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3.1.10 & 3.1.11 – Acid& base dissociation constants (HL)

📌 Strength of acids and bases

• Since the strength of acids and bases is based on the extent of dissociation, there are acid and base dissociation constants that can be used to measure the relative strength of an acid or base
• For weak acids/bases the dissociation is represented as a reversible reaction and the value of the constant will determine the relative strength.

• K_a is a fixed value for each specific acid at each specific temperature
• The higher the value of K_a, the stronger the acid is
• Using this same logic, we can ask get the following formula for a generic base ionisation

• The greater the value of K_b, the stronger the base is

📌 Calculations

• K_a is a fixed value for each specific acid at each specific temperature
• The higher the value of K_a, the stronger the acid is
• Using this same logic, we can ask get the following formula for a generic base ionisation

• The greater the value of K_b, the stronger the base is

3.1.9 – The pOH scale (HL)

📌 pOH as a measurement

• The pOH scale is a measurement of the concentration of hydroxide ions in an aqueous solution
• Similar to the pH scale, the pOH can be calculated using the following formula

• Since the two scales are inter-related we also know that pH + pOH = 14

3.1.8 – pH curves (SL)

📌 Titrations

3.1.7 – Neutralisation reactions

📌 Acid-base reactions

• When acids and bases react together, it is known as a neutralisation reaction
• The key concept behind a neutralisation reaction is that an acid and base react to form a salt and water
• Other reactions with acids also produce salts but not water
• For example, when acids react with metals the products are salt and hydrogen gas. Similarly, when acids react with carbonates, the prodcuts are salt, water and carbon dioxide gas
• The distinguishing feature of neutralisation reactions is that the ONLY products formed will be salt and water
• A well known example of a neutralisation reaction is the reaction between NaOH and HCl to give the salt NaCl

3.1.6 – Strong and weak acids and bases

📌 Strength of an acid/base is relative to extent of ionisation

• The strength of an acid is dependent on it’s extent of ionisation
• Strong acids are defined as those that can completely dissociate while weak acids only partially dissociate
• Most organic acids are considered to be ‘weak acids’ while acids like HCl are known as strong acids
• Similarly, bases are defined as strong or weak depending on how easily ionised they are

• The ease of dissociation is based on the strength of the bond between the hydrogen atom and the atom ‘X’
• For example, the strength of hydrogen halides increases down the group BECAUSE the bond strength decreases down the group

📌 Strong and weak acid/base properties

1. Strong acids and bases are better conductors of electricity than weak acids and bases (assuming the same concentrations are compared)
2. Strong acids and bases increase the rate of reaction while reactions will be slower with weaker acids/bases
3. Strong acids have lower pH than weak acids (all below 7) and strong bases have high pH than weak bases (all above 7)

3.1.5 – The ion product constant of water

📌 Ionisation of water

• Ionising water produces hydrogen ions and hydroxide ions (H⁺and OH^–)

• The constant of water K_W can be defined using this formula
• K_W = [H⁺][OH^–] which is fixed at 1.00 x 10^-14 at STP
• In pure water (distilled) we know that H⁺ = OH^– therefore giving us the concentration of H⁺ ions as 1.00 x 10^-7
• Knowing this, and applying the previously learnt formula, we can determine the pH of pure distilled water to be 7.0

📌 Inverse relationships between ions

• The relationship between hydrogen ions and hydroxide ions is inverse
• Thus, as hydrogen ions increase in concentration, hydroxide ions decrease the substance becomes more acidic
• Since K_W is temperature dependent, we can use the concentration of either hydrogen or hydroxide ions to calculate the value of K_W

📌 The water constant

• Given that K_W is temperature dependent and dissociation of water is endothermic then the followiung two statements musrt be true :
• 1. Increasing the temperature causes equilibrium to shift right, thus pH decreases as concentration of both ions increases
• 2. Decreasing the temperature causees equilibrium to shift left, thus pH increases as concentration of both ions decreases
• However, regardless of this change, the concentration of hydrogen ions is always equal to hydroxide ions, therefore despite the pH change, water does not become either acidic or basic

3.1.4 – The pH scale

📌 What is pH?

• The pH (potential of hydrogen) scale is the scale used to measure how acidic or how basic a certain substance is
• The scale ranges from 0 to 14, with 0 being the most acidic, 14 being the most basic and 7 being perfectly neutral
• The pH of a certain solution is determined by the concentration of hydrogen ions present in the solution
• The pH scale is the logarithmic expression of this concentration

• Some features of the pH scale include :

1. pH does not have units
2. The pH of a substance is inversely related to the hydrogen ion concentration (ie greater pH means lower concentration)
3. Increasing or decreasing the pH by 1 unit on the pH scale represents a 10x change in the concentration of hydrogen ions (due to logarithmic properties)

• The concentration of hydrogen ions is inverse to the concentration of hydroxide ions present in any aqueous solution, thus making the pH scale measurement a suitable way to test BOTH acidity and alkalinity

3.1.3 – Amphiprotic species

📌 What is an amphiprotic species?

• Certain species can act as both a Brønsted-Lowry acid AND a base depending on the substance they are interacting with. These are known as amphiprotic
• One such example of an amphiprotic species is water (H₂O)
• To be defined as an amphiprotic species, a substance must have both a lone pair of electrons (so it c an accept an H⁺ ion) as well as an H⁺ ion already in the substance (so it can donate it)
• To determine what species might be ampjoteric, we notice a trend in the periodic table
• Metal oxides are all basic, while non-metal oxides are all acidic. The acidic nature of an oxide therefore increases from left to right across a period.
• Using this logic, we can determine that amphoteric oxides form with many of the ‘metalloid’ elements (such as aluminium)

R3.1.1 & R3.1.2 – Brønsted-Lowry acids and bases

📌 Transferring H+ ions

• The Brønsted-Lowry acid base theory defines acids and bases as substances based on the exhcange of protons (aka H+ ions)
• An acid can be defined as a proton donor while a base is defined as a proton acceptor
• This theory also suggests that every acid has a ‘conjugate base’ and every base has a ‘conjugate acid’
• A conjugate is a substance differing by exacatly one H+ ion
• A Brønsted-Lowry acid would be a reactant with the general formula XH while it’s conjugate base would be X^–
• Similarly, a Brønsted-Lowry base would be a reactant with the general formula Y while it’s conjugate acid would be YH

1.3.4 – Biofuels

📌 Ethanol

• Used as a liquid biofuel
• Made from biomass by fermenting plants which are rich sources of carbohydrates
• This reaction is catalysed by yeast which perform anaerobic respiration which has waste products of ethanol and carbon dioxide
• Ethanol is then combined with unleaded gasoline (1:9 ratio) to produce a mixture called gasohol
• Ethanol does have a major disadvantage as it can cause corrosion. Additionally, ethanol forms hydrogen bonds with water molecules in the air which causes it to separate from the hydrocarbons in the mixture

📌 Advantages and disadvantages of biofuels