Introducing Chemical Reactions (OCR GCSE Combined Science A (Gateway): Chemistry): Flashcards

Stock notation is a system using Roman numerals in a compound's name to show the charge of the metal ion. For example, copper(II) oxide tells you the copper ion has a charge of 2+.

Elements in Group 7 form ions with a charge of 1−. They gain one electron to achieve a full outer shell — for example, chloride is Cl^-.

To deduce the formula of an ionic compound, you balance the ion charges so that the overall charge is zero. The number of ions needed becomes the subscript in the formula.

True.

The ammonium ion (NH₄⁺) is a positive ion made entirely of non-metal atoms. It is one of the few exceptions to the rule that positive ions come from metals.

The formula is Al₂O₃. Two Al³⁺ ions give a total charge of 6+ and three O^2- ions give 6−, so the overall charge is zero.

Brackets are used when more than one compound ion is needed in the formula. In Cu(OH)₂ there are two hydroxide ions, so brackets group the OH unit before adding the subscript 2. In NaCl there is only one of each ion so no brackets are needed.

The formula of magnesium nitrate is Mg(NO₃)₂. Two nitrate ions (NO₃^-) are needed to balance the 2+ charge on the magnesium ion, so brackets are used around NO₃ with a subscript 2.

The formula is Mg(NO₃)₂. One Mg²⁺ ion requires two NO₃^- ions to balance the charges, giving an overall charge of zero. Brackets are used because two nitrate ions are needed.

Elements in Group 2 form ions with a charge of 2+ and elements in Group 6 form ions with a charge of 2−.

The Law of Conservation of Mass states that matter cannot be created or destroyed in a chemical reaction. This means the total mass of reactants equals the total mass of products.

You can only change the coefficients — the numbers placed in front of each formula. You must never change the subscripts within a formula, as that would change the identity of the substance.

True.

On the left, 2Fe₂O₃ contains 2 × 3 = 6 oxygen atoms. On the right, 3CO₂ contains 3 × 2 = 6 oxygen atoms. The equation is balanced.

When a compound ion such as NO₃^- appears unchanged on both sides of an equation, it should be counted as one entity rather than as individual atoms.

The four state symbols are:

1. (s) — solid

2. (l) — liquid

3. (g) — gas

4. (aq) — aqueous (dissolved in water)

False.

A precipitate is an insoluble solid that forms in solution, so it is given the state symbol (s), not (aq).

In the equation NaOH (aq) + HCl (aq) ⟶ NaCl (aq) + H₂O (l), the three aqueous state symbols show dissolved substances and (l) shows that water is a liquid.

Oxygen must be written as O₂ because it is a diatomic element — it exists in nature as two oxygen atoms bonded together. Writing O would represent a single oxygen atom, which does not reflect the actual substance.

If the ionic compound is dissolved in water it is given (aq); if it is a solid it is given (s). As a general guide, ionic compounds are usually solids unless the question states they are in solution.

Spectator ions are ions that appear on both sides of a full ionic equation unchanged. They take no part in the reaction and are cancelled out when writing the net ionic equation.

A net ionic equation shows only the species that actually change during the reaction. It removes spectator ions, making it clearer which particles are involved in the chemical change.

When writing an ionic equation, you first identify the spectator ions and then cancel them from both sides of the equation.

False.

Solids are not split into ions in an ionic equation. Only substances with the state symbol (aq) are written as separate ions; solids, liquids and gases are written as their full formula.

The net ionic equation is:

2I^- (aq) + Cl₂ (aq) → 2Cl^- (aq) + I₂ (aq)

The K⁺ ions are spectator ions and are cancelled from both sides.

A half equation represents what happens to one reactant in a reaction. It shows either the loss or gain of electrons (e^-) and must be balanced in both atoms and charges.

The half equation for sodium is: Na → Na⁺ + e^-

Sodium loses one electron, forming a positive ion. The charges balance: 0 on the left equals 1+ plus 1− on the right.

False.

Half equations do not have to use the same coefficients as the overall equation. Each half equation only needs to be balanced in its own atoms and charges.

The half equation for chlorine gaining electrons is: Cl₂ + 2e^- → 2Cl^-

Each chlorine atom gains one electron, so two electrons are needed for the diatomic Cl₂ molecule.

The Avogadro constant is the number of elementary entities (atoms, molecules, or ions) in one mole of a substance. Its value is 6.02 x 10²³ mol^-1.

One mole of sodium contains 6.02 x 10²³ atoms. This is the Avogadro constant — the same number of entities is present in one mole of any substance.

True.

One mole of any substance contains the same number of entities: 6.02 x 10²³. This is the Avogadro constant and applies regardless of the substance.

The Avogadro constant has a value of 6.02 x 10²³ mol^-1, and it represents the number of elementary entities in one mole of a substance.

Molar mass is the mass of one mole of a substance, measured in g mol^-1. For an element it equals the relative atomic mass in grams; for a compound it equals the relative formula mass in grams.

Mass of 1 atom = molar mass ÷ Avogadro constant. Dividing by 6.02 x 10²³ gives the mass of a single particle in grams.

Atoms are too small to count individually. The mole allows us to relate measurable masses (in grams) to the number of particles using the Avogadro constant, making calculations practical.

To find the number of moles from a mass, you divide the mass by the molar mass. To find the mass, you multiply the moles by the molar mass.

True.

The molar mass of an element is numerically equal to its relative atomic mass but is expressed in g mol^-1. For example, the molar mass of carbon is 12.0 g mol^-1.

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.

False.

In a closed system, no matter can enter or leave. The Law of Conservation of Mass means the total mass remains constant throughout the reaction.

Chemical equations must be balanced because of the Law of Conservation of Mass. Atoms are rearranged but never created or destroyed, so the number of each type of atom must be the same on both sides.

In a chemical reaction, the total mass of the reactants equals the total mass of the products, because mass is always conserved.

The reaction produces carbon dioxide gas (a gaseous product). In an open system, the CO₂ escapes into the air. Mass appears to decrease because the gas is no longer weighed as part of the system.

Magnesium reacts with oxygen from the air, which is a gaseous reactant. The product, magnesium oxide, contains the mass of both the magnesium and the oxygen, so the solid appears heavier.

True.

When a gaseous product escapes from an open system, it is no longer part of the measured mass. Mass is still conserved overall, but it appears to decrease because the gas cannot be weighed.

When iron reacts with oxygen, the mass of the product increases because oxygen is a gaseous reactant drawn from the air.

Carry out the reaction in a closed system and measure the mass before and after. For example, mixing calcium chloride and sodium sulfate solutions produces solid calcium sulfate (a precipitate), but the total mass is unchanged.

Stoichiometry refers to the numbers (coefficients) placed in front of reactants and products in a chemical equation. These coefficients represent the molar ratios of each substance in the reaction.

1. Convert each mass to moles by dividing by the molar mass.

2. Identify the simplest whole-number ratio by dividing all values by the smallest.

3. Use the molar ratio as the coefficients in the balanced equation.

True.

The coefficients (stoichiometric numbers) in a balanced equation show the ratio of moles of each substance. For example, 2:3:2:4 in 2CH₃OH + 3O₂ ⟶ 2CO₂ + 4H₂O.

To find the coefficients of a balanced equation from masses, first convert each mass to moles by dividing by the molar mass.

The limiting reactant is the reactant that is completely used up first. It limits the duration of the reaction and determines the maximum amount of product that can form.

The reaction stops and the excess reactant remains unreacted. The amount of product formed is determined entirely by the amount of the limiting reactant present.

Having fewer moles does not automatically make a reactant the limiting reactant. The balanced equation shows the ratio in which substances react, so you must compare the moles available with the moles required by the molar ratio.

True.

The amount of product is directly proportional to the amount of the limiting reactant. Doubling the limiting reactant doubles the yield, provided the other reactant remains in excess.

In a reaction, the limiting reactant is completely used up, while the excess reactant remains at the end of the reaction.

1. Convert the given mass to moles (divide by molar mass).

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

3. Convert moles of product to grams (multiply by molar mass).

The molar ratio is the ratio of the coefficients of reactants and products in a balanced equation. It shows how many moles of each substance react or are produced relative to one another.

To calculate the mass of a reactant or product, first convert mass to moles using the molar mass, then apply the molar ratio from the balanced equation.

False.

Mass ratios from a balanced equation apply to any consistent unit of mass (grams, kilograms, tonnes). For example, the ratio for 2Mg + O₂ ⟶ 2MgO is the same whether you work in grams or tonnes.

The molar ratio of Mg to MgO is 2:2, which simplifies to 1:1. For every mole of magnesium burned, one mole of magnesium oxide is produced.

The balanced equation gives the correct molar ratio of reactants and products. Using an unbalanced equation gives the wrong ratio and therefore the wrong mass calculation.

The mass of one mole of a compound is found by calculating its relative formula mass and expressing it in grams per mole.

False.

A molar ratio of 1:2 means 1 mole of A produces 2 moles of B, not 1 gram producing 2 grams. The actual masses depend on the molar masses of A and B.

Divide the mass by the molar mass of the substance. For example, 6.0 g of Mg (molar mass 24.0 g mol^-1) = 6.0 ÷ 24.0 = 0.25 mol.