aqa chemistry paper

Atoms and Compounds

Definition of Substances

• Substances are forms of matter with uniform composition and properties, made of atoms.

• Elements are pure substances that cannot be broken down by chemical means and are represented by chemical symbols on the periodic table, such as H for hydrogen and O for oxygen.

Compounds

• Compounds are pure substances formed when two or more different types of atoms chemically bond together.

• Example: Water (H2O) consists of two hydrogen atoms and one oxygen atom.

• The invisible number (1) is implied when a subscript is not written, indicating one atom is present.

Chemical Reactions

• In chemical reactions, atoms are neither created nor destroyed, but rather rearranged, adhering to the law of conservation of mass.

• Atoms in the reactants must be balanced with those in the products, hence chemical equations must be balanced.

• Balancing Equations Pro-tip: Begin by balancing the number of atoms in compounds rather than single elements for efficiency.

Mixtures and States

Mixtures

• A mixture consists of various elements and compounds that are combined physically but not chemically bonded.

• Examples include air (a mixture of gases) and salt water (a solution of salt in water).

Separation Techniques

1. Filtration: Used to separate insoluble solid particles from liquids (e.g., sand can be filtered from water).

2. Crystallization: Involves evaporating the solvent to retrieve the solute as crystals, such as recovering salt from seawater by evaporation. Process of Crystallization:

   1. Saturation: The solution is heated to increase the solubility of the solute, allowing more of it to dissolve. This process often involves boiling the solution.

   2. Cooling: The hot solution is then allowed to cool slowly. As it cools, the solubility of the solute decreases, leading to the formation of crystals.

   3. Nucleation: Tiny clusters of solute particles, known as nuclei, begin to form as the temperature drops, providing a starting point for crystal growth.

   4. Crystal Growth: Over time, more solute molecules attach to the nucleation sites, leading to the growth of larger crystals.

   5. Separation: Once the crystals have formed, they are collected by filtration, generally using a filter paper within a funnel. The remaining liquid, or mother liquor, can be removed.

   6. Washing and Drying: The crystals are often washed with a small amount of cold solvent to remove impurities without dissolving the crystals. Finally, they are allowed to dry.

3. Distillation: Separates liquid components of a mixture by utilizing differences in boiling points; fractional distillation is used for complex mixtures, such as crude oil.

States of Matter

• The three states of matter include solids, liquids, and gases, with behaviors differing significantly among them.

• Energy must be supplied to change states (e.g., melting ice to water or evaporating water to steam), which are physical changes rather than chemical changes.

Historical Models of Atoms

• JJ Thompson: Proposed the plum pudding model, suggesting that atoms are made of a positively charged 'soup' with negative electrons embedded throughout.

• Ernest Rutherford: Conducted the gold foil experiment, leading to the discovery of the nucleus and demonstrating that most of the atom's volume is empty space.

• Niels Bohr: Introduced a model with electrons in defined paths (orbitals) around the nucleus, quantizing the energy levels of electrons.

• James Chadwick: Discovered neutrons, neutral particles in the nucleus, which contribute to atomic mass but not charge.

Isotopes

• Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons, affecting their mass.

Element Arrangement

• Elements were first arranged by increasing atomic weight but later by Mendeleev based on their chemical properties, effectively predicting the existence of undiscovered elements.

Electron Shells

• Electrons are organized into shells around the nucleus, filling from the innermost shell outward, typically following the 2, 8, 8, 2 rule for the first four shells.

Groups in the Periodic Table

1. Metals (left side): Tend to lose electrons easily and are often highly reactive.

2. Nonmetals (right side): Gain electrons, generally becoming less reactive down the group.

3. Alkali Metals (Group 1): Reactivity increases going down the group due to greater distance from the nucleus.

4. Halogens (Group 7): Reactivity decreases as you move down the group, being more reactive at the top.

5. Noble Gases (Group 0): Highly unreactive due to having full outer electron shells, making them stable.

Bonding

Metallic Bonding

• Occurs in metals where ions form a lattice structure and electrons are delocalized, allowing for high electrical conductivity and malleability.

Ionic Bonding

• Formed when a metal atom donates electrons to a non-metal atom (e.g., lithium donates one electron to chlorine to form Li+ and Cl-).

• Ionic compounds can be represented visually with dot and cross diagrams to depict electron transfer.

Covalent Bonding

• Involves the sharing of electrons between non-metal atoms (e.g., Cl2, O2, N2), forming molecules with generally low boiling points due to weak intermolecular forces.

• Unique structures like giant covalent arrangements exist in substances such as diamond and graphite, showcasing distinct physical properties.

Quantitative Chemistry

Conservation of Mass

• Total mass remains unchanged in chemical reactions; therefore, balancing equations is essential for accurate representations of reactions.

Moles

• Link the amounts of substances in grams to moles, using the formula moles = mass (g) / relative atomic/formula mass (RAM). This calculation is vital for stoichiometric calculations in reactions.

Stoichiometry

• Understanding stoichiometry is essential for determining ratios of reactants and products within chemical reactions, ensuring the correct proportion of substances in formulations.

Chemical Changes and Energy Changes

Reactivity

• Determining the relative reactivity of elements through displacement reactions, and the reactivity series that includes metals, hydrogen, and carbon.

Reduction/Oxidation

• Reduction involves gaining electrons, while Oxidation involves losing electrons. The mnemonic OIL RIG (Oxidation Is Loss, Reduction Is Gain) is valuable for memorization.

Acid-Base Reactions

• Reactions between acids and alkalis produce salt and water; the products exhibit neutral pH (e.g., sodium hydroxide reacts with hydrochloric acid to yield sodium chloride and water).

• The pH scale is logarithmic, meaning stronger acids have significantly lower pH values than weaker ones.

Titrations

• A technique used to determine unknown solution concentrations by neutralizing them with a solution of known concentration until a visible reaction endpoint is reached.

Electrolysis

Electrolysis is a chemical process that employs electrical current to drive a non-spontaneous chemical reaction, making it possible to separate elements from ionic compounds.

Process of Electrolysis

1. Setup: The process generally occurs in an electrolytic cell, which consists of two electrodes: the anode (positive electrode) and cathode (negative electrode).

2. Electrolyte: An electrolytic solution must be present, which contains ions allowing conduction of electricity. This can be a molten ionic compound or an aqueous solution of an ionic compound.

3. Electric Current: When electrical current is passed through the electrolyte, it causes the ions to move towards the electrodes. Positive ions (cations) travel to the cathode, where they gain electrons (reduction), while negative ions (anions) migrate to the anode, where they lose electrons (oxidation).

4. Separation of Elements: This movement results in the breaking of chemical bonds in the ionic compound, allowing for the separation of elements. The specific products formed at each electrode depend on the composition of the electrolyte and the nature of the electrodes used.

Applications of Electrolysis

• Electroplating: A process used to deposit a layer of metal onto a surface to enhance appearance or prevent corrosion.

• Extraction of Metals: Electrolysis is essential in extracting metals from their ores, particularly for reactive metals like aluminum and lithium, which cannot be extracted through traditional methods.

• Water Splitting: Electrolysis can be utilized to split water (H2O) into hydrogen and oxygen gases, presenting a method for generating clean hydrogen fuel.

Importance in Chemistry

Electrolysis is a crucial aspect of chemistry and materials science, facilitating processes that require the breaking down of compounds into their constituent elements. Its applications in sustainable energy production and industrial processes underscore its significance in modern technology and research.

Bonus: Batteries and Cells (Triple Only)

Chemical Reactions

• Batteries work by producing voltage through chemical reactions; rechargeable batteries can reverse the reaction when an external current is supplied.

• Hydrogen fuel cells function by separating hydrogen and oxygen gases and combining them to produce energy, emphasizing clean energy concepts in modern chemistry.

Required practical paper 1

1. Investigating the Effect of pH on the Rate of Reaction:

Preparation of a Salt: Involves acid-base neutralization to create a soluble salt from an acid and a base. This process typically requires the careful addition of an acid to a base until a neutral point is reached, indicated by the pH level approaching 7.

• Example: The reaction between hydrochloric acid and sodium hydroxide to produce sodium chloride (table salt) and water can be represented by the equation: [ HCl + NaOH \rightarrow NaCl + H_2O ]

• This reaction releases heat, illustrating the exothermic nature of the neutralization process and should be conducted under controlled conditions to prevent excessive heat from affecting the reaction.

Investigating the Effect of pH on the Rate of Reaction: This experiment uses sodium thiosulfate and hydrochloric acid to measure the time taken for a cross beneath a beaker to disappear as the solution turns cloudy. The rate of reaction can be influenced by the pH level, as variations in acidity can affect the reactivity of the reactants.

• A typical procedure involves mixing solutions of sodium thiosulfate of varying pH levels and hydrochloric acid in a conical flask and timing how long it takes for the cross to no longer be visible. This allows for the observation of how changes in pH can affect the reaction rate.

• Determination of the Density of a Solid or Liquid:

Determination of the Density of a Solid or Liquid: To calculate density, first measure the mass using a balance. Then, measure the volume of the liquid using a graduated cylinder or the volume of a solid using water displacement in a measuring cylinder or calculated geometric formulas for regular shapes. Density is calculated using:

• [ Density = \frac{Mass}{Volume} ] This experiment helps understand the physical properties of materials.

Titration to Determine the Concentration of an Acid or Base: This technique involves using a burette to deliver a solution of known concentration (the titrant) to a solution of unknown concentration (the analyte) until the endpoint is reached, which is often indicated by a color change if an indicator is used. The volume of titrant used gives the necessary information to calculate the unknown concentration using the formula:

• [ C_1V_1 = C_2V_2 ] Where C is concentration and V is volume.

Heating a Substance to Observe Physical and Chemical Changes: This involves heating various materials and observing the changes they undergo. Look for changes in color, state (solid to liquid, liquid to gas), and any alterations in composition indicating a chemical change. Record the temperatures at which changes occur to analyze the thermal properties of different substances.

Investigating the Temperature Change in Neutralization Reactions: This experiment involves mixing an acid with an alkali and measuring the resulting temperature change using a thermometer. This data helps determine the exothermic or endothermic nature of the reaction, along with calculating the enthalpy change. An example involves reacting hydrochloric acid with sodium hydroxide and monitoring the temperature change to observe if energy is released or absorbed during the reaction.

• Ensure to take multiple readings for accuracy and repeat the experiment to confirm results.

• Analyze the data to compare with theoretical values of enthalpy changes for neutralization reactions.