The Nature of Substances & Chemical Reactions (WJEC GCSE Science (Double Award): Chemistry): Flashcards

True.

The seven diatomic elements are H₂, N₂, O₂, F₂, Cl₂, Br₂ and I₂. All seven contain non-metal atoms only.

In a chemical formula, the subscript number after a chemical symbol shows the number of atoms of that element present in the molecule.

One molecule of H₂SO₄ contains 7 atoms in total: 2 hydrogen atoms, 1 sulfur atom and 4 oxygen atoms.

An ionic compound consists of a giant lattice of positive and negative ions held together by strong electrostatic forces of attraction. Ionic compounds commonly form when a metal (such as sodium) reacts with a non-metal (such as chlorine), but they can also contain polyatomic ions such as ammonium (NH₄⁺).

To use the swap-and-drop method, swap the size of the charge on one ion to become the number of the other ion in the formula. For example, Cu²⁺ and Cl^- gives CuCl₂, because the 2 from Cu²⁺ becomes the number of Cl atoms.

False.

Ionic compounds are not simple molecules. Simple molecules are formed when non-metal atoms bond together. Ionic compounds form when a metal and a non-metal react and transfer electrons.

Brackets are used because more than one ammonium ion (NH₄⁺) is needed to balance the sulfate ion (SO₄^2-). The subscript 2 outside the brackets shows that the whole NH₄ group is doubled.

Simple molecules can be shown using:

1. Chemical symbols from the Periodic Table to identify each atom type directly.

2. Differently coloured spheres with a key to identify each type of atom.

Both styles show the number of atoms and their arrangement in the molecule.

An ionic compound has no overall charge because the positive and negative ions cancel each other out.

Relative atomic mass (A_r) is the average mass of the atoms of an element compared to 1/12 the mass of a carbon-12 atom. It is found in the Periodic Table above the element symbol.

Relative molecular mass (M_r) is calculated by adding together the relative atomic masses of all atoms present in the formula. For example, M_r(H₂O) = (2 × 1) + 16 = 18.

Carbon-12 is used as the standard atom. The relative atomic mass of an element compares the mass of its atoms to 1/12 the mass of one carbon-12 atom.

False.

For hydrogen, A_r(H) = 1 and the atomic number is also 1, so they are equal. For all other elements the relative atomic mass is larger than the atomic number.

The M_r of K₂CO₃ is 138.

(2 × 39) + (1 × 12) + (3 × 16) = 78 + 12 + 48 = 138

Relative formula mass is the sum of the relative atomic masses of all atoms in the formula of an ionic compound. It is used instead of relative molecular mass for ionic substances.

True.

Relative atomic mass compares the mass of an atom to carbon-12. A_r(Mg) = 24 means magnesium atoms are twice as heavy as carbon-12 atoms.

Mass number is the total number of protons and neutrons in the nucleus of one specific atom and is always a whole number. Relative atomic mass is the average mass of all isotopes of an element compared to carbon-12 and can be a decimal.

To calculate M_r, add together the relative atomic masses of all the atoms in the formula of the substance.

Percentage composition is the mass of one element in a compound expressed as a percentage of the total mass of the compound.

Percentage composition = (total mass of element / total mass of compound) × 100

The total percentage by mass of all elements in a compound must add up to 100%.

The percentage composition of hydrogen in water, H₂O, is calculated by dividing the total mass of hydrogen by the total mass of water and multiplying by 100.

Percentage of nitrogen = 35%

M_r(NH₄NO₃) = 14 + 4 + 14 + 48 = 80

Total mass of nitrogen = 2 × 14 = 28

(28 / 80) × 100 = 35%

False.

A percentage composition greater than 100% is impossible. It usually means the calculation is inverted — the total mass of the compound should be the denominator, not the numerator.

Percentage of carbon = 40%

M_r(C₆H₁₂O₆) = 72 + 12 + 96 = 180

Total mass of carbon = 6 × 12 = 72

(72 / 180) × 100 = 40%

True.

For example, ammonium nitrate (NH₄NO₃) contains two nitrogen atoms. Both must be included in the total mass of nitrogen used in the calculation.

Subtract the known percentages from 100%. The remaining percentage belongs to the element not listed. This works because all percentage compositions in a compound must sum to 100%.

A mixture is a combination of two or more substances that are not chemically joined together. Mixtures can be separated by physical processes and their components retain their own properties.

Filtration is used to separate an insoluble solid from a liquid. The liquid (filtrate) passes through the filter paper while the solid is left behind as the residue.

True.

Distillation works because the pure water evaporates when heated, the vapour condenses in the condenser and is collected, leaving the dissolved salt behind.

In chromatography, the pencil line must be placed above the level of the solvent so that samples do not wash into the solvent container.

The R_f value (retention factor) is the ratio of the distance travelled by a substance to the distance travelled by the solvent front. It is a number between 0 and 1 with no units.

R_f = distance travelled by substance / distance travelled by solvent

For example, if a substance travels 3 cm and the solvent travels 6 cm, R_f = 3/6 = 0.5.

Pencil is used because pencil marks do not dissolve in the solvent. Ink would dissolve and travel up the chromatography paper along with the samples, interfering with the results.

True.

A pure substance contains only one component, so it travels at one speed and produces a single spot. An impure substance or mixture produces more than one spot.

Different substances separate in chromatography because they have different solubilities in the solvent. Substances with higher solubility are carried further up the paper, producing spots at different heights.

1. Filtration — insoluble solid from a liquid

2. Evaporation / crystallisation — dissolved solid from a solution

3. Distillation — pure liquid from a solution or mixture of liquids

4. Chromatography — substances with different solubilities in a solvent

The Law of Conservation of Mass states that no matter is lost or gained during a chemical reaction. The total mass of the reactants is always equal to the total mass of the products.

All chemical equations must be balanced because the Law of Conservation of Mass requires the same number of each type of atom on both sides of the equation. Atoms are rearranged but not created or destroyed during a reaction.

True.

The gaseous product escapes into the surroundings, so the mass measured inside the container decreases. The total mass including the gas is still conserved overall.

In a chemical reaction, atoms are rearranged but not created or destroyed. This means the total mass of reactants equals the total mass of products.

When two solutions react in a closed system to form an insoluble precipitate, no substance can enter or leave. The total mass before the reaction equals the total mass after, demonstrating that mass is conserved.

False.

Sodium chloride has quite different properties from sodium and chlorine because a chemical reaction has caused the atoms to rearrange into a new substance with different chemical and physical properties.

The reaction produces carbon dioxide gas (CO₂ (g)) which escapes from the open flask into the surroundings. Mass is still conserved overall, but the gas is no longer inside the flask so the measured mass decreases.

The arrangement of atoms changes, producing new substances with different properties. The number and type of atoms stays the same, so the total mass is conserved throughout the reaction.

Effervescence is the fizzing observed during a chemical reaction, caused by the rapid production of a gas. It is evidence that a chemical reaction has taken place.

Three signs of a chemical reaction are:

1. Colour change — e.g. copper displacing silver from silver nitrate solution turns the solution blue

2. Temperature change — exothermic reactions increase temperature; endothermic reactions decrease it

3. Effervescence — gas bubbles produced, e.g. acid reacting with a carbonate

False.

An exothermic reaction releases heat energy, causing the temperature of the surroundings to increase. It is endothermic reactions that cause the temperature to decrease.

When copper metal is added to silver nitrate solution, the solution changes from colourless to blue and the solid changes from orange-brown to silver.

The reaction of sodium with water shows two signs of a chemical reaction: the sodium effervesces (fizzes) as hydrogen gas is produced and the reaction is exothermic, so the temperature rises. Both are observable evidence of a chemical change.

Dissolving solid ammonium chloride in water is endothermic. It is endothermic because heat energy is absorbed from the surroundings during the process, causing the temperature to decrease. This principle is used in cold packs.

False.

A colour change can indicate a chemical reaction, but physical changes such as dissolving or mixing can also produce colour changes. Other evidence should be considered alongside it.

Two further signs of a chemical reaction are:

1. Change of physical state — e.g. a liquid becoming a solid precipitate

2. Smell produced — e.g. a pungent odour released during a reaction

A word equation shows the reactants and products of a chemical reaction using their full chemical names, with the reactants on the left and the products on the right of an arrow.

sodium hydroxide + hydrochloric acid → sodium chloride + water

Sodium hydroxide is the base and hydrochloric acid is the acid. The products are a salt and water in this neutralisation reaction.

False.

You must never change the subscripts in a formula because this changes the identity of the substance. Only the coefficients (numbers in front of formulas) can be changed to balance an equation.

A symbol equation must be balanced because the Law of Conservation of Mass states that atoms are neither created nor destroyed in a chemical reaction. There must be the same number of each type of atom on both sides of the equation.

2Al (s) + 3CuO (s) → Al₂O₃ (s) + 3Cu (s)

Step 1: 2 Al needed to match Al₂O₃. Step 2: 3 O atoms on the right requires 3 CuO. Step 3: 3 Cu atoms produced on the right.

False.

The seven diatomic non-metals — H₂, N₂, O₂, F₂, Cl₂, Br₂ and I₂ — must be written as molecules in symbol equations.

In a word equation, the reactants are written on the left of the arrow and the products are written on the right. The arrow means "to form (or produces)".

S (s) + O₂ (g) → SO₂ (g)

One atom of sulfur reacts with one molecule of oxygen to produce one molecule of sulfur dioxide. The equation is already balanced as written.

If a polyatomic group such as NO₃^- appears unchanged on both sides of the equation, count the whole group as one unit rather than counting individual atoms. This simplifies balancing and reduces errors.

Percentage yield is the actual yield of a product expressed as a percentage of the theoretical yield. It compares what was actually obtained to the maximum amount that could have been produced.

Percentage yield = (actual yield / theoretical yield) × 100

The actual yield is what you measure in the experiment. The theoretical yield is the maximum amount calculated from the balanced equation.

In practice, 100% yield is never achieved because some product may be lost during separation and purification stages, or because the reaction may be reversible.

Percentage yield = 80%

Percentage yield = (1.6 / 2.0) × 100 = 80%

The student obtained 80% of the maximum possible amount of copper(II) sulfate.

False.

A percentage yield greater than 100% is impossible. If you calculate a value above 100%, the calculation is incorrect — the most common error is dividing the theoretical yield by the actual yield instead of the other way round.

1. Reactants left behind in equipment

2. Reaction is reversible, so products convert back to reactants

3. Product lost during separation or purification (e.g. filtration or distillation)

4. Side reactions between reactants and impurities or gases in the air

True.

A high percentage yield means more product is made from the same amount of reactants, which reduces costs and waste and increases profits for the company.

A reversible reaction reduces percentage yield because the products continuously convert back into reactants. The reaction reaches an equilibrium where both reactants and products are present, so 100% conversion to products is never achieved.

The simplest formula (empirical formula) is the whole number ratio of atoms of each element present in the compound. It is determined from reacting masses or percentage composition data.

1. Write the mass of each element.

2. Divide each mass by its A_r to find the number of moles.

3. Divide all values by the smallest to get the molar ratio.

4. Round to the nearest whole number to give the simplest formula.

To find the simplest formula from reacting masses, you divide the mass of each element by its A_r and then divide all values by the smallest value to get the whole number ratio.

False.

The simplest formula gives the ratio of atoms; the molecular formula gives the actual numbers. For example, ethene has a simplest formula of CH₂ but a molecular formula of C₂H₄.

1. Calculate the moles of the known substance (mass / M_r).

2. Use the molar ratio from the balanced equation to find moles of the unknown substance.

3. Calculate the mass of the unknown substance (moles × M_r).

Mass of MgO = 10 g

Moles of Mg = 6.0 / 24 = 0.25 mol. Ratio Mg : MgO = 1 : 1, so 0.25 mol MgO produced. Mass = 0.25 × 40 = 10 g.

True.

By converting masses to moles and finding the simplest whole number ratio, the coefficients in the balanced equation can be determined.

Simplest formula = H₂O

Moles: H = 10/1 = 10; O = 80/16 = 5. Ratio: H = 10/5 = 2; O = 5/5 = 1. Simplest formula is H₂O.

To calculate reacting masses, you use the molar ratio from the balanced equation together with the relative masses of the substances.

The mole (symbol: mol) is the SI unit of amount of substance. One mole of any substance contains 6.02 × 10²³ particles (atoms, molecules or ions).

The Avogadro constant is 6.02 × 10²³ mol^-1. It is the number of particles (atoms, molecules or ions) in one mole of any substance.

True.

One mole of any substance contains 6.02 × 10²³ particles. Both H₂ and CH₄ contain 6.02 × 10²³ molecules per mole, in line with Avogadro's Law.

At room temperature and pressure (RTP), one mole of any gas occupies 24 dm³ or 24 000 cm³.

Volume (dm³) = moles × 24

For example, 3 moles of hydrogen gas occupies 3 × 24 = 72 dm³ at RTP.

Moles = 0.25 mol

Moles = volume / molar volume = 6 / 24 = 0.25 mol

At RTP, molar volume = 24 dm³ mol^-1.

Molar mass is the mass of one mole of a substance in grams. For an element it equals the relative atomic mass in grams; for a compound it equals the relative molecular (or formula) mass in grams.

True.

This is Avogadro's Law: at the same temperature and pressure, equal amounts (moles) of gases occupy the same volume, so equal volumes contain the same number of moles.

The molar mass of an element in grams equals its relative atomic mass, and the molar mass of a compound equals its relative molecular (or formula) mass in grams.