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

Stock notation uses Roman numerals in a compound's name to show the charge on a transition metal ion. For example, copper(II) means the copper ion has a 2+ charge.

A Group 7 ion carries a 1− charge because it gains one electron to complete its outer shell. For example, bromide is Br^-.

False.

Most positive ions are metals, but there are non-metal exceptions. The ammonium ion (NH₄⁺) and the hydrogen ion (H⁺) are positive ions formed from non-metals.

To balance the formula of aluminium oxide, you need 2 aluminium ions (3+) for every 3 oxide ions (2−), giving Al₂O₃. The total positive and negative charges must cancel to zero.

Brackets are used around the hydroxide ion (OH^-) when more than one of that compound ion is needed. Without brackets, it would be unclear how many atoms belong to the polyatomic ion.

A polyatomic ion (also called a compound ion) is a charged particle made up of more than one type of atom bonded together. Sulfate (SO₄^2-) and nitrate (NO₃^-) are common examples.

The formula is Mg(NO₃)₂. Magnesium has a 2+ charge and nitrate has a 1− charge, so two nitrate ions are needed to balance one magnesium ion.

True.

Group 2 metals lose 2 electrons to form 2+ ions. For example, magnesium forms Mg²⁺ in all its ionic compounds.

The formula of ammonium sulfate is (NH₄)₂SO₄ because two ammonium ions (1+) are needed to balance one sulfate ion (2−). Brackets show that the whole ammonium group is multiplied by 2.

Find the charges on the positive and negative ions, then adjust the number of each ion until the total charges balance to zero. Write the ratio as a formula, using brackets around any compound ions where more than one is needed.

A coefficient is the number placed in front of a formula in a chemical equation to show how many units of that substance are involved. Changing coefficients is how equations are balanced.

A chemical equation must be balanced because atoms are conserved in a reaction — they are neither created nor destroyed. The same number of atoms of each element must appear on both sides.

False.

Subscripts in a formula are fixed and define the compound itself. You balance an equation by changing the coefficients in front of each formula only.

The state symbol (aq) means aqueous, meaning the substance is dissolved in water. The other state symbols are (s) for solid, (l) for liquid and (g) for gas.

The four state symbols are (s) for solid, (l) for liquid, (g) for gas and (aq) for aqueous (dissolved in water).

The state symbol (g) indicates that the substance is in the gaseous state at the conditions described in the equation.

True.

These are examples of diatomic elements that naturally exist as two atoms bonded together. The halogens (F₂, Cl₂, Br₂, I₂) are also diatomic and must be written with a subscript 2 in equations.

To balance Al + CuO → Al₂O₃ + Cu, the coefficient in front of CuO must be 3. The fully balanced equation is 2Al + 3CuO → Al₂O₃ + 3Cu.

Work across the equation from left to right, adjusting coefficients until the number of atoms of each element is equal on both sides. Balance elements that appear on their own last.

False.

A precipitate is an insoluble solid, so it is assigned the state symbol (s). It forms when two solutions react and an insoluble product is produced.

A spectator ion is an ion that appears unchanged on both sides of an ionic equation and takes no part in the reaction. It is removed when writing the net ionic equation.

Ionic compounds dissociate into their component ions when dissolved in water. For example, HCl (aq) splits into H⁺ (aq) and Cl^- (aq).

False.

A half equation only needs to be balanced in atoms and charges. Its coefficients may differ from those in the overall equation.

Ions that appear unchanged on both sides of an ionic equation are called spectator ions and are removed from the final equation. Their removal gives the net ionic equation.

Both the number of atoms and the total charge must be equal on each side. Electrons are added to the appropriate side to achieve charge balance.

A half equation represents the change that occurs to one reactant in a reaction, showing the electrons gained or lost. For example, Na → Na⁺ + e^-.

True.

Soluble salts are ionic compounds that dissociate into their component ions when dissolved in water, which is why they are written as separate ions in ionic equations.

The net ionic equation gives: 2I^- (aq) + Cl₂ (aq) → 2Cl^- (aq) + I₂ (aq). The K⁺ ions are spectator ions and are removed.

Write all dissolved ionic compounds as their separate ions, then identify and cancel any spectator ions that appear unchanged on both sides. What remains is the net ionic equation.

The symbol e^- represents a free electron — a negatively charged particle. It is added to half equations to balance the charge on each side of the reaction.

A mole is the SI unit of amount of substance. One mole of any substance contains exactly 6.02 × 10²³ elementary entities (atoms, molecules or ions).

The Avogadro constant is 6.02 × 10²³ mol^-1. It gives the number of elementary entities per mole of substance.

True.

Any one mole of a substance always contains 6.02 × 10²³ entities, regardless of the element or compound.

One mole of H₂ contains 6.02 × 10²³ molecules of hydrogen.

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.

Moles = mass (g) ÷ molar mass (g mol^-1). Rearranged: mass = moles × molar mass.

Divide the molar mass by the Avogadro constant: mass of 1 atom = molar mass ÷ 6.02 × 10²³.

False.

The molar mass of H₂O is (2 × 1.0) + 16.0 = 18.0 g mol^-1. Only the oxygen atom has a mass of 16.0 g mol^-1.

To find the number of moles in a sample, divide the mass (in grams) by the molar mass.

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

Chemical equations must be balanced because mass is conserved in every reaction. The total mass of reactants equals the total mass of products, so atoms cannot appear or disappear.

True.

In a closed system no substances can enter or leave, so mass is conserved and the total mass stays the same.

If a reaction flask is open and a gaseous product escapes, the measured mass of the flask will decrease.

The metal reacts with oxygen from the air, which is a gaseous reactant. The oxygen adds to the solid metal, so the product (metal oxide) has a greater mass than the metal alone.

True.

Thermal decomposition of calcium carbonate produces carbon dioxide gas, which escapes into the air. The solid mass decreases because the CO₂ is no longer weighed.

A precipitation reaction demonstrates conservation of mass. Two solutions react to form an insoluble precipitate, and if the container is closed the total mass before and after the reaction is unchanged.

A reaction appears to lose mass when a gaseous product is produced in an open container and escapes into the surroundings.

State symbols show whether a substance is a gas, liquid, solid or aqueous solution. A gaseous product (g) in an open system can escape, causing an apparent mass change — state symbols alert you to this possibility.

Stoichiometry refers to the numbers in front of reactants and products in a balanced chemical equation. These numbers (called coefficients) show the molar ratio in which substances react and are produced.

Convert the mass of each substance into moles by dividing by its molar mass. The resulting mole values give the ratio of coefficients in the balanced equation.

False.

Coefficients must be the smallest whole numbers. Multiply all values by 2 to give 2 : 4 : 5.

The molar ratio from a balanced equation is the same as the coefficients (stoichiometric numbers) of each substance in the reaction.

A limiting reactant is the reactant that is completely used up first in a reaction. It limits the amount of product that can form because once it is gone the reaction stops.

The amount of product is directly proportional to the amount of limiting reactant. Doubling the limiting reactant doubles the product; halving it halves the product, provided the other reactants are in excess.

You need the amount in moles of each reactant and the molar ratio from the balanced chemical equation. Comparing these tells you which reactant is not present in excess.

False.

The limiting reactant is determined by comparing moles to the molar ratio from the balanced equation, not simply by which reactant has fewer moles.

In a two-reactant system, if one reactant is in excess then the other is by default the limiting reactant.

1. Convert the given mass into moles (mass ÷ molar mass).

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

3. Convert moles into grams (moles × molar mass).

True.

Mass ratios are determined by the balanced equation and molar masses, not the units used. You can scale up from grams to tonnes and the ratios remain constant.

To find the moles of a substance from its mass, divide the mass by the molar mass.

Moles of Mg = 6.0 ÷ 24.0 = 0.25 mol

Molar ratio Mg : MgO = 1 : 1, so moles MgO = 0.25 mol

Mass of MgO = 0.25 × 40.0 = 10.0 g

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

The balanced equation gives the correct molar ratio of reactants and products. Using an unbalanced equation gives incorrect mole conversions and therefore incorrect mass calculations.

Once the moles of a product are known, multiply by the molar mass to convert to mass in grams.

False.

The molar ratio is 2 Al₂O₃ : 4 Al, so only two moles of Al₂O₃ are needed to produce four moles of Al.