KimiaChemistry

Experiments that release toxic vapours or pungent gases such as chlorine, ammonia and bromine must be done in a fume chamber. The safety shower and eyewash are for washing after an accident and a fire extinguisher is for putting out fires.

The type of metal is the manipulated variable (what is changed). The volume of hydrogen released in 2 minutes is what is measured, so it is the responding variable. Temperature, acid concentration and acid volume are fixed variables.

Neutrons = nucleon number - proton number = 23 - 11 = 12.

Isotopes have the same number of protons but different numbers of neutrons. P and Q both have 17 protons but 18 and 20 neutrons. R has 18 protons and S has 19, so they are different elements (S, with 18 electrons, is an ion).

Molecules = 0.20 × 6.02 × 10²³ = 1.2 × 10²³. Each NH₃ molecule has 4 atoms, so atoms = 4 × 1.2 × 10²³ = 4.8 × 10²³. 1.2 × 10²³ counts molecules, not atoms.

n(Al) = 2.7 ÷ 27 = 0.10 mol. From the equation 2 mol Al gives 3 mol H₂, so n(H₂) = 0.10 × 3/2 = 0.15 mol. Volume = 0.15 × 24 = 3.6 dm³. 2.4 dm³ uses a 1 : 1 ratio.

5 valence electrons in a main group from 13 to 18 means group = 5 + 10 = 15. Three shells are filled, so it is in Period 3.

Group 1 atoms donate one electron to form M⁺, so the oxide is M₂O (Rb₂O). Down the group reactivity increases, melting point decreases and atomic radius increases, so A, B and D are all wrong.

Transition elements form coloured ions and compounds (e.g. Cu²⁺ blue, Fe³⁺ brown), act as catalysts, have more than one oxidation number and form complex ions. They are hard, dense metals with high melting points.

All Period 3 atoms have three filled shells. Across the period the number of protons increases, so the nuclear charge is higher and pulls the electrons in the same shells closer. Valence electrons increase, not decrease.

CCl₄ is formed from two non-metals (carbon and chlorine) that share electrons, so it has covalent bonds. NaCl, MgO and K₂O are formed from a metal and a non-metal and have ionic bonds.

Mg (2.8.2) donates 2 electrons to become Mg²⁺ (2.8). Each Cl (2.8.7) receives 1 electron to become Cl⁻ (2.8.8), so two Cl atoms are needed: MgCl₂. A metal and a non-metal form an ionic bond by electron transfer, not sharing.

Z is 2.4 (4 valence electrons) and Y is 2.8.6 (6 valence electrons). Both are non-metals, so they share electrons. Z needs 4 more electrons and each Y needs 2, so one Z atom shares two electrons with each of two Y atoms: ZY₂ with two double bonds.

pH = -log [H⁺], so [H⁺] = 10⁻⁴ = 1.0 × 10⁻⁴ mol dm⁻³. 1.0 × 10⁻¹⁰ is the [OH⁻] (pOH = 10).

An insoluble salt is made by double decomposition: two soluble salt solutions, one with Ba²⁺ and one with SO₄²⁻. Barium nitrate and potassium sulphate are both soluble, and they give BaSO₄ plus soluble KNO₃. B uses an insoluble carbonate, C gives barium chloride and D gives soluble BaCl₂.

n(H₂SO₄) = 0.150 × 20.0 ÷ 1000 = 0.0030 mol. The ratio acid : alkali = 1 : 2, so n(NaOH) = 0.0060 mol. V = 0.0060 × 1000 ÷ 0.200 = 30.0 cm³. 15.0 cm³ ignores the 1 : 2 ratio.

Powder has a larger total surface area than lumps of the same mass, so more zinc atoms are exposed to the acid and the frequency of effective collisions rises. Activation energy, acid concentration and particle energy do not change.

n(HCl) = 0.50 × 50 ÷ 1000 = 0.025 mol, giving 0.0125 mol CO₂ = 0.0125 × 24 000 = 300 cm³ (curve I). Curve II ends at 600 cm³, so twice as many moles of acid reacted: the acid concentration was doubled to 1.00 mol dm⁻³. Higher temperature, smaller chips or a catalyst would make the curve steeper but would not change the final volume, because the acid is the limiting reactant.

Borosilicate glass contains boron oxide, which gives it a low expansion coefficient and heat resistance, so it is used for beakers and flasks. Soda-lime glass cracks with sudden temperature changes.

Concrete is strong under compression but weak when stretched. Steel has high stretching strength. Together they resist both compression and stretching (textbook Section 8.4).