventually the fermentation stops when the concentration of ethanol is about 12%.

 
i)
What is an enzyme?
 
[1]
 
ii)
Pasteur said that fermentation was respiration in the absence of air. Suggest a definition of respiration.
 
[2]
 
iii)
On a large scale, the reaction mixture is cooled. Suggest a reason why this is necessary.
 
[1]
 
iv)
Why does the fermentation stop? Suggest two reasons.
 
[2]
 
v)
When the fermentation stops, there is a mixture of dilute aqueous ethanol and yeast. Suggest a technique which could be used to remove the cloudiness due to the yeast.
 
[1]
 
vi)
Name a technique which will separate the ethanol from the ethanol/water mixture.
 
[1]

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3a5 marks

Extended Only

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.

 
i)
Write an ionic equation for the reaction at the negative electrode.
 
[2]
 
ii)
Complete the ionic equation for the reaction at the positive electrode.
 
2O2– → ....... + .......
 
[2]
 
iii)
Is the reaction exothermic or endothermic? Explain your answer.
 
[1]

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3b5 marks

Extended Only

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

 
mgso4-cuso4-electrochemical-cell
 
i)
Is the reaction in the cell exothermic or endothermic? Explain your answer.
 
[1]
 
ii)
Explain why the mass of the magnesium electrode decreases and the mass of the copper electrode increases.
 
[2]
 
iii)
How could you use this cell to determine which is the more reactive metal, magnesium or manganese?
 
[2] 

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3c2 marks

Extended Only

The combustion of propane, C3H8, is exothermic.

Give an equation for the complete combustion of propane.

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3d2 marks

Separate: Chemistry Only

Photosynthesis is an unusual endothermic reaction.

 
i)
Where does the energy for photosynthesis come from?
 
[1]
 
ii)
Give the word equation for photosynthesis.
 
[1]

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4a3 marks

Separate: Chemistry and Extended Only


The balanced equation for the combustion of ethene is

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

Bond Bond energy (kJ / mol) 
C=C 614
C-H 414
O-H 463
C=O 804
O=O 498

Use the information in the table to calculate the following:

i)
Energy required to break the bonds
[1]
ii)
Energy released when bonds are made
[1]
iii)
Energy change for the reaction
[1]

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4b3 marks

Extended Only

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

5-1-4b-h-energy-level-diagram

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4c5 marks

Separate: Chemistry and Extended Only

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

 
C2H4 (g) + H2O (g) rightwards harpoon over leftwards harpoon CH3CH2OH (g)   incrementH = -45 kJ / mol
 

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

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5a4 marks

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
Nidentical toN +945
O-H +463
O-O +144


The structures of the reactants and products are shown. 

5-1-5a-h-hydrazine-reaction-figure-5-1

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

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5b3 marks

Draw a dot cross diagram for hydrazine. 

Use x to represent electrons from nitrogen atoms

Use o to represent electrons from hydrogen atoms

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5c4 marks

Extended Only

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.

5-1-5ch-energy-diagram-q5c-a

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1a4 marks

Separate: Chemistry and Extended Only

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

 
i)
What is meant by the phrase photochemical reaction?
 
[1]
 
ii)
The products of this reaction include two isomers, one of which has the following structural formula.
 
1-chloropropane
 
Draw the structural formula of the other isomer.
 
[1]
 
iii)
Explain why these two different compounds are isomers.
 
[2]

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1b
Sme Calculator3 marks

Separate: Chemistry and Extended Only

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.

 
7-1-displayed-reaction-of-chlorpropane-m2b-
 

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

 
bond bond energies in kJ / mol
C-Cl 338
C-H 412
Cl-Cl 242
H-Cl 431
C-C 348

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1c5 marks

Separate: Chemistry and Extended Only

i)
Chloropropane can be hydrolysed to propanol, CH3CH2CH2OH, by sodium hydroxide.
 
Write the equation for this reaction.
 
[2]
 
ii)
Propanol can be dehydrated. It loses a water molecule to form a hydrocarbon.
 
Give the name and structural formula of this hydrocarbon.
 
name .................................................
 
structural formula
 
[2]
 
iii)
Propanol is oxidised to a carboxylic acid by acidified potassium manganate(VII).
 
Deduce the name of this acid.
 
[1]

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1d
Sme Calculator5 marks

Extended Only

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

 
CH3CH2CH2OH + HCOOH → HCOOCH2CH2CH3 + H2O
 

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

 
i)
Calculate the Mr of methanoic acid = ........................................
 
[1]
 
ii)
Calculate the Mr of propanol = ........................................
 
[1]
 
iii)
Determine which one is the limiting reagent. Show your reasoning.
 
[2]
 
iv)
Calculate the maximum yield in grams of propyl methanoate, Mr = 88.
 
[1]

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2a2 marks

Separate: Chemistry and Extended Only


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.

ICl (l) +  Cl2 (g) ⇌  ICl3 (s)
dark brown   yellow

 

What do you understand by the term equilibrium?

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2b1 mark

Separate: Chemistry and Extended Only

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.

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2c2 marks

Separate: Chemistry and Extended Only

The pressure on the equilibrium mixture is decreased.

 
i)
How would this affect the position of equilibrium? Give a reason for your choice.
 
It would move to the ................................................
 
reason ................................................
 
[1]
 
ii)
Describe what you would observe.
 
[1]

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2d
Sme Calculator3 marks

Separate: Chemistry and Extended Only

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

 
I2 + Cl2 → 2ICl
 
Bond  Energy / kJ per mol
I–I 151
Cl–Cl 242
I–Cl 208
 

Show your working.

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2e2 marks

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

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3a
Sme Calculator3 marks

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 ................................................

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3b4 marks

Extended Only

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

 
i)
Complete the equation for the reduction of lanthanum ions at the negative electrode (cathode).
 
La3+ + ................ → ................
 
ii)
Name the three products formed by the electrolysis of aqueous caesium bromide. 

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3c2 marks

Extended Only

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

 
metal + water → ................................. + .............................

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3d2 marks

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.

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3e2 marks

Extended Only

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

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3f4 marks

Extended Only

The reactions of these metals with oxygen are exothermic.

 
2Ba (s) + O2 (g) → 2BaO (s)
 
i)
Give an example of bond forming in this reaction.
[1]
 
ii)
Explain using the idea of bond breaking and forming why this reaction is exothermic.
 
[3] 

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4a1 mark

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.

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4b
Sme Calculator3 marks

Separate: Chemistry and Extended Only

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

q6bii-0620-s20-qp-42

Some bond energies are shown in the table.

bond bond energy
in kJ/mol
C–C 347
C=C 612
C–H 413
C–Cl 339
Cl–Cl 242

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.

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4c3 marks

Extended Only

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.

4408c8cf-15a4-44cc-8a9b-a20ae8be3d12

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5a2 marks

Iodine reacts with aqueous sodium thiosulfate, Na2S2O3.

Balance the chemical equation for this reaction.

..... Na subscript 2 straight S subscript 2 straight O subscript 3 space plus space straight I subscript 2 rightwards arrow space Na subscript 2 straight S subscript 4 straight O subscript 6 space plus space..... NaI

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5b1 mark

Extended Only

The energy level diagram for this reaction is shown. 

q4dii-0620_s19_qp_31

Explain how this diagram shows that the reaction is exothermic.

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5c1 mark

Heat is given out when iron reacts with sulfuric acid.


What term describes a reaction which gives out heat?

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6a1 mark

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


What is meant by the term exothermic?

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6b3 marks

Extended Only

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.

q5b-0620_s19_qp_43

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