Chemical Energetics

Chemistry · 11th Class (Intermediate Part 1) · BISE Lahore

70 questions

Tests for this chapter

Tests run in your chosen medium — the sample below shows both.

Chemical Energetics Test

10 questions · Chemical Energetics × 10

Full Book Test

20 questions · Periodic Table and Periodic Properties × 1, Atomic Structure × 1, Chemical Bonding × 2, Stoichiometry × 1, States and Phases of Matter × 1, Chemical Energetics × 2, Reaction Kinetics × 1, Chemical Equilibrium × 1, Acid-Base Chemistry × 1, Electrochemistry × 2, Hydrocarbons × 2, Nitrogen and Sulfur × 1, Halogens × 1, Atmosphere × 1, Basic Separation Techniques × 1, Lab Safety and Practical Skills × 1

Sample questions

Showing 10 of 70 questions from this chapter. The full bank is in the chapter test below.

1
The sum of all the possible potential and kinetic energies of a system is called its
- A.activation energy
- B.entropy
- C.enthalpy (correct answer)
- D.lattice energy
Why: Enthalpy, also called heat content, is the sum of all possible potential and kinetic energies of a system. Activation energy is only the minimum energy needed to start a reaction, while entropy measures the disorder of a system.
2
In a reaction in which the total heat content of the products is lower than that of the reactants, the reaction
- A.evolves heat and has a negative $\Delta H$ (correct answer)
- B.absorbs heat and has a positive $\Delta H$
- C.evolves heat and has a positive $\Delta H$
- D.absorbs heat and has a negative $\Delta H$
Why: $\Delta H$ = HP - HR, so when the products have the lower heat content the difference is negative and heat is given out to the surroundings. Absorbing heat with a positive $\Delta H$ happens when the products have the higher heat content.
3
When ammonium chloride is dissolved in water the mixture becomes cold and $\Delta H$ = +16.2 kJ/mol. This substance is used in cold packs because
- A.it releases heat to the surroundings as it dissolves
- B.it dissolves only at very low temperatures
- C.it reacts with water to produce a gas
- D.it absorbs heat from the surroundings as it dissolves (correct answer)
Why: A positive $\Delta H$ means the dissolution is endothermic, so heat is taken from the surroundings and a cooling sensation is produced. A substance that releases heat would warm the pack instead of cooling it.
4
Which of the following changes is endothermic?
- A.Combustion of carbon in oxygen
- B.Dissolution of ammonium chloride in water (correct answer)
- C.Neutralization of hydrochloric acid with sodium hydroxide
- D.Formation of water from hydrogen and oxygen
Why: Dissolving ammonium chloride absorbs heat, so $\Delta H$ is +16.2 kJ/mol and the process is endothermic. Combustion, neutralization and the formation of water from its elements all release heat.
5
The minimum amount of energy that reactant molecules must possess to cross the energy barrier during a collision is called the
- A.enthalpy of reaction
- B.bond dissociation energy
- C.activation energy (correct answer)
- D.lattice energy
Why: Activation energy, Ea, is the minimum energy the colliding reactant molecules need to overcome the energy barrier. It is read from an energy profile diagram as the height from the reactants up to the peak, not from reactants to products.
6
On an energy profile diagram, the activation energy of a reaction is measured as the energy difference between
- A.the reactants and the products
- B.the reactants and the highest point of the curve (correct answer)
- C.the products and the highest point of the curve
- D.two successive peaks on the curve
Why: The energy barrier lies between the reactants and the products, so Ea is the gap from the reactant level up to the top of the curve. The gap between reactants and products is $\Delta H$ , not Ea.
7
For the reaction $\ce{N2(g) + 3H2(g) -> 2NH3(g)}$ , the enthalpy change is obtained from an energy diagram as
- A.enthalpy of reactants divided by enthalpy of products
- B.enthalpy of reactants minus enthalpy of products
- C.sum of the enthalpies of reactants and products
- D.enthalpy of products minus enthalpy of reactants (correct answer)
Why: Enthalpy change is defined as HP minus HR, which for this reaction is negative because ammonia has the lower heat content. Reactants minus products would give the same magnitude with the opposite sign.
8
Enthalpy changes of different reactions are compared under standard conditions, which are
- A.298 K and 1 atm (correct answer)
- B.273 K and 1 atm
- C.298 K and 2 atm
- D.273 K and 2 atm
Why: Standard conditions are 25 °C (298 K) and 1 atm (101 kPa). A temperature of 273 K or a pressure of 2 atm is not the standard set of conditions.
9
By definition, the standard enthalpy change of formation of an element in its standard state is
- A.equal to its enthalpy of combustion
- B.always negative
- C.zero (correct answer)
- D.equal to its enthalpy of atomization
Why: No enthalpy change is involved in forming an element from itself in its most stable form, so $\Delta H_{f}$ is taken as zero for every element in its standard state. Values such as equal to the enthalpy of atomization apply to a change of state, not to formation.
10
Which equation represents the standard enthalpy change of formation of $\ce{C2H4}$ ?
- A. $\ce{2C(diamond) + 2H2(g) -> C2H4(g)}$
- B. $\ce{2C(graphite) + 2H2(g) -> C2H4(g)}$ (correct answer)
- C.C(graphite) + $\ce{H2(g)}$ → 1/2 $\ce{C2H4(g)}$
- D. $\ce{2C(graphite) + 4H(g) -> C2H4(g)}$
Why: Formation means one mole of the compound made from its elements in their standard states, so carbon must be graphite and hydrogen must be $\ce{H2}$ gas. Hydrogen atoms and the diamond allotrope are not standard states.

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