Leaving Cert Chemistry — Example Quiz

Mass number is protons plus neutrons: 11 + 12 = 23. Electrons are not counted, as their mass is negligible. The atomic number is 11, which identifies the element as sodium.

A metal loses electrons to form a positive ion and a non-metal gains them to form a negative ion; the resulting electrostatic attraction is an ionic bond. Covalent bonding involves shared electrons between non-metals, and metallic bonding occurs between metal atoms.

Ionic compounds conduct only when the ions are free to move — molten or in solution, not in the solid lattice. They are generally water-soluble and have high melting points because of the strong electrostatic forces throughout the lattice.

Moles = mass ÷ relative molecular mass = 36 ÷ 18 = 2 mol. Each mole of water contains 6.02 × 10²³ molecules, so this sample contains roughly 1.2 × 10²⁴ molecules.

True. Each successive element down a group has an additional occupied energy level, so the outer electrons sit further from the nucleus. Increased shielding by inner electrons also reduces the effective nuclear pull on them.

pH below 7 is acidic, and 3 is strongly so. The scale is logarithmic, so pH 3 has ten times the hydrogen ion concentration of pH 4 and a hundred times that of pH 5.

Higher temperature gives particles more kinetic energy, so collisions are more frequent and more of them exceed the activation energy. A catalyst lowers activation energy. Higher concentration means more frequent collisions. Decreasing surface area slows a reaction down — increasing it speeds it up.

Alkanes are saturated hydrocarbons with the formula CₙH₂ₙ₊₂ — methane is CH₄, ethane C₂H₆. CₙH₂ₙ is the alkene general formula, and CₙH₂ₙ₋₂ is that of the alkynes.

The number of protons is the atomic number, and it defines the element. Changing the neutron count gives an isotope of the same element; changing the electron count gives an ion of the same element. Change the proton count and it is a different element entirely.

Apparatus is rinsed with the solution it will hold, so residual water cannot dilute it. The burette is then filled with the standard solution. Indicator is added to the measured aliquot in the conical flask, the titration is carried out, and the reading at the end point is recorded.

Hydrogen is +1 in most compounds. The exception is in metal hydrides such as sodium hydride (NaH), where the more electronegative hydrogen takes −1. Uncombined H₂ has an oxidation number of 0.

False. A catalyst speeds up the forward and reverse reactions equally, so equilibrium is reached sooner but its position is unchanged — the yield is the same. Only changes in concentration, pressure or temperature shift the position of equilibrium.

Combustion releases heat to the surroundings, so it is exothermic. Melting, photosynthesis and thermal decomposition all absorb energy and are endothermic.

Isotopes have the same atomic number but different mass numbers, which means the same protons and a different number of neutrons. Chlorine-35 and chlorine-37 both have 17 protons; they differ by two neutrons.

Acid plus carbonate gives a salt, water and carbon dioxide, which turns limewater milky. Hydrogen is the gas produced when a dilute acid reacts with a reactive metal, which is the distinction being tested here.

Argon and neon are in group 0 (group 18), with full outer energy levels, which is why they are so unreactive. Chlorine is a halogen in group 7, and nitrogen is in group 5.

The cathode is negative, so it attracts positive ions. Sodium ions are reduced there by gaining an electron to form sodium metal. Chloride ions are oxidised at the anode to give chlorine gas.

True. Chlorine occurs as roughly 75% chlorine-35 and 25% chlorine-37. The relative atomic mass is the weighted average of the isotopic masses, which lands at about 35.5 — no individual chlorine atom has that mass.

The hydroxyl group −OH defines an alcohol, as in ethanol (C₂H₅OH). −COOH is a carboxylic acid, −CHO an aldehyde, and −NH₂ an amine.

Litmus is red in acid and blue in base. A useful memory aid is that both "base" and "blue" begin with B. Universal indicator gives a fuller range, running through green at neutral to purple in strong base.