AQA A-Level Chemistry Question Cards Questions and Answers

AQA A-Level Chemistry Question Cards Questions and Answers What is meant by activation energy? The minimum energy that a particle must have in order for a reaction to occur; the energy (enthalpy) difference between the reactants and the transition state. What is an aldehyde? An organic compound with the general formula RCHO. What is an alkane and what isthe general formula for the alkanes? A hydrocarbon with C-C and C-H single bonds only. General Formula = CnH2n+2 What are allotropes? Give examples of allotropes of a non-metal. Pure elements which can exist in different in physical forms in which their atoms are arranged differently. Examples include the allotropes of carbon: diamond, graphite and buckminsterfullerene. What is meant by anaerobic respiration? The process by which energy is released and new compounds are formed in living things in the absence of oxygen. What is meant by atom economy and how do you find the atom economy for the production of a particular species in a given reaction? This describes the efficiency of a chemical reaction by comparing the total number of atoms in the product with the total number of atoms in all of the starting materials. % atom economy = mass of desired product/total mass of reactants x 100 What is an atomic orbital? Describe the general shape of the s, p and d orbitals. A region of space around the nucleus of an atom where there is a high probability of finding an electron. s = spherical p = dumb-bell shaped d = variable: dxy, dxz, dyz, dx²-y², dz² What is the Avogadro constant and what value does it take? The total number of particles in a mole of a substance. Also called the Avogadro number. Symbol = L L = 6.022 x 10²³ (atoms, molecules or ions) What is bond dissociation enthalpy? The enthalpy change required to break a covalent bond with all species in the gaseous state. ΔH = Bond Breaking - Bond Making What is a calorimeter? An instrument for measuring the heat changes that accompany chemical reactions. What is a catalyst and how does it work? A substance that alters the rate of a chemical reaction but is not used up in the reaction. It provides an alternate reaction pathway with a lower activation energy. e.g. Fe is a catalyst in the Haber-Bosch process (ammonia etc) What is catalytic cracking and why is it done? The breaking, with the aid of a catalyst, of long-chain alkane molecules (obtained from crude oil) into shorter-chain hydrocarbons (some of which are alkenes) for which there is higher demand. Catalyst = zeolite catalyst Temp. = 800-1000 °C What is a carbocation? An organic ion in which one of the carbon atoms has a positive charge (C⁺). What does the term carbon-neutral mean? A process, or series of processes, in which as much carbon dioxide is absorbed from the air as is given out e.g. biofuels and photosynthesis/combustion What does the term chemical feedstock mean? The starting materials in an industrial chemical process. What is coordinate bonding? Give an example of such bonding. Covalent bonding in which both the electrons in the bond come from one of the atoms in the bond; also called dative bonding. e.g. formation of H₃O⁺ (the hydronium ion) What is covalent bonding? Give an example of a covalently-bonded species. Describes a chemical bond in which a pair of electrons are shared between atoms. examples: H₂O, CH₄, NH₃....... What is meant by dative covalent bonding? Covalent bonding in which both the electrons in the bond come from one of the atoms in the bond; also called dative bonding. e.g. formation of H₃O⁺ (the hydronium ion) What are delocalised electrons? Electrons which are spread over several atoms and help to bond them together e.g. in metallic bonding. Also found in species such as graphite and allow such materials to conduct electricity as the electrons are able to move through the structure and carry a charge. What are dipole-dipole forces? Give an example of a species which exhibits such forces. An intermolecular force that results from the attraction betwen molecules with permanent dipoles. e.g. between molecules of HCl. What is a displacement reaction? Give an example of such a reaction. A chemical reaction in which one atom or group of atoms replaces another in a compound. e.g. Zn + CuO → ZnO + Cu In Organic Chemistry, what is meant by the displayed formula of a compound? The formula of a compound drawn out so that each atom and each bond is shown. In redox chemistry, what is meant by disproportionation? Describes a redox reaction in which the oxidation number of some atoms of a particular element increases and that of other atoms of the same element decreases. What is meant by dynamic equilibrium? Give an example of dynamic equilibrium. A situation (reaction) in which the composition of a constant concentration reaction mixture does not change because both forward and backward reactions are proceeding at the same time and at the same rate. What is meant by the term electron density? The probability of electrons being found in a particular volume of space. What is Valence-Shell- Electron-Pair-Repulsion (VSEPR) Theory? A theory which explains the shapes of simple molecules by assuming that pairs of electrons around a central atom repel each other and thus take up positions as far away as possible from each other in space; thereby minimising repulsions. BP-BP LP-BP LP-LP Define electronegativity The relative attraction that an atom in a molecule has for the shared pair of electrons in a covalent bond. or The power of an atom to attract the electrons in a covalent bond. What is an electrophile? An electron-deficient atom, ion or molecule that takes part in an organic reaction by attacking areas of high electron density in another reactant; an electron-pair acceptor. What is electrophilic addition? Give an example of such a reaction. A reaction in which a carboncarbon double bond is saturated, by the C=C bond attacking an electrophile. e.g. addition of Br₂ to ethene to form 1,2-dibromoethane What are electrostatic forces? Where would you find such forces? The forces of attraction and repulsion between electrically charged particles. e.g. in ionic compounds What is an elimination reaction? Give an example of an eleimination reaction. A reaction in which an atom or group of atoms is removed from a reactant. e.g. dehydration of alcohols to form alkenes Define empirical formula. The simplest whole-number ration of the atoms of each element present in a compound. e.g. the empirical formula for glucose (C₆H₁₂O₆) = CH₂O What is an endothermic reaction? Describes a reaction in which heat is taken in as the reactants change to products; the temperature of the system thus drops i.e. heat is taken from the surroundings and a thermometer would measure a decrease in temperature e.g. coldpacks Define enthalpy change A measure of heat energy given out or taken in when a chemical or physical change occurs at constant pressure. What are enthalpy (reaction profile) diagrams? Diagrams in which the enthalpies (energies) of the reactants and products of a chemical reaction are plotted on a vertical scale to show their relative levels. What is meant by an equilibrium mixture? The mixture of reactants and products formed when a reversible reaction is allowed to proceed in a closed container until no further change occurs. The forward and backward reactions are occurring at the same time and rate. Concentrations of all species remain constant. What is an exothermic reaction? Describes a reaction in which heat is given out as the reactants change to products; the temperature of the system thus rises i.e. heat is given out to the surroundings and a thermometer would measure an increase in temperature. In Infrared Spectroscopy, what is the fingerprint region? What is the use of this region? The area of an infrared spectrum below about 1500 cm⁻¹. It is caused by complex vibrations of the whole molecule and is characteristic of a particular molecule. Use: distinguishing between isomers, purity testing, C-O bond. In Organic Chemistry, what is a fraction? A mixture of hydrocarbons collected over a particlar range of boiling points during the fractional distillation of crude oil. In a given fraction, the hydrocarbons have similar chain lengths etc. Define the term free radical A chemical species with an unpaired electron - usually highly reactive. In Organic Chemistry, what is a functional group? What is the functional group present in ethanol? An atom or group of atoms that gives an organic compound its characteristic properties. Functional group = ROH Why do elements in a given group of the periodic table have similar reactivity? They have the same number of electrons in their outer (valence) shell. e.g. Group 2 elements all have two electrons in their valence shell and will want to lose these 2 electrons when they react. In redox chemistry, what is meant by a half-equation? Give an example of such an equation. An equation for a redox reaction which considers just one of the species involved and shows explicitly the electrons transferred to or from it. e.g. Br₂ + 2e⁻ → 2Br In Organic Chemistry, what is a homologous series? Give an example of such a series. A family of organic compounds with the same functional group but different carbon chain length. -general formula e.g. CnH2n+2 -each member differs by CH₂ -gradation in physical properties as chain length increases What is hydrogen bonding? Give an example of a species which exhibits hydrogen bonding. A type of intermolecular force in which a hydrogen atom (Hδ+) interacts with a more electronegative atom with a δ⁻ charge. Occurs between H and atoms of N, O and F and acts as a bridge between 2 electronegative atoms What is meant by incomplete combustion? A combustion reaction in which there is insufficient oxygen for all the carbon in the fuel to burn to CO₂. Carbon monoxide (CO) and/or carbon (soot) are formed. What is Ionic Bonding? Give an example of an ionically bonded species. Describes a chemical bond in which an electron or electrons are transferred from one atom to another, resulting in the formation of +ve and -ve ions with electrostatic forces of attraction between them. e.g. bonding between metals and non-metals: NaCl, K₂O etc. Define Ionisation Energy The energy required to remove a mole of electrons from a mole of isolated, gaseous atoms or ions. 2nd IE is always greater than 1st IE as you are trying to remove an electron from a +ve ion. Na(g) → Na⁺(g) + e⁻ Describe the trend in ionisation energy across a period and down a group in the periodic table. Increases across a period and decreases down a group. Describe the trend in electronegativity across a period and down a group in the periodic table. Increases across a period and decreases down a group. F is the most electronegative element. What is an isomer? Give an example of a hydrocarbon which exhibits isomerism and state the type of isomerism. One of two (or more) compounds with the same molecular formula but different arrangement of atoms in space. e.g. butan-1-ol and butan-2-ol; position isomerism. What is a ketone? Give an example of a ketone and an everyday use of that ketone. An organic compound with the general formula R₂CO in which there is a C=O bond. e.g. acetone (propanone) Use = nail polish remover What is a lattice? A regular three-dimensional arrangement of atoms, ions or molecules. e.g. diamond (covalent) and NaCl (ionic) both have lattice structures. In Organic Chemistry, what is a leaving group? In an organic substitution reaction, the leaving group is an atom or group of atoms that is ejected from the starting material, normally taking with it an electron pair and forming a negative ion. e.g. nucleophilic substitution of haloalkanes What is meant by the term lone pair? Give an example of a species with at least one lone pair of electrons. A pair of electrons in the outer shell of an atom that is not involved in bonding. e.g. H₂O (2 l.p.), NH₃ (1 l.p.) In Kinetics, what is a Maxwell-Boltzmann distribution? The distribution of energies (and therefore speeds) of the molecules in a gas or liquid. x- axis = energy (E) y-axis = fraction of particles with energy E highest point on curve = most probable energy Define Mean Bond Enthapy The average value of the bond dissociation enthalpy for a given type of bond taken from a range of different compounds. Unit = kJ mol⁻¹