Section 1: MCQs
- Identify the exothermic reaction from the following.
a) 2H₂(g) + O₂(g) → 2H₂O(l) + 571.6 kJ b) H₂(g) + I₂(s) → 2HI(g) − 53.08 kJ c) N₂(g) + O₂(g) → 2NO(g) − 180.6 kJ d) Melting of ice - What effect does a catalyst have on activation energy?
a) Increases activation energy b) Decreases activation energy c) Stops the reaction d) Does not affect the reaction - Which statement correctly describes an endothermic reaction?
a) A reaction where energy is released b) A reaction that absorbs heat from the surroundings c) A reaction that produces heat d) A reaction that needs no energy - Which statement about bond formation is correct?
a) It is an exothermic process b) It is an endothermic process c) It absorbs energy d) It does not release energy - Which of the following is the main product of anaerobic respiration?
a) Oxygen b) Ethanol and carbon dioxide c) Water d) Glucose - In what way does enthalpy differ from heat?
a) Heat is a form of energy, while enthalpy is the total heat content of a system b) Heat is measured in joules, while enthalpy is measured in calories c) Heat is essential to a system, while enthalpy is not d) Heat is absorbed, while enthalpy is released - Which of the following reactions has the lowest activation energy?
a) H₂(g) + ½O₂(g) → H₂O(g) b) C(s) + O₂(g) → CO₂(g) c) NaCl(aq) + AgNO₃(aq) → AgCl(s) + NaNO₃(aq) d) H₂(g) + I₂(s) → 2HI(g) - In a chemical reaction, what do the surroundings refer to?
a) Only the air b) Anything outside the system c) Only the reactants d) Only the products - During an exothermic reaction, what happens to the surroundings?
a) They get cooler b) They get warmer c) They remain the same d) They absorb energy - Which option correctly defines enthalpy?
a) The energy required to break a bond b) The total amount of heat energy present in a system c) The energy released during a reaction d) The energy absorbed during a reaction - Which statement explains how self-heating or self-cooling packs work?
a) They contain reactants that undergo exothermic or endothermic reactions b) They contain catalysts that speed up reactions c) They contain enzymes that break down food d) They contain lipids that store energy - Which of the following makes up the system in a chemical reaction?
a) Only the reactants b) Only the products c) Reactants, products, catalyst, and solvent d) Only the surroundings - Why does washing clothes at 60°C consume more energy than washing at 30°C?
a) More heat is required to break bonds b) Less heat is needed c) More water is used d) No energy difference exists - What is the energy change when bonds are broken?
a) Energy is absorbed b) Energy is released c) No energy change occurs d) It depends on the reaction - Which method is used to calculate the enthalpy change (ΔH)?
a) By measuring the initial and final temperature b) By adding up all bond energies c) By measuring energy absorbed and released d) By counting the number of molecules - Which statement defines aerobic respiration?
a) Respiration that occurs in the absence of oxygen b) Respiration that occurs in the presence of oxygen c) Respiration that produces ethanol d) Respiration that produces lactic acid - Select the endothermic reaction from the following.
a) 2H₂(g) + O₂(g) → 2H₂O(l) + 571.6 kJ b) C(s) + O₂(g) → CO₂(g) + 393.5 kJ c) H₂(g) + I₂(s) → 2HI(g) − 53.08 kJ d) Combustion of methane - Which statement defines activation energy?
a) The energy released during a reaction b) The energy absorbed during a reaction c) The minimum energy required for a successful collision d) The total energy of the reactants - The reaction H₂ + Cl₂ → 2HCl is exothermic. What is the source of the energy needed to break the bonds of H₂ and Cl₂?
a) By collisions between the molecules b) From sunlight c) From the surroundings d) By collisions of the molecules with the walls of the container - What is the energy efficiency of traditional electric bulbs?
a) 90% of energy is used as light b) 50% of energy is wasted c) They have 100% efficiency d) 90% of energy is wasted as heat - What function does a catalyst perform in a chemical reaction?
a) It increases the activation energy b) It changes the products of the reaction c) It absorbs energy from the reaction d) It decreases the activation energy - Which set lists the products of anaerobic respiration?
a) ATP + CO₂ + H₂O b) CO₂ + H₂ c) ATP + Ethanol + H₂O d) Ethanol + H₂O - Which of the following is the main source of energy in fireworks?
a) Exothermic reactions b) Endothermic reactions c) Catalysts d) Enzymes - How does the activation energy change when a catalyst is added?
a) It increases b) It decreases c) It remains the same d) It becomes zero - Which statement about bond breaking is correct?
a) It is an exothermic process b) It is an endothermic process c) It releases energy d) It does not require energy - What is the function of ATP in the body?
a) It stores energy for long-term use b) It provides immediate energy for metabolic activities c) It acts as a catalyst in reactions d) It is a byproduct of anaerobic respiration - Which of the following defines a system in chemistry?
a) The surrounding environment b) The energy released during a reaction c) The catalyst used in a reaction d) The physical or chemical change under study - Which of the following is an everyday example of an exothermic reaction?
a) Evaporation of water b) Burning wood c) Melting of ice d) Dissolving salt in water - What makes the combustion of fuel an exothermic reaction?
a) It absorbs more energy than it releases b) It releases more energy than it absorbs c) It has no energy change d) It only forms weak bonds - Which statement defines anaerobic respiration?
a) Respiration that occurs in the absence of oxygen b) Respiration that occurs in the presence of oxygen c) Respiration that produces water d) Respiration that produces ATP - Boiling water in a beaker is an example of which type of change?
a) Exothermic change b) Isothermic change c) Adiabatic change d) Endothermic change - Identify the endothermic reaction among the following.
a) Freezing water b) Burning wood c) Decomposition of calcium carbonate d) Condensation of steam - How are the surroundings defined in a chemical reaction?
a) The reactants and products b) The energy absorbed during the reaction c) The catalyst used in the reaction d) Everything outside the system - Which statement describes the transition state in a chemical reaction?
a) The state where reactants are fully converted to products b) The state where products are fully converted to reactants c) The state where no energy is involved d) The state where reactants have the highest energy - The average bond energies of O-O and O=O are 146 and 496 kJ mol⁻¹, respectively. Calculate the enthalpy change in kJ for the reaction H₂O₂ → H₂O + ½ O₂.
a) −102 kJ b) +102 kJ c) +350 kJ d) +394 kJ - What does it indicate when a chemical reaction is exothermic?
a) The bonds which break are weaker than those which are formed. b) The bonds which break are stronger than those which are formed. c) The exothermic nature of the reaction is not concerned with bond formation or bond breakage. d) It shows that the reactants are more stable than the products. - What part does lightning play in the formation of nitric oxide?
a) It provides the necessary activation energy for the reaction b) It acts as a catalyst c) It absorbs energy from the reaction d) It breaks down nitric oxide - What is the value of the bond dissociation energy of H₂?
a) 435 kJ/mol b) 498 kJ/mol c) 484 kJ/mol d) 568 kJ/mol - Which one of the following is an exothermic reaction?
a) Boiling water b) Photosynthesis c) Combustion of fuel d) Electrolysis of water - The temperature rises when NaOH and HCl are mixed. What type of reaction is this?
a) Endothermic with a positive enthalpy change. b) Endothermic with a negative enthalpy change. c) Exothermic with a positive enthalpy change. d) Exothermic with a negative enthalpy change. - Which of the following describes activation energy?
a) The total heat content of a system b) The energy required to start a reaction c) The energy released in an exothermic reaction d) The final energy of the products - Which statement correctly defines an exothermic reaction?
a) A reaction that absorbs heat b) A reaction that evolves heat c) A reaction with no energy change d) A reaction that requires light energy - Which of the following makes use of exothermic reactions?
a) Cooking food b) Melting ice c) Photosynthesis d) Electrolysis - What is the reason power plants rely on combustion?
a) It is an exothermic reaction that produces heat b) It absorbs energy c) It is an endothermic reaction d) It cools down the environment - Which option correctly defines enthalpy (H)?
a) Total amount of heat content in a compound b) A form of light energy c) The energy required to break bonds d) The energy stored in the surroundings - What is the effect of adding a catalyst to a reaction?
a) The reaction speeds up b) The reaction stops c) The energy required increases d) The reaction slows down - The average bond dissociation energy of the C-H bond is 412 kJ mol⁻¹. For which of the following processes will the enthalpy change be close to 412 kJ?
a) CH₄(g) → C(g) + 2H₂(g) b) CH₄(g) → CH₃(g) + H c) CH₄(g) → CH₂(g) + H₂ d) CH₄(g) → CH(g) + H₃ - The change C (Diamond) → C (Graphite), ΔH = −3 kJ mol⁻¹, is exothermic. Why does it not occur?
a) The structure of diamond is more stable than that of graphite. b) Diamond has stronger covalent bonds than graphite. c) The change from diamond to graphite has a high activation energy. d) The density of graphite is less than that of diamond. - What is the energy change when new bonds are formed?
a) Energy is absorbed b) Energy is released c) Energy is neutralized d) The reaction stops - In what form is glucose mainly stored in the body?
a) Lipids b) Glycogen c) ATP d) Pyruvate - What is the enthalpy change for the reaction 2H₂(g) + O₂(g) → 2H₂O(g)?
a) −568 kJ b) +568 kJ c) −284 kJ d) +284 kJ - The transfer of energy from the surroundings to the system is called:
a) Exothermic b) Isothermic c) Adiabatic d) Endothermic - Which hydrogen halide is formed endothermically from its elements?
a) HCl b) HF c) HBr d) HI - Aerobic respiration belongs to which type of reaction?
a) Endothermic b) Exothermic c) Neutral d) Photochemical - Which unit is used to measure enthalpy?
a) J/s b) kJ mol⁻¹ c) Calories d) Watts - What function do lipids perform in the body?
a) They are the primary source of immediate energy b) They serve as an energy reserve c) They are used to produce ATP directly d) They are not involved in energy storage - Enthalpy is expressed in which unit?
a) Joules b) kJ mol⁻¹ c) Calories d) Watts - During an endothermic reaction, what happens to the surroundings?
a) They get cooler b) They get warmer c) They remain the same d) They release energy - In which part of the cell does aerobic respiration take place?
a) Cytoplasm b) Mitochondria c) Nucleus d) Cell membrane - Which type of respiration releases more energy?
a) Anaerobic respiration b) Aerobic respiration c) Fermentation d) None of these - What occurs when energy moves from the surroundings to the system?
a) The change is called exothermic b) The change is called endothermic c) The system cools down d) The surrounding absorbs heat
Short Questions
(i) Differentiate between heat and enthalpy.
(ii) How is enthalpy change measured in a chemical reaction?
(iii) Why do chemical reactions require activation energy?
(iv) Give an example of an exothermic reaction used in daily life.
(v) What happens to energy when a bond is formed?
(vi) Define an endothermic reaction with an example.
(vii) How does energy transfer occur between a system and its surroundings?
(viii) Why do vehicles rely on combustion reactions?
(ix) Write the chemical equation for anaerobic respiration.
(x) How does respiration release energy?
(xi) What is meant by heat content?
(xii) Name one example of a catalyst used in industry.
(xiii) Why is breaking of a bond an endothermic process?
(xiv) What is the role of mitochondria in respiration?
(xv) Who was the first scientist to use the word ‘energy’ in physics?
(xvi) How does activation energy affect the rate of a reaction?
(xvii) Write the chemical equation for aerobic respiration.
(xviii) How does glycolysis produce ATP?
(xix) Why is enthalpy important in chemical reactions?
(xx) How does the combustion of petrol generate energy?
(xxi) Is boiling water an endothermic or exothermic change? Why?
(xxii) Define an exothermic reaction with an example.
(xxiii) Define aerobic respiration.
(xxiv) What is the function of a catalyst in a chemical reaction?
(xxv) What is the role of glycogen in our body?
(xxvi) What is the difference between enthalpy and enthalpy change?
(xxvii) Give an example of an endothermic reaction used in daily life.
(xxviii) Why does melting ice require energy while freezing releases energy?
(xxix) Why does adding a catalyst not affect the enthalpy change (ΔH) of a reaction?
(xxx) Why does lightning cause nitrogen to react with oxygen in the atmosphere?
(xxxi) How are exothermic reactions used in power plants?
(xxxii) What is the significance of energy profile diagrams?
(xxxiii) Why is it essential to cook some food items while others can be eaten raw?
(xxxiv) What happens to the surroundings during an endothermic reaction?
(xxxv) Why is bond formation an exothermic process?
(xxxvi) What is a system in chemistry?
(xxxvii) Why do fireworks look spectacular? What type of chemical compounds undergo chemical reactions during this activity?
(xxxviii) What type of reaction occurs when energy is transferred from surroundings to a system?
(xxxix) What is the role of exothermic reactions in fireworks?
(xl) What is the sign of enthalpy change (ΔH) for an endothermic reaction?
(xli) What is the unit of enthalpy?
(xlii) What happens to energy when a bond is broken?
(xliii) Why does the temperature of a container increase in an exothermic reaction?
(xliv) Why does the reaction between sodium metal and water proceed violently?
(xlv) What is meant by the standard enthalpy of reaction (ΔH°)?
(xlvi) What would happen if all chemical reactions were endothermic?
(xlvii) What is the sign of enthalpy change (ΔH) for an exothermic reaction?
(xlviii) Draw the reaction profiles for two exothermic reactions, one of which moves faster than the other.
(xlix) Define enthalpy (H).
(l) What is meant by surroundings in a chemical reaction?
(51) Why does the reaction between atmospheric gases oxygen and nitrogen not take place under normal conditions, but in the presence of lightning, these gases react to give NO. Does the reaction stop as soon as the lightning stops?
(52) Depict the transition state for the following reaction: H₂ + Cl₂ → 2HCl
(53) What is a transition state in a chemical reaction?
(54) Can an exothermic reaction be reversed? Explain.
(55) Why is the energy of the transition state higher than reactants or products?
(56) How do lipids serve as an energy reserve?
(57) What determines whether a reaction is overall exothermic or endothermic?
(58) Why do strong bonds release more energy when formed?
(59) How does a catalyst affect the activation energy of a reaction?
(60) What is glycogen? Where is it stored in the human body?
(61) Why is enthalpy change (ΔH) important in chemical reactions?
(62) What is activation energy?
(63) Define thermodynamics in the context of chemistry.
(64) Define anaerobic respiration.
(65) What happens when fuels like gas and coal burn?
(66) Why can’t we measure total enthalpy directly?
(67) Why is bond breaking an endothermic process?
(68) Reaction between natural gas (CH₄) and atmospheric oxygen does not take place when mixed, but starts immediately when a burning matchstick is introduced, continuing until the reactants are used up. Explain
Long Questions
- Describe the importance of exothermic reactions in generating energy for daily activities.
- Define system and surroundings. Explain their role in energy transfer with examples.
- Define exothermic and endothermic reactions. Explain with energy profile diagrams.
- Define the following terms: (a) Activation energy (b) Transition state (c) Aerobic respiration.
- Explain the role of lipids in our body.
- Given the bond energies:
H₂ = 436 kJ/mol
I₂ = 151 kJ/mol
HI = 299 kJ/mol
Calculate the enthalpy change for the reaction:
H₂ + I₂ → 2HI
H₂ = 436 kJ/mol
I₂ = 151 kJ/mol
HI = 299 kJ/mol
H₂ + I₂ → 2HI - In an industrial reaction, the decomposition of CaCO₃ absorbs 178 kJ/mol of energy. Determine whether this reaction is endothermic or exothermic and explain why.
- Discuss aerobic and anaerobic respiration with chemical equations.
- Explain the role of combustion reactions in everyday life.
- Explain why the formation of a bond is always an exothermic process.
- Explain the difference between the terms heat and enthalpy.
- What is enthalpy? How is it different from heat? Discuss with examples.
- Find the enthalpy change of the following reaction using the given data:
N₂ + O₂ → 2NO
Bond dissociation energy of N₂ = 958.38 kJ/mol.
Bond dissociation energy of O₂ = 498 kJ/mol.
Bond formation energy of NO = −626 kJ/mol - How is energy transferred in chemical reactions? Explain bond breaking and bond formation with examples.
- Determine the energy required to break all bonds in one mole of water molecules if the bond dissociation energy of O-H is 484 kJ/mol.
- Explain activation energy and transition state with the help of energy profile diagrams.
- Calculate the enthalpy change for the formation of one mole of gaseous water, given:
Bond energy of H₂ = 435 kJ/mol
Bond energy of O₂ = 498 kJ/mol
Bond formation energy of O-H = 484 kJ/mol
Bond energy of H₂ = 435 kJ/mol
Bond energy of O₂ = 498 kJ/mol
Bond formation energy of O-H = 484 kJ/mol - Calculate the enthalpy change for the reaction:
C + O₂ → CO₂
Given that the bond energy of O=O is 498 kJ/mol and the bond energy of C=O is 799 kJ/mol.
ری ایکشن کے لیے اینتھالپی چینج معلوم کریں:
C + O₂ → CO₂
O=O کی بانڈ انرجی 498 کلو جول/مول ہے اور C=O کی بانڈ انرجی 799 کلو جول/مول ہے۔ - How do catalysts affect chemical reactions? Explain with an example.
Answer Key
- a) 2H₂(g) + O₂(g) → 2H₂O(l) + 571.6 kJ
- b) Decreases activation energy
- b) A reaction that absorbs heat from the surroundings
- a) It is an exothermic process
- b) Ethanol and carbon dioxide
- a) Heat is a form of energy, while enthalpy is the total heat content of a system
- c) NaCl(aq) + AgNO₃(aq) → AgCl(s) + NaNO₃(aq)
- b) Anything outside the system
- b) They get warmer
- b) The total amount of heat energy present in a system
- a) They contain reactants that undergo exothermic or endothermic reactions
- c) Reactants, products, catalyst, and solvent
- a) More heat is required to break bonds
- a) Energy is absorbed
- c) By measuring energy absorbed and released
- b) Respiration that occurs in the presence of oxygen
- c) H₂(g) + I₂(s) → 2HI(g) − 53.08 kJ
- c) The minimum energy required for a successful collision
- b) From sunlight
- d) 90% of energy is wasted as heat
- d) It decreases the activation energy
- c) ATP + Ethanol + H₂O
- a) Exothermic reactions
- b) It decreases
- b) It is an endothermic process
- b) It provides immediate energy for metabolic activities
- d) The physical or chemical change under study
- b) Burning wood
- b) It releases more energy than it absorbs
- a) Respiration that occurs in the absence of oxygen
- d) Endothermic change
- c) Decomposition of calcium carbonate
- d) Everything outside the system
- d) The state where reactants have the highest energy
- b) +102 kJ
- a) The bonds that break are weaker than those which are formed.
- a) It provides the necessary activation energy for the reaction
- a) 435 kJ/mol
- c) Combustion of fuel
- d) Exothermic with a negative enthalpy change.
- b) The energy required to start a reaction
- b) A reaction that evolves heat
- a) Cooking food
- a) It is an exothermic reaction that produces heat
- a) Total amount of heat content in a compound
- a) The reaction speeds up
- b) CH₄(g) → CH₃(g) + H
- c) The change from diamond to graphite has high activation energy.
- b) Energy is released
- b) Glycogen
- a) −568 kJ
- d) Endothermic
- d) HI
- b) Exothermic
- b) kJ mol⁻¹
- b) They serve as an energy reserve
- b) kJ mol⁻¹
- a) They get cooler
- b) Mitochondria
- b) Aerobic respiration
- b) The change is called endothermic