AQA Chemistry A-level Physical Chemistry Definitions and Concepts

A series of flashcards covering essential definitions and concepts for the AQA Chemistry A-level Physical Chemistry.

What is acceleration in TOF spectrometry?

The second stage of TOF spectrometry where positively charged ions are accelerated by an electric field to have the same kinetic energy.

What is an atom?

The smallest part of an element that can exist, with all substances made up of atoms.

Define atomic nucleus.

The positively charged object composed of protons and neutrons at the center of every atom, with one or more electrons orbiting it.

What does atomic number refer to?

The number of protons in the nucleus of an atom.

What is an electron?

A negatively charged subatomic particle that orbits the nucleus at various energy levels, with a relative mass of 1/2000.

What is the electron configuration of calcium (Ca)?

1s22s22p63s23p64s2.

Define electron impact ionisation.

A method of ionisation in TOF spectrometry where a vaporised sample is bombarded with high energy electrons to knock off an electron from each particle.

What is electrospray ionisation?

A method of ionisation in TOF spectrometry where a sample is dissolved and pushed through a fine nozzle at high pressure, gaining an H+ ion.

What is first ionisation energy?

The energy required to remove 1 mole of electrons from 1 mole of gaseous atoms to form 1 mole of gaseous 1+ ions.

Describe the ion detection stage in TOF spectrometry.

The fourth stage where a negatively charged plate detects charged particles, producing a mass spectrum.

What is ion drift?

The third stage of TOF spectrometry where ions drift through a region with no electric field; lighter ions drift faster.

What is ionisation in TOF spectrometry?

The first stage where the sample can be ionised by methods like electrospray ionisation or electron impact ionisation.

Define isotope.

Atoms of the same element with the same number of protons but different numbers of neutrons.

What does mass number refer to?

The total number of protons and neutrons in the nucleus.

What is a mass spectrometer?

An instrument that provides accurate information about relative isotopic mass and the relative abundance of isotopes.

What is mass spectrometry?

A technique used to identify elements and determine relative molecular mass.

Define neutron.

A neutral subatomic particle present in the nucleus of the atom, with a relative mass of 1.

What is nuclear charge?

The total charge of all protons in the nucleus, having the same value as the atomic number.

Describe second ionisation energy.

The energy required to remove 1 mole of electrons from 1 mole of gaseous 1+ ions to form 1 mole of gaseous 2+ ions.

What are sub-shells (orbitals)?

Divisions of electron shells that have slightly different energy levels, comprising different numbers of orbitals.

What is a Time of Flight (TOF) spectrometer?

A mass spectrometry method where the mass-to-charge ratio of ions is determined by measuring their time of flight.

What is atom economy?

The measure of the amount of starting materials that convert into useful products.

Define Avogadro's constant.

The number of atoms, molecules, or ions in one mole of a substance.

What is concentration (in chemistry)?

The amount of substance per unit volume, typically expressed in g/dm3 or mol/dm3.

What is an empirical formula?

The smallest whole number ratio of atoms of each element in a compound.

Define limiting reactant.

The reactant that is completely used up in a reaction, limiting the amount of products formed.

What is a mole?

The mass of a substance containing the same number of units as there are atoms in exactly 12 g of carbon-12.

What is a molecular formula?

The actual ratio of atoms of each element present in a compound.

Define percentage by mass.

A value representing the concentration of an element in a compound or a component in a mixture.

What is percentage yield?

The percentage ratio of the actual yield of a product from a reaction compared with the theoretical yield.

What is relative atomic mass?

The average mass of an atom of an element compared to 1/12th the mass of an atom of carbon-12.

Define relative molecular mass.

The average mass of one molecule of an element or compound compared to 1/12th the mass of an atom of carbon-12.

What is a co-ordinate bond?

A bond that contains a shared pair of electrons with both electrons supplied by one atom.

What defines a covalent bond?

A bond formed by a shared pair of electrons between two non-metals.

Define dipole in chemistry.

The difference in charge between two atoms in a covalent bond due to a shift in electron density.

What does electron pair repulsion refer to?

The repulsion between electron pairs that positions them as far apart as possible around a central atom.

What is electronegativity?

The power of an atom to attract the electron density in a covalent bond towards itself.

Define electrostatic forces.

The strong forces of attraction between oppositely charged ions.

What is hydrogen bonding?

An interaction between a hydrogen atom and an electronegative atom, commonly nitrogen, fluorine, or oxygen.

What are intermolecular forces?

Forces that exist between molecules, affecting physical properties like boiling and melting points.

Define ion.

An atom or molecule with an electric charge due to the loss or gain of electrons.

What is an ionic bond?

A bond formed when a metal atom loses electrons to form a positively charged ion and a non-metal atom gains those electrons.

What characterizes an ionic compound?

A compound formed of ions held together by strong electrostatic forces.

Define lattice in chemistry.

A repeating regular arrangement of atoms, ions, or molecules, occurring in crystal structures.

What is a macromolecular crystal structure?

Giant covalent structures with very high melting points due to many strong covalent bonds.

What is a metallic bond?

The bond present in metals between positive metal ions and negatively charged electrons.

Define permanent dipole-dipole forces.

Intermolecular forces produced when molecules with polar covalent bonds interact with other dipoles.

What is a polar bond?

A covalent bond where electrons are unevenly distributed, causing a slight charge difference.

Define simple molecular crystal structure.

Structures where atoms are joined by strong covalent bonds, but weak intermolecular forces lead to low melting and boiling points.

What are Van der Waals forces?

Forces existing between all molecules, arising from fluctuations of electron density.

What is VSEPR theory?

Valence shell electron pair repulsion theory used to deduce molecular geometry based on electron pair arrangements.

What is calorimetry?

The process of measuring the amount of energy absorbed or released during a chemical reaction.

Define endothermic reaction.

A reaction that absorbs energy from the surroundings, resulting in a decrease in surrounding temperature.

What is enthalpy change (∆H)?

The heat energy change measured under conditions of constant pressure.

What characterizes an exothermic reaction?

A reaction that transfers energy to the surroundings, causing an increase in surrounding temperature.

What is Hess’s law?

The law stating that the enthalpy change of a reaction is independent of the route taken.

Define mean bond enthalpy.

The enthalpy change when one mole of a specified covalent bond is broken, averaged across several compounds.

What is molar enthalpy change?

The enthalpy change per mole of substance.

What are standard conditions in chemistry?

100 kPa and generally a temperature of 298K.

Define standard enthalpy of combustion (∆cHƟ).

The enthalpy change when one mole of a substance is burned in excess oxygen under standard conditions.

What is standard enthalpy of formation (∆fHƟ)?

The enthalpy change when one mole of a substance in its standard state is formed from its elements under standard conditions.

What is activation energy?

The minimum amount of energy needed for particles to collide successfully for a reaction to take place.

Define catalyst.

A substance that increases the rate of a reaction without being changed in composition, providing an alternative pathway with lower activation energy.

What is collision theory?

The theory stating that reactions can occur only when particles collide with sufficient energy.

How does concentration affect reaction rate?

Increasing concentration brings particles closer together, leading to more frequent collisions and a faster reaction rate.

What effect does pressure have on reaction rate?

Increasing pressure brings gaseous particles closer, increasing collision frequency and reaction rate.

How does temperature affect reaction rate?

Higher temperature increases particle kinetic energy and speed, resulting in more frequent and successful collisions.

What is the Maxwell-Boltzmann distribution?

A representation showing the distribution of molecular energies in a gas at a constant temperature; the area under the curve indicates total particle number.

How is rate of reaction defined?

The measure of the amount of product formed or reactant used over time, measured in g/s, cm3/s, or mol/s.

What is a closed system in chemistry?

A system where only heat exchange occurs with surroundings and no matter can enter or exit.

What is dynamic equilibrium?

A state reached when the rate of the forward reaction equals the rate of the backward reaction, keeping reactant and product concentrations constant.

How does changing concentration affect equilibrium?

Increasing reactant concentration shifts equilibrium towards product formation to restore balance.

How does changing pressure affect equilibrium?

Increasing pressure shifts equilibrium towards the side with fewer molecules; decreasing pressure shifts towards the side with more.

What is the impact of changing temperature on equilibrium?

Increasing temperature favors endothermic reaction product formation and reduces exothermic products.

Define equilibrium constant (KC).

A value expressing the relationship between reactant and product concentrations present at equilibrium in a reversible reaction.

What differentiates a heterogeneous system from a homogeneous system?

A heterogeneous system contains chemicals in different phases, while a homogeneous system contains all in the same phase.

What is Le Chatelier’s principle?

The principle stating that a system at equilibrium will shift to counteract changes in concentration, temperature, or pressure.

What is a reversible reaction?

Reactions where products can react together to regenerate the original reactants, with direction change possible by altering conditions.

Define half equation in redox reactions.

An equation that represents either oxidation or reduction in a redox reaction, useful for balancing.

What does oxidation entail?

The process of losing electrons, resulting in an increase in oxidation number.

What is oxidation state?

The charge of an ion or a theoretical charge of an atom in a covalently bonded compound assuming ionic character.

Define oxidising agent.

Substances that accept electrons during a redox reaction, resulting in their own reduction.

What characterizes a redox reaction?

A reaction in which both oxidation and reduction processes occur simultaneously.

What is a reducing agent?

Substances that donate electrons during a redox reaction, resulting in their own oxidation.

What does reduction involve?

The process of gaining electrons, resulting in a decrease in oxidation number.