1.1 The Nature of Substances & Chemical Reactions (WJEC GCSE Chemistry): Flashcards

False.

Bromine exists as diatomic molecules, Br₂. It is one of the seven elements that naturally exist as molecules made of two atoms of the same element bonded together.

The chemical formula of a simple molecule tells you the types of atom present (shown by chemical symbols) and the number of each type of atom (shown by subscript numbers after each symbol).

In the formula of copper(II) chloride, CuCl₂, the Cu²⁺ ion has a charge of 2+ and the Cl^- ion has a charge of 1−. Two chloride ions are needed to balance the charge.

The swap-and-drop method is used when ions have different charges. The size of the charge on each ion becomes the subscript number for the other ion, signs are removed and the ratio is simplified to the lowest whole numbers. For example, Fe³⁺ and Cl^- gives FeCl₃.

A simple molecule is a substance made from two or more non-metal atoms chemically joined together. It can be an element (e.g. O₂) or a compound (e.g. H₂O).

True.

In an ionic compound, the positive charges on cations and the negative charges on anions cancel out so the compound has no overall charge. The formula is written to reflect this balanced ratio.

A carbonate ion has the formula CO₃^2- and carries a 2− charge. It is a polyatomic ion containing one carbon atom bonded to three oxygen atoms.

The formula of ammonium sulfate is (NH₄)₂SO₄ because the NH₄⁺ ion has a charge of 1+ and the SO₄^2- ion has a charge of 2−. Two ammonium ions are needed to balance the charge.

Relative atomic mass (A_r) is the mass of an atom compared to 1/12 of the mass of a carbon-12 atom. The A_r of each element can be found above its symbol in the Periodic Table.

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

False.

The relative atomic mass can be a decimal because it is the average mass of the naturally occurring isotopes of an element. The mass number (protons + neutrons) is always a whole number.

The M_r of water (H₂O) is 18.

M_r = (2 × 1) + (1 × 16) = 18

Magnesium has a relative atomic mass of 24, meaning it is twice as heavy as a carbon-12 atom. Hydrogen, with A_r = 1, has one-twelfth of the mass of a carbon-12 atom.

The M_r of calcium hydroxide (Ca(OH)₂) is 74.

M_r = (1 × 40) + (2 × 16) + (2 × 1) = 40 + 32 + 2 = 74

True.

For ionic compounds, the term relative formula mass is used rather than relative molecular mass, since ionic compounds do not exist as discrete molecules. The calculation method is the same. Add the A_r values of all atoms in the formula.

The M_r of potassium carbonate (K₂CO₃) is 138.

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

A percentage composition is the mass of one element expressed as a percentage of the total mass of a compound. It is calculated as: (total mass of element ÷ total mass of compound) × 100.

The equation for percentage composition is:

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

True.

The percentage composition accounts for every element in the compound. Adding the percentages of all elements present must give exactly 100%.

The percentage of hydrogen in water (H₂O) is 11.1%.

M_r(H₂O) = (2 × 1) + (1 × 16) = 18

Percentage of H = (2 × 1 ÷ 18) × 100 = 11.1%

A percentage composition greater than 100 is not possible. This error usually occurs when the numbers in the calculation are placed upside down.

The percentage of nitrogen in ammonium nitrate (NH₄NO₃) is 35%. NH₄NO₃ contains two nitrogen atoms, giving M_r = 80.

Percentage of N = (2 × 14 ÷ 80) × 100 = 35%

False.

NH₄NO₃ contains two nitrogen atoms: one in the NH₄ group and one in the NO₃ group. Both must be included. Counting only one is a common exam error.

The percentage of carbon in glucose (C₆H₁₂O₆) is 40%.

M_r = (6 × 12) + (12 × 1) + (6 × 16) = 72 + 12 + 96 = 180

Percentage of C = (6 × 12 ÷ 180) × 100 = 40%

A mixture contains two or more substances that are not chemically joined together. The substances in a mixture keep their individual properties and can be separated by physical methods such as filtration, distillation and chromatography.

False.

Filtration only separates insoluble solids. Salt dissolved in water passes through the filter paper along with the water. Evaporation or distillation must be used instead.

Filtration is a separation technique used to separate an insoluble solid from a liquid or solution. The solid is trapped by the filter paper as the residue and the liquid passes through as the filtrate.

Simple distillation is used to obtain pure water from salt water by heating the solution. The water evaporates and the vapour rises into a condenser, where it cools and condenses back to liquid and is collected. The salt remains in the flask because it has a much higher boiling point.

In chromatography, the R_f value is calculated by dividing the distance travelled by the substance by the distance travelled by the solvent.

Chromatography is a separation technique used to separate substances with different solubilities in a solvent. Substances with greater solubility in the solvent travel further up the paper.

The pencil baseline must be above the solvent level so that sample spots are not submerged in the solvent. If the baseline is below the solvent level, the spots dissolve directly into the solvent and wash away rather than being carried up the paper by capillary action.

False.

A substance with higher solubility is carried further by the solvent and travels a longer distance up the paper, giving a higher R_f value.

Crystallisation is used to obtain a dissolved solid from a solution. The solution is heated to evaporate solvent until saturated; on cooling, the solid comes out of solution as crystals, which are collected by filtration, washed with distilled water and dried.

True.

A pure substance contains only one component and so produces a single spot. An impure substance produces multiple spots because different components separate out at different rates.

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

Chemical equations must be balanced because the Law of Conservation of Mass states that atoms cannot be created or destroyed in a reaction. The same atoms present in the reactants are present in the products, only their arrangement changes.

False.

During a chemical reaction, atoms are only rearranged — the same number of each type of atom is present before and after. No atoms are created or destroyed.

In a chemical reaction, atoms are rearranged rather than created or destroyed. This means the total mass of the reactants equals the total mass of the products.

The mass decreases in an open system because the gaseous product escapes into the surroundings and is no longer measured. For example, when hydrochloric acid reacts with calcium carbonate, carbon dioxide gas is released. In a closed system, the mass stays the same.

True.

In a closed system, no matter can enter or leave, so the total mass stays the same before and after the reaction. This is consistent with the Law of Conservation of Mass.

During a chemical reaction, atoms are rearranged to form new substances. The same atoms that were present in the reactants are present in the products — only the arrangement has changed. No atoms are gained or lost.

Observable signs that a chemical reaction has taken place include:

1. A colour change

2. A temperature change

3. Effervescence (fizzing)

4. A change of state

5. A smell being produced

6. Light being given out

7. A change in pH

Effervescence is the fizzing observed when a gas is produced during a chemical reaction. It is a common sign that a reaction is taking place, often seen when acids react with metals or carbonates.

False.

An exothermic reaction causes an increase in temperature as energy is released to the surroundings. A decrease in temperature indicates an endothermic reaction, where energy is absorbed from the surroundings.

Two colour changes are observed when copper is added to silver nitrate solution. The solid changes from orange-brown to silver as copper displaces silver from solution. The solution changes from colourless to blue as copper ions form.

A reaction that releases energy to the surroundings is called an exothermic reaction. A reaction that absorbs energy from the surroundings is called an endothermic reaction.

True.

A change in pH is one of several signs that a chemical reaction is occurring. It is particularly useful as evidence when acids or alkalis are involved, such as during a neutralisation reaction.

One example is the thermal decomposition of copper(II) carbonate. Green copper(II) carbonate decomposes on heating to form black copper(II) oxide, giving a clear colour change from green to black. This shows that a chemical reaction has taken place.

A word equation shows the reactants and products of a chemical reaction using their full chemical names. Reactants are written on the left side of the arrow and products on the right. The arrow means "produces" or "to form".

A symbol equation uses the chemical formulae of the reactants and products to show what happens in a reaction. It must be balanced so there are equal numbers of each type of atom on both sides.

False.

When balancing an equation, only the coefficients (large numbers in front of formulae) may be changed. Changing subscript numbers would change the substance itself. For example, changing H₂O to H₂O₂ changes water to hydrogen peroxide.

The four state symbols are: (s) = solid, (l) = liquid, (g) = gas, (aq) = aqueous (dissolved in water). State symbols are written in brackets after the formula of each substance in a symbol equation.

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

True.

A balanced equation obeys the Law of Conservation of Mass. The same number of atoms of each element must appear on both sides since atoms cannot be created or destroyed in a reaction.

The correct state symbol for a substance dissolved in water is (aq), which stands for aqueous. It indicates that the substance is in solution, dissolved in water.

A reactant is a substance that takes part in a chemical reaction. Reactants are shown on the left-hand side of the reaction arrow in a word or symbol equation.

A percentage yield is a measure of how much of the expected product is actually obtained in a reaction. It is calculated as: (actual yield ÷ theoretical yield) × 100.

The equation for percentage yield is:

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

False.

A percentage yield greater than 100% is impossible. If you calculate a yield above 100%, you have made an error. The most common mistake is dividing the theoretical yield by the actual yield rather than the other way round.

Reasons that the percentage yield is never 100% include:

1. Reactants may be left behind in the equipment

2. The reaction may be reversible (products convert back to reactants)

3. Products can be lost during separation and purification

4. There may be side reactions consuming some of the reactants

A student collects 1.6 g of copper(II) sulfate. The theoretical yield is 2.0 g. The percentage yield is 80%. (1.6 ÷ 2.0) × 100 = 80%

True.

In a reversible reaction, products continuously convert back to reactants, so a 100% yield is never possible. This is one of several reasons why percentage yield is always less than 100%.

A theoretical yield is the maximum amount of product that could be obtained from a reaction, calculated from the balanced equation and the masses of reactants used. It assumes perfect conditions with no losses at any stage.

The simplest formula (empirical formula) of a compound shows the smallest whole number ratio of atoms of each element present. It is found by dividing the moles of each element by the smallest mole value to give the ratio.

The three steps are: (1) calculate the moles of the given substance using moles = mass ÷ M_r; (2) use the molar ratio from the balanced equation to find the moles of the product; (3) calculate the mass using mass = moles × M_r.

True.

The simplest formula of ethane is CH₃, which is the lowest whole number ratio of carbon to hydrogen atoms (1:3). The molecular formula, C₂H₆, is a multiple of the simplest formula.

To find the simplest formula: (1) write the mass of each element; (2) divide each mass by the A_r to get the moles of each element; (3) divide all mole values by the smallest; (4) if any ratio is not a whole number, multiply all values by the appropriate integer.

To calculate the mass of a product, first calculate the moles of the reactant, then use the molar ratio from the equation, and finally convert back to mass using the M_r.

False.

The molar ratio gives the ratio of moles, not masses. To find masses, moles must be converted using the relative atomic or formula masses of each substance.

The simplest formula shows the lowest whole number ratio of atoms in a compound. The molecular formula shows the actual number of each type of atom in a molecule. For example, the simplest formula of ethene is CH₂ but its molecular formula is C₂H₄.

A molar ratio is the ratio of the amounts in moles of each substance in a balanced chemical equation, shown by the coefficients in front of each formula. For example, in 2Mg + O₂ → 2MgO, the molar ratio of Mg to MgO is 1:1.

The mole is the SI unit of amount of substance. One mole of any substance contains 6.02 × 10²³ particles, which may be atoms, molecules or ions. The symbol for the mole is mol.

The Avogadro constant is the number of particles in one mole of any substance. Its value is 6.02 × 10²³ particles per mole. This applies equally to atoms, molecules and ions.

False.

One mole of water (H₂O) contains 6.02 × 10²³ molecules, not atoms. Each water molecule contains three atoms, so one mole of water contains 3 × 6.02 × 10²³ atoms in total.

The molar mass of a substance is the mass of one mole in grams. For an element, it equals the relative atomic mass (A_r) in g/mol. For a compound, it equals the relative molecular mass (M_r) or relative formula mass in g/mol.

At RTP, one mole of any gas occupies a volume of 24 dm³ or 24,000 cm³. This value is known as the molar gas volume.

The volume of 3 moles of hydrogen gas at RTP is 72 dm³.

Volume = moles × molar gas volume = 3 × 24 = 72 dm³

False.

According to Avogadro's Law, equal moles of any gas at the same temperature and pressure occupy the same volume. At RTP, one mole of any gas occupies 24 dm³, regardless of the type of gas.

The number of moles in 6 dm³ of carbon dioxide gas is 0.25 mol.

Moles = volume ÷ molar gas volume = 6 ÷ 24 = 0.25 mol