QCM : Fundamentals of Chemical Reactions and Energy — 8 questions

Questions et réponses du QCM

1. What is the main purpose of balancing a chemical equation?

To ensure the number of atoms for each element is the same on both sides of the equation
To predict the products formed in a reaction
To verify the reaction occurs at the correct temperature
To calculate the energy change during the reaction

To ensure the number of atoms for each element is the same on both sides of the equation

Explication

The primary purpose of balancing a chemical equation is to ensure the conservation of matter, which means the same number of atoms for each element appears on both sides of the equation. This reflects the law of conservation of mass. The other options relate to different aspects of chemical reactions but are not the main purpose of balancing equations.

2. What is the main principle behind balancing a chemical reaction equation?

Matter is conserved
Atoms can be created or destroyed
Subscripts can be changed to balance
Only oxygen atoms need balancing

Matter is conserved

Explication

Balancing equations is based on the principle of matter conservation, meaning atoms are neither created nor destroyed during a reaction, so their counts must be equal on both sides.

3. What is a key feature that distinguishes a redox half equation from the overall redox reaction?

It shows the transfer of electrons explicitly.
It does not include any electrons.
It balances atoms but not charge.
It only applies to oxidation processes.

It shows the transfer of electrons explicitly.

Explication

A redox half equation explicitly shows the transfer of electrons, either as products or reactants, which is essential for representing oxidation or reduction processes separately from the overall reaction.

4. Why should subscripts in chemical formulas remain unchanged when balancing equations?

Subscripting affects atom counts
Changing subscripts alters chemical identities
Coefficients are easier to adjust
Subscripts are only for ions

Changing subscripts alters chemical identities

Explication

Subscripts define the chemical identity of compounds; changing them would alter the substance itself, unlike coefficients which are adjusted for balancing.

5. In the reaction 2Fe + O₂ → 2FeO, what is being balanced?

Iron (Fe) atoms
Oxygen atoms
Subscripts in formulas
Molecular weights

Oxygen atoms

Explication

Oxygen atoms are balanced in this reaction by adjusting the oxygen molecules (O₂) to match the oxygen in iron oxide.

6. Which statement correctly describes a redox half equation?

It shows atom balancing only
It represents either oxidation or reduction process separately
It combines oxidation and reduction in one step
It only applies to acids

It represents either oxidation or reduction process separately

Explication

A half equation separately represents either oxidation (loss of electrons) or reduction (gain of electrons), not both together.

7. In redox reactions, what happens during oxidation and reduction?

Oxidation gains electrons, reduction loses electrons
Both processes involve the loss of electrons
Oxidation involves loss of electrons, reduction involves gain
Electrons are not involved in redox reactions

Oxidation involves loss of electrons, reduction involves gain

Explication

Oxidation involves the loss of electrons, while reduction involves the gain of electrons, which is fundamental to redox reactions.

8. Which of the following correctly describes Avogadro’s constant?

It is approximately 6.02 x 10²³
It is the number of molecules in one mole
It measures the mass of one atom
It is used to calculate the energy of reactions

It is approximately 6.02 x 10²³

Explication

Avogadro’s constant, approximately 6.02 x 10²³, is the number of particles (atoms, molecules, ions) in one mole of a substance.

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Chemical Reaction Balancing — principle?

Matter is conserved; only coefficients are changed.

Matter is conserved — principle?

Atoms are neither created nor destroyed in reactions.

Redox Half Equations — electrons?

Show electron transfer; oxidation loses, reduction gains electrons.

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