Acids

Describe how acids are formed

Acids are made up of polar bonds. Polar bonds are bonds that are covalent (the sharing of electrons between gas atoms), although there is an uneven share of electrons). The covalent bond is formed because an acid is made from positively charged hydrogen ions (H+), and negatively charged gas ions such as (Cl-). Due to oppositely charged particles attracting to each other, both form a polar covalent bond.

Explain what happens when an acid reacts with water

In each water molecule, (H₂O), the oxygen is negatively charged. Due to opposite charges attracting to each other, when the acid enters the aqueous solution (a solution where water is the solvent), the positively charged hydrogen ions are stripped from the acid molecules and bond to the negatively charged oxygen in the water molecules. Leaving negatively charged gas atoms to be floating around the solution. This is called ionization.

Explain the neutralisation reaction in terms of pH

An acid has positively charged hydrogen ions (H+), a base has negatively charged hydroxide ions (OH-). Due to an acid having a pH (hydrogen concentration) of 0-6.9, and a base having a pH of 7.1-14, the pH cancels out. Additionally, the pH cancels out because H+ + OH- makes H₂O (water, a neutral liquid). Because the negatively and positively charged hydrogen atoms cancel out, it leaves oxygen to bond to the hydrogen atoms, to form a polar bonded water molecule.

Explain neutralisation in terms of electrons

Following the Bronsted-Lowry theory of bases, bases are proton exceptors. The acids hydrogen ions (H+ also known as protons), needs 2 transferred electrons to become stable. The bases hydroxide ions (OH-) have delocalised electrons (electrons not fixed to a bond) to transfer to the acids hydroxide ions (H+). This then forms a coordinative covalent bond (a bond where all electrons transferred go towards 1 atom).

What are the theories of bases?

The Arrhenius theory (1884), The Bronsted-Lowry theory (1932), The Lewis theory (1932)

What is the Arrhenius theory of bases?

A base is a substance that ionizes in water to produce hydroxide ions (OH-) increasing the concentration in an aqueous solution (a solution where water is the solvent; for example sodium hydroixde).

What is the limitation of the Arrhenius theory of bases?

It only applies to aqueous solutions, and can’t define basic behaviour in non-aqueous solutions (a solution where water is not a solvent).

Define a weak acid

An acid that when it enters a solvent, it only partially ionizes (splits apart).

Define a strong acid

An acid that when it enters a solvent, it completely ionizes.

What is the strength of an acid determined by?

The ease of it to ionise and release hydrogen ions (H⁺/protons) when added to an aqueous solution.

What is the concentration of an acid determined by?

Concentration of acids are determined by the amount of water added to an aqueous solution that contains an acid.

What type of acid are hydrocarbon acids?

Weak acids

Define concentration

The mass of the solute per given volume of solvent.

What is the unit for volume?

Cm³

What is the unit for mass?

Grams (g) or kilograms (kg)

What is the unit of concentration?

g/cm³ OR kg/cm³

What is the equation of concentration?

Concentration=Mass (of solute) / Volume (of solvent)

What does pH stand for?

Potential for hydrogen

What is the range of the pH scale?

0-14

Define the pH scale

A scale representing the measurements of how acidic or alkaline a solution is based on the concentration of hydrogen ions (H⁺)

What are the two ways of measuring the pH of a solution?

1. Chemical indicators

2. pH probes

What are pH chemical indicators?

Chemical dyes that indicate the pH of a solution, when added to a solution. It indicates the pH value of the solution by colour change.

What do chemical indicators that contain a mixture of dyes do?

They slowly change colour over a wide range of pH levels.

What is a universal indicator?

A common chemical pH dye that indicates colour change, by changing the solution red, green or blue. Red indicates a strong acidic nature present in the solution (pH 0-3), and orange and yellow represent slightly lower pH values, meaning slightly weaker acidic nature (pH 4-6). Green indicates a neutral pH (7). Whilst blue indicates an alkaline nature present in the solution (pH 8-11), and a lilac or purple colour indicates a highly alkaline nature (pH 12-14).

Explain what are pH probes?

A pH probe is a device that has a probe attached to a pH meter. The pH probe is dipped into the solution that the pH value is being identifyed from. Then, the pH meter electronically measures the pH of the solution. This has several advantages such as removing the need for human judgements.

What are the advantages of the pH probe?

1. Much more precise than chemical indicators

2. Much more accurate then chemical indicators

3. Much more reliable as it removes the need for human judgements

Define an acid

Any substance that contains hydrogen ions (H⁺), has a pH less than 7 and ionises to release hydrogen ions in aqueous solutions (solutions where water is the solvent). An aqueous solution becomes acidic because the hydrogen ions that are released form hydronium ions (H₃O⁺), which as they are positively charged they are acidic.

Define a base

Any substance that has a pH greater than 7, releases hydroxide ions (OH⁻) in aqueous solutions (solutions where water is the solvent). These are insoluble, meaning that they don’t completely dissolve/ionise when in contact with a solvent. They also accept protons because H⁺ (a proton) + H⁻ (from the hydroxide ion) → H₂, following the Brownsted-Lowry theory of bases.