Acids, Alkalis and pH Scale

Acids

• Acids added to water form positively charged hydrogen ions (H+).
• The presence of H+ ions makes a solution acidic.
• Alkalis added to water form negative hydroxide ions (OH-).
• The presence of OH- ions makes a solution an alkali.
• The pH scale is a numerical scale to show how acidic or alkaline a solution is.

The pH Scale

• The pH scale ranges from 0-14.
• Extremely acidic substances can have values below 0.
• Acids have pH values below 7, while alkalis have pH values above 7.
• Lower pH indicates a more acidic solution.
• Higher pH indicates a more alkaline solution.
• A pH of 7 is neutral.
• The pH scale is logarithmic, so each change represents a tenfold change in hydrogen ion concentration.
• An acid with a pH of 3 has ten times the H+ concentration of an acid with a pH of 4.
• An acid with a pH of 2 has 100 times the H+ concentration of an acid with a pH of 4.

Indicators

• Two color indicators are used to distinguish between acids and alkalis.
• Litmus is a common indicator from lichens.
• Synthetic indicators are organic compounds that change color based on acidity.
• Phenolphthalein and methyl orange are frequently used synthetic indicators.
• Synthetic indicators show the endpoint in titrations due to their sharp color change when an acid is neutralized by an alkali.
• Litmus changes color gradually and is not suitable for titrations.
• Litmus paper (red and blue versions) is useful for testing gases and solutions.

Hydrogen Ions & pH

• Acids contain hydrogen ions in solution.
• More hydrogen ions means a stronger acid and a lower pH.
• Higher hydroxide ion concentration results in a higher pH.
• pH measures H+ concentration, having an inverse relationship.

Core Practical: Investigating pH

• The aim is to investigate pH changes in a fixed volume of dilute HCl by adding various amounts of solid base.
• Materials include dilute HCl, solid base (CaO or Ca(OH)2), a conical flask, a volumetric pipette, a glass rod, a spatula, a weighing boat, and pH probe or universal indicator paper.
• A graph of the amount of base (x-axis) vs. pH (y-axis) should be plotted, use a neat table.
• The graph indicates a sudden pH change, corresponding to the vertical section.
• Adding more solid base will increase the pH, thus neutralizing the acid.
• Lab safety is also discussed.
• Copper(II) oxide can cause serious eye irritation and is a skin irritant, harmful if swallowed/inhaled, and toxic to aquatic life.
• Dilute hydrochloric acid is not classified as hazardous but can harm eyes or skin.
• Avoid contact with skin and eyes, and prevent powder inhalation.

Acid Strength & Concentration

• Acids are either strong or weak based on how many ions they produce.
• Acids ionise or dissociate in water to make H+ ions.
• Strong acids (HCl, H2SO4) dissociate completely, resulting in high H+ concentration and low pH.
• Weak acids (ethanoic acid, CH3COOH, hydrofluoric acid, HF) ionize partially, with pH values between 4 and 6.
• Weak acids establish an equilibrium between molecules and ions after being added to water.

Strong & Weak Acids

• Equilibrium favors intact acid molecules in weak acids.
• Strong/ weak indicates the ability to dissociate, not concentration.
• Dilute strong acids can have lower pH than concentrated weak acids due to complete dissociation.
• Dilute solutions contain small amounts of solute.
• Concentrated solutions contain large amounts of solute.

Bases

• Bases neutralize acids, forming a salt and water.
• Not all bases are alkalis, but all alkalis are bases.
• An alkali is a water-soluble base.
• Alkalis have pH values above 7.
• Under alkaline conditions red litmus paper turns blue.
• Bases are usually oxides, hydroxides, or carbonates of metals.
• One unusual base is ammonia solution.
• Ammonia and ammonium hydroxide are the same.
• Terminology: Ammonia (NH3) is a gas, ammonium (NH4+) is an ion in ammonium compounds.

Reactions of Acids

• Metals react with acids to form a salt and hydrogen gas.
• Must be above hydrogen in the reactivity series.
• More reactive metals react more vigorously.
• The general equation is: metal + acid → salt + hydrogen.
• Acids react with oxides/hydroxides in neutralization reactions.

Tests for Hydrogen & Carbon Dioxide

• Test for hydrogen: hold a burning splint at the open end of a test tube of gas.
• If the gas is hydrogen: It burns with a loud "squeaky pop."
• The rapid hydrogen and oxygen combustion produces this effect and water.
• In carbon dioxide the limewater turns milky or cloudy. This occurs by bubbling gas through calcium hydroxide.
• Distinguishing the tests for hydrogen and oxygen with the memory aid of lighted splint having "H" for hydrogen, and Oxygen gas is "O".

Core Practical: Preparing Copper Sulfate

• Aim is to prepare a pure, dry sample of hydrated copper(II) sulfate crystals.
• Materials provided are: 1.0 mol/dm³ dilute sulfuric acid, Copper(II) oxide, Spatula & glass rod, Measuring cylinder & 100 cm³ beaker, Bunsen burner, Tripod, gauze & heatproof mat, Filter funnel & paper, conical flask, Evaporating basin and dish.
• The practical process involves heating acid, adding metal/base/carbonate, stirring, filtering, and evaporator.
• Safety is mentioned to be aware of the material's corrosive / Irritant / Dangerous for the environment, that can occur during the preparation of a salt.

Preparing a Salt by Titration

• If salts must be made from an acid and a soluble reactant, then the titration technique is required.
• The goal of a titration is finding the perfect volume of acid and soluble reactant and mixing them together so that the only products remaining are salt and water. Lab safety is discussed.
• During the handling of lead salts they should be considered as a toxin and approached with care.

Solubility Rules

• Compared to covalent ones, ionic compounds aretypically soluble in water, but there areexceptions.
• With an adequate understanding of this, you'll be able to use the most accurate preparations.
• Whether the salt being formed ends up being soluble or insoluble, the salt is used to decide the laboratory.
• Balanced equations help find the identity of a salt product.

Preparing an Insoluble Salt

• Lead(II) sulfate is formed to prepare a sample of dry insoluble salt.
• Water and reagents of a known concentration: Mix 10 cm³ of the 0.05 M solutions.
• Allow any precipitate to settle to the bottom of the container (a test tube is ideal as it is easier to see through).
• The precipitate must be washed during filtration with deionized water.
• Weigh the filter paper, transfer the damp crystals onto it, and dry in a warm oven, then reweigh and subtract.