0% found this document useful (0 votes)

26 views17 pages

Resouce of Energy From Chemicals

The document describes the syllabus for a chemical energy topic, including describing exothermic and endothermic reactions, combustion of fuels, and energy changes involved in bond breaking and making. It also provides short notes on related concepts like energy profiles, enthalpy changes, and fossil fuels.

Uploaded by

borglingchompers

We take content rights seriously. If you suspect this is your content, claim it here.

Available Formats

Download as PDF, TXT or read online on Scribd

0% found this document useful (0 votes)

26 views17 pages

Resouce of Energy From Chemicals

Uploaded by

borglingchompers

We take content rights seriously. If you suspect this is your content, claim it here.

Available Formats

Download as PDF, TXT or read online on Scribd

You are on page 1/ 17

Energy from Chemicals

Syllabus:

Candidates should be able to:

(a) Describe the meaning of enthalpy change in terms of exothermic (ΔH negative) and endothermic (ΔH
positive) reactions

(b) *Represent energy changes by energy profile diagrams, including reaction enthalpy changes and activation
energies (see 6.1(c))

(d) *Explain overall enthalpy changes in terms of the energy changes associated with the breaking and making
of covalent bonds

(e) Describe combustion of fuels as exothermic, e.g. wood; coal; oil; natural gas; hydrogen

(f) Describe hydrogen, derived from water or hydrocarbons, as a potential fuel for use in future, reacting with
oxygen to generate electricity directly in a fuel cell (details of the construction and operation of a fuel cell are
not required) and discuss the advantages and disadvantages of this

(g) Name natural gas, mainly methane, and petroleum as sources of energy

(h) Describe petroleum as a mixture of hydrocarbons and its separation into useful fractions by fractional
distillation

(i) Name the following fractions and state their uses

(i) petrol (gasoline) as a fuel in cars

(ii) naphtha as feedstock for the chemical industry

(iii) paraffin (kerosene) as a fuel for heating and cooking and for aircraft engines

(iv) diesel as a fuel for diesel engines

(v) lubricating oils as lubricants and as a source of polishes and waxes

(vi) bitumen for making road surfaces

(j) Describe photosynthesis as the reaction between carbon dioxide and water in the presence of chlorophyll,
using sunlight (energy) to produce glucose and explain how this can provide a renewable energy source.
Short Notes:
Law of Conservation of Mass and Energy: Mass and energy can neither be created nor can be
destroyed but can be transformed from one form to the other form.
Exothermic reaction:
 Reaction that gives out heat energy to the surrounding.
 Direction of flow of heat is from system towards surroundings.
 Temperature of reaction mixture increases container feels hot.
Examples:
Combustion of fuels, respiration, Condensation, Freezing, Dissolution of concentrated acid
Enthalpy Change:
 Amount of energy involved in a reaction is known to be heat energy or enthalpy change.
 It is measured in kilo joule and represented by ΔH
 Enthalpy change or heat change is negative ΔH =-ve (EXO)
Information from energy profile diagram:
 Endothermic or Exothermic reaction
 Energy of product is higher or lower than reactant
 Bond breaking energy is higher or lower than bond making
 Amount of activation energy needed to cross the barrier
 Difference in level of hump for catalyzed and unanalyzed path
Endothermic Reaction:
 Reaction that absorbs heat from the surrounding
 Direction of flow of heat is from surroundings towards system.
 Temperature of reaction mixtures decreases container feels cold.
Examples:
Evaporation, Photosynthesis, Melting, Dissolution of some ionic compound
Enthalpy change
 Amount of energy involved in a reaction is known to be heat energy or enthalpy change.
 It is measured in kilo joule and represented by ΔH
 is ΔH =+ve (ENDO)
Bond breaking and bond making process:
Bond breaking:
 All chemical reactions involve breaking of old bond and making of new bond
 Bond breaking is always endothermic process
 ΔH bond breaking > ΔH bond making
 Energy absorbed during bond breaking > energy released during bond making hence extra energy is
released so the reaction is exothermic.
Bond Making:
 Bond making is exothermic process
 ΔH bond breaking < ΔH bond making
 Energy absorbed during bond breaking < energy released during bond making hence extra energy is
needed so the reaction is endothermic.
Bond energy:
 Amount of energy required to break a chemical bond is equal to the amount of energy released to form
a chemical bond.
 Stronger bonds needs high energy to break so its bond energy will be higher

H-H 2H ΔH = +436 KJ bond breaking

2H H-H ΔH = -436 KJ bond making

Calculation of enthalpy change ΔH:

ΔH = Bond breaking – bond making

Energy of Activation (Ea):

 Minimum amount of energy which reactant must possess for a chemical reaction to take place is called
Energy of activation
 For a chemical reaction to take place reactant particles must collide with each other
 Collisions which are less energetic do not lead towards product formation
 Reactant must possess activation energy which is bunch of energy to initiate the reaction.
 Collisions with enough E equal to activation E and taking place in proper orientation lead towards
product formation

Fossil fuels:

Advantages:

 Are good source of energy on burning in air they release lot of heat energy
 e.g: wood, charcoal, coal, petroleum, H2, CH4
 When coal burns in air, reaction is highly exothermic ,and gives carbon dioxide and steam

Disadvantages:

 Nonrenewable
 On burning releases toxic gases which causes environmental pollution

Fuel Cells:

 A chemical cell in which reactants (fuel + O) are continuously supplied to produce electricity directly is
called fuel cell.
 Reactants are not combined within the cell rather continuously supplied from external reservoir

Advantages of Fuel Cells:

 Hydrogen is fuel for future

 Renewable source of energy
 Pollution free because only water is produced on reaction of H 2 and O2

Exam Favorite Topics:

 Difference of Exothermic and Endothermic reaction
 Energy Profile diagram
 Bond breaking and Bond making Energy
 Energy of Activation and Enthalpy change
Multiple Choice Questions
1. The table compares the strengths of the bonds for reactions of the type below.

X2 + Y2 → 2XY
Which reaction is most exothermic?

2. Which process is endothermic?

A adding water to anhydrous copper(II) sulphate B burning magnesium to make the oxide
C heating water to make steam D neutralising acidic industrial waste

3. Which diagrams show a process in which an exothermic change is taking place?

A 1 and 2 only B 1 and 3 only C 2 and 3 only D 1, 2 and 3

4. Are hydrogen and uranium oxidized when used as a source of energy?

5. The diagrams show the molecules of three elements.

Which of these elements are present in water?

A 1 and 2 only B 1 and 3 only C 2 and 3 only D 1, 2 and 3
6. Coal, methane and hydrogen are burned as fuels. Which descriptions of this process are correct?
7. Two reactions involving water are shown.

Which of these reactions are reversible by heating?

8. The diagrams show some pieces of laboratory equipment.

Which equipment is needed to find out whether dissolving salt in water is an endothermic process?
A 1 only B 1 and 3 C 2 and 3 D 3 only

9. Which is an endothermic process?

A burning hydrogen B distilling petroleum
C reacting potassium with water D using petrol in a motor car engine

10. Which fuel needs oxygen in order to produce heat energy and which type of reaction produces the energy?

11. The reaction C2H4 + 3O2→2CO2 + 2H2O is exothermic because

A more bonds are broken than are formed. B more bonds are formed than are broken.
C the energy needed to break the bonds is greater than that released on forming new bonds.
D the energy needed to break the bonds is less than that released on forming new bonds.

12. Which reaction profile shows the fastest exothermic reaction?

13 The equation below shows an exothermic reaction.
Mg(s) + 2HCl(aq) →MgCl2(aq) + H2(g)
Which statement about this exothermic reaction is not correct?
A Magnesium chloride is soluble in water. B Magnesium is above hydrogen in the reactivity series.
C One mole of magnesium produces one mole of hydrogen gas.
D The total energy of the products is greater than that of the reactants.

14. The diagram shows the energy profile for a chemical reaction.

What is the correct description of the reaction?

15. The energy diagram for the reaction between sodium hydroxide and hydrochloric acid is shown.
What can be deduced from the diagram?
A Heat is needed to start the reaction. B The products contain less energy than the reactants.
C The reaction is rapid. D The OH– ions have more energy than the H+ ions.

16. Which pair of statements about the combustion of a carbohydrate and its formation by photosynthesis is
not correct?

17. Why is nickel used in the addition of hydrogen to alkenes?

A It increases the yield of products. B It lowers the activation energy of the reaction.
C It makes the reaction more exothermic. D It prevents a reverse reaction from occurring.

18. The energy profile diagram for the reaction between hydrogen and chlorine is shown.

What information about this reaction does the diagram show?

19. The energy profile diagram shows the pathways for a reaction with and without a catalyst. Which energy
change is the activation energy for the catalyzed reaction?

20. Which of these processes are both endothermic?

A combustion, cracking B combustion, fermentation
C cracking, photosynthesis D fermentation, photosynthesis

21. Sulfur is burnt in air. Which statement about this reaction is correct?
A The gas formed turns aqueous potassium dichromate (VI) from green to orange.
B The product is used as a food preservative.
C The reaction is endothermic. D The reaction is reversible.

22. Bond breaking is an endothermic process and bond making is an exothermic process. For which change is
it not possible, from the equation, to deduce whether the reaction is endothermic or exothermic?
A Cl2(g) →2Cl (g) B H2(g) + Cl2(g) →2HCl (g)
C H2O(g) →2H(g) + O(g) D H(g) + Cl(g) →HCl(g)

23. It has been suggested that the cars of the future could be powered by fuel cells. One type of fuel
cell uses the chemical reaction between oxygen and hydrogen to produce electricity.
What would be a disadvantage of using this type of fuel cell to power a car?
A A car cannot be powered by electricity.
B The hydrogen tank might split in an accident, leading to an explosion.
C The product of the reaction between oxygen and hydrogen is toxic.
D The oxygen would need to be obtained from air.

24. Why does an increase in pressure increase the rate of reaction between the gases nitrogen and
hydrogen in the manufacture of ammonia?
A The activation energy is lowered. B The molecules collide more frequently.
C The molecules have more energy. D The reaction is more exothermic.

25. What is the effect of a catalyst on the activation energy and on the enthalpy change, ΔH, of a reaction?

26. A researcher notices that atoms of an element are releasing energy. Why are the atoms releasing energy?
A The atoms are absorbing light. B The atoms are evaporating.
C The atoms are radioactive. D The atoms react with argon in the air.
27. Increasing the number of atoms in one molecule of a hydrocarbon increases the amount of energy
released when it burns. What is the correct order

28. Which statements about exothermic and endothermic reactions are correct?
1 During an exothermic reaction, heat is given out.
2 The temperature of an endothermic reaction goes up because heat is taken in.
3 Burning methane in the air is an exothermic reaction.
A 1, 2 and 3 B 1 and 2 only C 1 and 3 only D 2 and 3 only
29. A power station was designed to burn gaseous fuels only. Which two substances could be used?

A carbon dioxide and hydrogen B carbon dioxide and 235U

C hydrogen and methane D methane and 235U

30. The diagrams show the difference in energies of the reactants and products in two types of reaction.

Which diagram and which type of energy change apply to a fuel burning in air?

31. Some reactions are listed.

methane + oxygen → carbon dioxide + water
sodium + water → sodium hydroxide + hydrogen
magnesium + hydrochloric acid → magnesium chloride + hydrogen
Which word correctly describes all of these reactions?
A combustion B endothermic C exothermic D neutralisation
32.The diagram shows a match.

By striking the match, a chemical reaction takes place. Which statements about the chemical reaction are
correct?

33. Which process is not exothermic?

A burning a fossil fuel B obtaining lime from limestone
C radioactive decay of 235U D reacting hydrogen with oxygen

34. Nitrogen monoxide is an atmospheric pollutant that is formed in car engines by the reaction between
nitrogen and oxygen.
N2(g) + O2(g) → 2NO(g) ΔH = +66 kJ / mol
Which diagram represents the energy profile for this reaction?

Structured Questions
A 1. Hydrogen-oxygen fuel cells are used to generate electricity.

The overall reaction in a hydrogen-oxygen fuel cell is shown below.

2H2(g) + O2(g) 2H2O(g)

This reaction is exothermic.

(a) Explain the meaning of the term exothermic.
..........................................................................................................................................

..................................................................................................................................... [1]

(b) Explain, in terms of the energy changes associated with bond breaking and bond forming, why the reaction
is exothermic.
..........................................................................................................................................

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

..................................................................................................................................... [2]
(c) A hydrogen-oxygen fuel cell uses 2000 dm3 of hydrogen measured at room temperatureand pressure.
Calculate the volume of oxygen, measured at room temperature and pressure, used by the fuel cell.
[One mole of any gas at room temperature and pressure occupies a volume of 24 dm 3.]
..........................................................................................................................................

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

volume of oxygen = ......................................... dm3 [2]

(d) The electrode reactions in an oxygen-hydrogen fuel shell are shown below.

Equation 1 O2 (g) + 2H2O (l) + 4e– 4OH–(aq)

Equation 2 H2 (g) + 2OH– (aq) 2e– + 2H2O(l)

Explain why the reaction in a fuel cell involves both oxidation and reduction.

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

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

..................................................................................................................................... [2]

(e) Name one source of the hydrogen needed for a fuel-cell.

..................................................................................................................................... [1]

(f) State one advantage and one disadvantage of using an oxygen-hydrogen fuel cell.
advantage ........................................................................................................................

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

disadvantage ...................................................................................................................

..................................................................................................................................... [2]
[Total: 10]

A 2.Nitric oxide, NO, is an atmospheric pollutant formed inside car engines by the reaction between nitrogen
and oxygen.
N2(g) + O2(g) 2NO(g) ΔH = +66 kJ mol–1
This reaction is endothermic.

(a) Explain the meaning of the term endothermic.

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

...................................................................................................................................... [1]

(b) Complete the energy profile diagram for the reaction between nitrogen and oxygen.
On your diagram label the
• product, • activation energy, Ea, • enthalpy change for the reaction ΔH.

[3]
(c) Calculate the mass of nitric oxide formed when 100 g of nitrogen reacts completely with oxygen.
mass of nitric oxide = ................................... g [3]
(d) Explain how the speed of reaction between nitrogen and oxygen changes when the pressure of the
gaseous mixture is increased from 1 atmosphere to 10 atmospheres.
..........................................................................................................................................

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

...................................................................................................................................... [3]
[Total: 10]

A 3. (a) Exothermic reactions produce heat energy.

An important fuel is methane, natural gas. The equation for its combustion is as follows.
CH4 + 2O2 CO2 + 2H2O
(i) In chemical reactions bonds are broken and new bonds are formed. Using this reaction give an example of

a bond that is broken,

a bond that is formed. [2]

(ii) Explain, using the idea of bonds forming and breaking, why this reaction is exothermic, that is it produces
heat energy. [2]

A 4.Oxides of nitrogen are atmospheric pollutants. Nitrogen monoxide, NO, is formed in an internal combustion
engine when nitrogen and oxygen react together.

N2(g) + O2(g) 2NO(g)

The diagram shows the energy profile for this reaction.

(a) Identify the energy changes X and Z. [2]

(b) The reaction between nitrogen and oxygen is endothermic.

(i) Explain how you can tell from the diagram that the reaction is endothermic. [1]

(ii) Explain, using ideas about bond breaking and bond making, why the overall reaction is endothermic. [3]

A 5.Hydrazine, N2H4, is a liquid that has been used as a rocket fuel. It reacts with oxygen as shown in the
equation.
N2H4 + O2 N2 + 2H2O
This reaction is highly exothermic.
(a) Suggest why the combustion of hydrazine has very little environmental impact.
..........................................................................................................................................

.................................................................................................................................... [1]

(b) Explain, in terms of the energy changes which occur during bond breaking and bond forming, why the
combustion of hydrazine is exothermic.
..........................................................................................................................................

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

.................................................................................................................................... [2]

(c) (i) Calculate the volume of oxygen, measured at room temperature and pressure, needed to completely
combust 1.00 tonne of hydrazine.
[One tonne is 106 grams. One mole of any gas at room temperature and pressure occupies a volume of 24
dm3.]

volume of oxygen = .............................. dm3 [3]

(ii) A rocket burns hydrazine in an atmosphere of oxygen. Both hydrazine and oxygen are stored in the rocket
as liquids. Suggest why oxygen is stored as a liquid rather than as a gas.
..................................................................................................................................

............................................................................................................................ [1]

(d) Hydrazine, N2H4, has similar chemical properties to ammonia.

(i) Hydrazine reacts with hydrochloric acid. Suggest the formula of the product of this reaction.

............................................................................................................................ [1]

(ii) Hydrazine is a covalent compound. Draw a ‘dot-and-cross’ diagram for hydrazine. [2]

[Total: 10]
A 6. Aqueous hydrogen peroxide is used to sterilise contact lenses.
At room temperature aqueous hydrogen peroxide decomposes very slowly to form water and oxygen.
The decomposition can be represented by the equation below.

(a) Explain why this reaction is exothermic in terms of the energy changes that take place during bond
breaking and bond making. [2]

(b) Draw the energy profile diagram for the decomposition of hydrogen peroxide.
Label on the diagram the activation energy and the enthalpy change. [3]

A 7. Methane, CH4, is used as a fuel. The complete combustion of methane can be represented by the
equation below.

(a) Explain why this reaction is exothermic in terms of the energy changes that take place during bond
breaking and bond making. [3]

(b) Calculate the energy released when 4.0 g of methane is completely combusted. [2]

(c) Draw the energy profile diagram for the complete combustion of methane. Label on the diagram the
activation energy and the enthalpy change. [3]
(d) Draw a ‘dot and cross’ diagram to show the bonding in methane. You only need to draw the outer (valence)
electrons of carbon. [2]

(e) Bond energy is the amount of energy, in kJ, which must be supplied to break one mole of the bond.

Use the data in the table to show that the following reaction is exothermic.

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

...........................................................................................................................................................................[3]

(f) The equation for the complete combustion of ethanol is shown.

Use the bond energies in the table to calculate the energy change, in kJ /mol, for the complete
combustion of ethanol.