1. a. Answer.
Exothermic reaction.
b. Answer.
Endothermic reaction.
c. i. Answer.
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
ii. Answer.
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)
2. Answer.
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.
3. Answer.As we can see the temperature of the reacting mixture rises from 19oC to 27oC, therefore we can infer that the reaction is an exothermic reaction and thus energy is transferred to the surroundings.
4. a. Answer.
MgCO3(s) → MgO (s) + CO2(g)
b. Answer.
When 16.8 g of MgCO3is thermally decomposed, 0.1992 moles of MgCO3 is broken down
7.2
1. a. Answer. i. In instant cold packs to treat sports injuries.
ii. To chill cans of drinks.
b. Answer. It often involves reaction between ammonium nitrate and water which absorbs energy from the surroundings as ammonium nitrate dissolves.
c. i. Answer. (NH4)(NO3)ii. Answer. Used in the agricultural industry as a major component of fertilizers. It provides nitrogen to the soil.
2. a. Answer.
Calcium oxide
b. Answer.
CaO (s) + H2O (l) → Ca(OH)2(s)
c. Answer.
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.
3. a. Answer.
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 surrounding (exothermic reaction). Sodium chloride (common salt) is used as catalyst. This can be used once but it lasts for many hours.
b. Answer.
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.
c. Answer. Disposable hand warmer:
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.d. Answer In food industry, exothermic reactions can be used to design self-heating food cans which can keep the food and drinks hot without providing external heat.
7.3
2. Answer.During a chemical reaction, bonds are broken and for breaking bonds energy is required. Therefore energy transfer takes place from surroundings to the reacting mixture. This makes bond breaking an endothermic reaction. Also, during making of bonds energy is released to the surroundings and thus bond making is exothermic reaction.
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.
b. Answer.
H2-⇡-C-⇡-H2 + O-⇡-O → O-C-O + H-O-H
O-⇡-O H-O-H
(⇡- Heat)
No. of bonds broken-6
No. of bonds formed- 6
Type of bonds: Covalent bonds.
7.4
1. Answer Endothermic reaction
2. Answer The energy required to break the bond between two atoms is called the bond energy for that bond. It is measured in kJ/mol.
3. Answer.
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.
4. Answer.
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
b. Answer 2H2 + O2 → 2H2O
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.
7.5
1. Answer.
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.
2. a. Answer.
b. Answer.
The metal which is reduced is iron.
c. Answer.
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]
3. Answer.
Two disadvantages of dry cell:
i. Once discharged, they cannot be recharged and has to be disposed off. Modern cells are rechargeable when discharged.
ii. The amount of power supplied is less than the modern cells.
1. a. Answer.
Hydrogen and oxygen.
b. Answer.
Water.
c. Answer Hydrogen + oxygen → water
2H2 + O2 → 2H2O
2. Answer 2H2(g) + 4OH–(aq) → 4H2O (l) + 4e–
O2(g) + 2H2O (l) + 4e–→ 4OH–(aq)
2H2(g) + O2(g) → 2H2O (l)
3. Answer Electrical car uses batteries which convert chemical energy into mechanical energy and make the car move. During this process no fossil fuels are burnt and no carbon dioxide is released into the atmosphere. However, making of batteries require electrolysis dependent on non-renewable sources of energy which releases carbon dioxide to the atmosphere. Thus, electrical cars that run on batteries indirectly contribute to global warming. Since, chemical energy is converted to mechanical energy and no fossil fuels are burnt, therefore they don’t directly contribute to global warming.
C7 Summary Questions
1. a. Answer.
Temperature vs time graph going first down and then back to starting temperature.
b. Answer.
Labeling / explanation: energy transferred into reaction mixture from surroundings so temperature falls, when reaction finished temperature rises as energy is transferred back to surroundings as reaction mixtures warms to room temperature, then remains at that temperature.
2. a. Answer.
Similar to C 7.3 Figure 2,with reactants nitric acid + sodium hydroxide and products sodium nitrate + water.
b. Answer.Similar to C 7.3 Figure 3,with reactants NH4NO3(s) + H2O(l) and products NH4+(aq) + NO3−(aq).
3. a. Answer.C12H22O11 + 12O2 → 12CO2 + 11H2O
b. Answer.
Break the bonds in reactants / to supply activation energy.
c. Answer.
Making new C=O bonds in carbon dioxide and H–O bonds in water transfers energy to surroundings.
d. Answer.
85 KJ.
a. Answer.
210 kJ, transferred to surroundings.
b. Answer.
Energy transferred into reaction mixture from surroundings (endothermic) in breaking bonds in hydrogen peroxide < energy transferred into surroundings (exothermic) when new bonds in water and oxygen are made.
a. i. Answer.
1st column = C
2nd column = A
3rd column =1.6
ii. Answer.
Similar to C 7.5, Figure 2, change zinc to C, and copper to A.
b. Answer.
C,B,A.
c. Answer.
A.
6.a. Answer.
Any soluble alkali, e.g., sodium hydroxide, NaOH / potassium hydroxide, KOH / lithium hydroxide, LiOH.
b.i. Answer.
2H2 + 4OH− à H2O + 4e−
ii. Answer.
Oxidation.
c. i. Answer.
Oxygen.
ii. Answer.
2H2 (g) + O2 (g) à 2H2O (l)
iii. Answer.
CH4 (g) + 2O2 (g) à CO2 (g) + 2H2O (l).
iv. Answer.
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.
C7 Practice Questions
01.1. Answer.
The line goes up before it goes down Ea correctly labelled (ignore direction of arrow if any) energy released labelled (arrow must point downwards).
01.2. Answer.
Energy is required to break bonds energy is released when bonds are formed more energy is released (in forming bonds in the products) than was taken in to break the bonds (in the reactants).
02.1. Answer.
Covalent bonding is (electrostatic) attraction between (the protons in) the two nuclei and the shared pair(s) of electrons energy is needed to break this attraction.
02.2. Answer.
Energy change = -185 kJ / mol
03.1. Answer
Potassium is too reactive or potassium would react rapidly with the water / salt solution the reaction would be dangerous / unsafe.
03.2. Answer.
the concentration of the salt solution
the temperature
03.3. Answer.
Any value between 0.80V and 1.09V.
03.4. Answer.
Silver is less reactive than copper therefore electrons flow in the opposite direction (through the voltmeter).
03.5. Answer.
Cu2+ + 2e– → Cu
Zn → Zn2+ + 2e–
(Cu2+ ions are reduced because) electrons are gained.