This page contains the AQA GCSE Chemistry C7 Energy changes Questions and kerboodle answers for revision and understanding Energy Changes.This page also contains the link to the notes and video for the revision of this topic.
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Exothermic reaction is a reaction that transfers energy to its surroundings.
Endothermic reaction is a reaction that takes in energy from the surroundings.
Combustion of methane gas. Methane burns and gets oxidised releasing the energy to its surroundings.
CH4(g) + 2O2(g) → CO2(g) + 2H2O (l)
Neutralisation reaction between sulfuric acid and potassium hydroxide. H2SO4 + 2KOH → K2SO4 + 2H2O
Thermal decomposition of calcium carbonate to form calcium oxide and carbon dioxide.
CaCO3(s) → CaO (s) + CO2(g)
Reaction between ammonium chloride (NH4Cl) is and water in a test tube, the tube becomes colder than before.
NH4Cl (s) + H2O (l) → NH4Cl (aq)
Since, dissolving of potassium nitrate in water is an endothermic process i.e. it absorbs energy from the environment. Thus, when you hold the beaker of water in your hand you will sense a cooling effect in your hand as the energy is transferred to the reacting substances.
MgCO3(s) → MgO (s) + CO2(g)
When 16.8 g of MgCO3is thermally decomposed, 0.1992 moles of MgCO3 is broken down
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Calcium oxide is used as a base in the self heating coffee cans
CaO (s) + H2O (l) → Ca(OH)2(s)
It is important that coffee stays out of contact with calcium oxide because calcium oxide will then react with water that has been added to coffee and will make it unfit for drinking.
Hand warmers which can be used once makes use of oxidation of iron into hydrated iron (III) oxide during which energy is transferred to the surroundings (exothermic reaction). Sodium chloride (common salt) is used as catalyst. This can be used once but it lasts for many hours.
Reusable hand warmers involve formation of crystal from saturated salt solutions. Usually the salt is sodium ethanoate CH3COO-Na+.
Supersaturated solution of salt is prepared by dissolving large amount of salt in hot water and is allowed to cool. There is a metal disc in the plastic packet which when pressed releases small particles of the metal which is required to start crystallisation reaction. The crystals spread throughout the solution and transfer energy to the surrounding in an exothermic change. This lasts for 30 Minutes. In order to reuse, the pack is put in boiling water which re-dissolves the crystals. Once it is cool, one can reuse it.
Advantage: It lasts for hours i.e. longer duration than reusable hand warmer.
Disadvantage: It cannot be used again and once used is waste.
Reusable hand warmer: Advantage: It can be used more than once. It is easy to start and stop the reaction.
Disadvantage: The heat only last for 30 min.
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a) It is an exothermic reaction:
b) It is an endothermic reaction
In an exothermic reaction, more energy is released in making the bonds of the products than it is used up in breaking the bonds of the reactants.
In an endothermic reaction, more energy is required to break the bonds of the reactants than it is released in making the bonds of the products.
3.a. Answer.
During a chemical reaction, the chemical bonds between atoms and ions in the reactants are broken and new chemical bonds are formed to make bonds. Breaking of bonds require energy and energy is absorbed from the environment. Thus bond breaking is an endothermic reaction.
H2-⇡-C-⇡-H2 + O-⇡-O → O-C-O + H-O-H
O-⇡-O H-O-H
(⇡- Heat)
No. of bonds formed- 6
Type of bonds: Covalent bonds.
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Energy required to break 1 mole of oxygen gas = 498 kJ/mol
1 mole of O2= 16g
Energy required for 16g of O2= 498 kJ/mol
For 1 g of O2= 498/ 16 kJ/mol
For 0.0960 g = (498/16) * 0.0960 =2.988 kJ mol will be required.
H2 (g) + Cl2 (g) → 2HCl (g)
Bond energy to break H-H =436 kJ/mol
Bond energy to break Cl-Cl =243 kJ/mol
Energy required to break H2and Cl2=436 kJ/mol + 243 kJ/mol=679 kJ/mol
Energy required to form 2 mole of HCl =2*432=864 kJ/mol
Overall energy change =679 kJ/mol – 864 kJ/mol=– 185 kJ/mol
–185 kJ/mol of energy is transferred to the surroundings. Therefore, the reaction is exothermic reaction
Bond energy to break 2 H-H = 2 * 436 kJ/mol =872 kJ/mol
Bond energy to break O-O =498 kJ/mol
Energy required to break 2H2and O2=872 kJ/mol + 498 kJ/mol=1370 kJ/mol
Energy required to form 2 mole of H2O (4 * H-O)=4* 464=1856 kJ/mol
Overall energy change =1370 kJ/mol – 1856 kJ/mol= –486 kJ/mol
-486 kJ/mol of energy is transferred to the surroundings. Therefore, the reaction is exothermic reaction.
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In a cell, two metals with the different order of reactivity are required. The difference in their reactivity will allow the flow of electrons and thereby generating voltage. If both the electrodes are made up of zinc then there will be no difference in reactivity, no flow of electrons and no current will be generated.
1.Answer.
The metal which is reduced is iron.
Zinc is more reactive and thus will act as electron donor. This will the negative terminal.
Reduction reaction: Fe2+(aq) + 2e– → Fe (s) [Fe2+reduced to Fe]
Oxidation reaction: Zn (s) → Zn2+(aq) + 2e–[Zn oxidised to Zn2+ions]
Two disadvantages of dry cell:
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1.a. Answer.
Hydrogen and oxygen are pumped into the fuel cell.
Water is the waste product for the fuel cell.
Hydrogen + oxygen → water
2H2 + O2 → 2H2O
2H2(g) + 4OH–(aq) → 4H2O (l) + 4e–
O2(g) + 2H2O (l) + 4e–→ 4OH–(aq)
2H2(g) + O2(g) → 2H2O (l)
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Temperature vs time graph.
The reaction starts at room temperature and since it is an endothermic reaction the temperature decreases with time as heat is taken from the surroundings. After the reaction is completed, the reaction takes heat from the surrounding and the temperature increases to the room temperature.
1.Answer.
The reaction starts at room temperature and since it is an endothermic reaction the temperature decreases with time as heat is taken from the surroundings. After the reaction is completed, the reaction takes heat from the surrounding and the temperature increases to the room temperature.
2.Answer. Endothermic Reaction reaction profile diagram
3.a. Answer. Sucrose oxidized with oxygen to form carbon dioxide and water.
C12H22O11 + 12O2 → 12CO2 + 11H2O
Energy is needed to break the bonds of the reactant. The bond breaking is an exothermic reaction. The energy is also required to provide activation energy to the reaction.
In the reaction above carbon dioxide and water is formed and bond making releases energy to the surrounding as bond making is an exothermic reaction. So forming C=O bonds and O-H bonds release energy.
1700 KJ of energy is required to break 100 g of Sugar
So 1 g of sugar will require 17 KJ of energy.
So 5 g of sugar will require 85 KJ of energy.
Bonds in the reactants : 4 (O-H)
=464*4
=1856 KJ/mol
2(O-O) = 2*144
=288 KJ/mol
Total Energy of the reactants = 2144 KJ/mol
Bonds in the products : 4 (O-H)
=4*464
=1856 KJ/mol
Bonds in the oxygen: (O=O)
=498
=498 KJ/mol
Total Energy of the products = 2354 KJ/mol
So Energy transferred to the surroundings = 2144-2354
=-210 KJ/mol
210 kJ, transferred to surroundings.
It is an exothermic reaction as more energy is released in making the bonds of the products then used up in breaking the bonds of the reactants.
Donator of electrons (attached to the negative terminal of the voltmeter) = C
Acceptor of electrons (attached to the positive terminal of the voltmeter) = A
Voltage in volts =1.6
C>B>A.
A is the least powerful reducing agent.
6.a. Answer.
In an alkaline hydrogen fuel cell, the alkali used as an electrolyte is sodium hydroxide, NaOH.
b.i. Answer. Reaction is the fuel where hydrogen generated electrons.
2H2 + 4OH− H2O + 4e−
Oxidation.
Oxygen is another gas with hydrogen which is required to operate the fuel cell.
2H2 (g) + O2 (g) = 2H2O (l)
iii. Answer. In the methane fuel cell, methane is oxidised to form carbon dioxide and water releasing energy.
CH4 (g) + 2O2 (g) = CO2 (g) + 2H2O (l).
Only waste product is water, whereas methane fuel cell also produces carbon dioxide (a greenhouse gas that contributes to global warming), methane is non-renewable fossil fuel, whereas hydrogen made using renewable sources of electricity.
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01.1. Answer.
01.2. Answer.
Energy is required to break the bonds of the reactants and the energy is released when bonds are formed. If more energy is released (in forming bonds in the products) than was taken in to break the bonds (in the reactants) then the reaction is exothermic. In the above reaction, less energy is used in breaking the bonds of ethene and bromine and more energy is released in making the bonds of bromoethane so the reaction is exothermic.
02.1. Answer.
Covalent bond is formed by the electrostatic force of attraction between the nucleus of each atom and the shared pair of electrons. Since there is an electrostatic forces of attraction that needs to be broken down in order to break the bond therefore energy is required to break the covalent bond.
02.2. Answer.
Energy required to break the bonds of the reactants = (H-H) + (Cl-Cl)
=436 + 243
= 679 KJ/mol
Energy required to make the bonds of the products = 2(H-Cl)
= 2*432
=864
So Energy change of the reaction = Reactant energy – Products energy
= 679 – 864
= -185 KJ/mol
Energy change = -185 kJ / mol
03.1. Answer
Potassium is highly reactive metal. If potassium is used in the above reaction the reaction will be too explosive as potassium will readily react with water making the reaction dangerous.
03.2. Answer.
In the above experiment,
the concentration of the salt solution and the temperature of the reaction mixture should be controlled.
03.3. Answer.
The Voltage produced will be intermediate between iron and zinc so 0.90 V (approx)
03.4. Answer.
As silver is less reactive than copper the electrons will move in the opposite direction generating a negative voltage.
03.5. Answer.
Half equations when zinc and copper are connected :-
Cu2+ + 2e– → Cu
Zn → Zn2+ + 2e–
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This page contains the detailed and easy notes for AQA GCSE Chemistry Energy Changes for revision and understanding Energy Changes.
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Law of conservation of energy states that energy is neither created nor destroyed. It just converts from one form to another.
| Exothermic Reactions | ENDOTHERMIC REACTIONS |
| Reactions that releases heat to the surroundings. | Reactions that takes in heat from the surroundings |
| The reaction is accompanied by increase in temperature of the surroundings as the heat is released. | The reaction is accompanied by decrease in temperature of the surroundings as the heat is absorbed. |
| The product have the lower energy than the reactants. | The products have higher energy than the reactants. |
| Example: Combustion reaction and Respiration | Example: Thermal decomposition and -Photosynthesis |
| In terms of bond breaking the energy released in making the product is more than energy used up in breaking the reactants. | In terms of bond breaking the energy used in breaking the bonds of reactants is more than the energy released in making up of the products. |
| Used in self heating cans and hand warmers | Used in Ice packs made for sports injuries |
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Exothermic Reaction
Endothermic Reaction
Defining Exothermic and Endothermic in terms of Bond making and Breaking
| Reactants | Products | ||
| Bonds are always broken in a reaction | Bonds are always made in a reaction. | ||
| Endothermic | Breaking of bonds of the reaction takes in heat | > | Making of bonds of the products Releases heat. |
| Exothermic | Breaking of bonds of the reaction takes in heat | < | Making of bonds of the products releases heat. |
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A2 + B2 2AB
A-A + B-B 2[A-B]
221+ 325 2 x 425
546 850
| Bond | Energy |
| A-A | 221 |
| B-B | 325 |
| A-B | 425 |
Energy used in breaking the bonds – 546 KJ/mol
Energy released in making the bonds – 850KJ/mol
Energy associated in overall reaction – 546- 850 = -304 KJ/mol
Is it Exothermic or Endothermic – Exothermic as energy is released in products is greater
At the Negative Electrode
2H2(g) + 4OH–(aq) 4H2O(l) + 4e–
At the Positive Electrode
O2 + 2H2O + 4e– 4 OH–(aq)
Overall – 2H2(g) + O2 (g) 2H2O(l)
ADVANTAGES
DISADVANTAGES
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Exothermic Reaction – The reaction which gives out heat to the surroundings. Example: Respiration or Combustion
Endothermic Reaction – The reactions which taken in heat from the surrounding. Example: Photosynthesis or Thermal decomposition
Reaction Profile – Diagramatic representation showing the relative energies of reactants and products in a reaction.
Activation – Minimum energy required to start a reaction.
Bond Energy – Heat energy contained in a bond between two atoms.
Chemical Cells – A device that converts chemical energy into electrical energy.
Fuel Cells – A chemical cells that uses a reaction between hydrogen and oxygen to form water and energy.
Batteries – collection of cells.
Label the reaction profile and classify it as Exothermic or Endothermic
ADVANTAGES
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DISADVANTAGES
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Disclaimer:
I have tried my level best to cover the maximum of your specification. But this is not the alternative to the textbook. You should cover the specification or the textbook thoroughly. This is the quick revision to help you cover the gist of everything. In case you spot any errors then do let us know and we will rectify it.
References:
BBC Bitesize
Wikipedia
Wikimedia Commons
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