Chemistry OCR A Level Chapter 4 - Acids and Redox

To what degree of accuracy is a burette reading measured?

-A burette reading is recorded to the nearest half division, with the bottom of the meniscus on a mark or between two marks. -Each burette reading is measured to the nearest +/- 0.05cm^3 so the reading always has two decimal places.

What is a titration?

-A titration is a technique used to accurately measure the volume of one solution that reacts exactly with another solution. -Titrations can be used for finding the concentration of a solution, identification of unknown chemicals, and finding the purity of a substance.

What is used to make up a standard solution?

-A volumetric flask is used to make up a standard solution very accurately. -Volumetric flasks are manufactured in various sizes and can measure volumes very precisely. -The volumetric flasks that you will use are manufactured to the typical tolerances below: A 100cm^3 volumetric flask: +/- 0.20cm^3 A 250cm^3 volumetric flask: +/- 0.3cm^3

What is a weak acid?

-A weak acid, such as ethanoic acid, CH3COOH, only releases a small proportion of its available hydrogen atoms into solution as H+ ions. -A weak acid partially dissociates in aqueous solution. -CH3COOH(aq) <----> H+(aq) + CH3COO-(aq) -The equilibrium sign (<---->) indicates that the forward reaction is incomplete. -Most organic acids, like ethanoic acid, are weak acids.

What is an acid?

-All acids contain hydrogen in their formulae. -When dissolved in water, an acid releases hydrogen ions as protons, H+, into the solution. -pH 7>

What is an alkali?

-An alkali is a base that dissolves in water releasing hydroxide ions (OH-) into the solution. -e.g. NaOH, KOH, NH3.

Titration calculations.

-From the results of a titration, you will know the following: both the concentration and reacting volume of one of the solutions. only the reacting volume of the other solution. -Step one - work out the amount, in mol, of the solute in the solution for which you know both the concentration and volume. -Step two - use the equation to work out the amount, in mol, of the solute in the other solution. -Step three - work out the unknown information about the solute in the other solution.

What are the special cases of oxidation number?

-H in metal hydrides - -1 - NaH, CaH2 -O in peroxides - -1 - H2O2 -O bonded to F - +2 - F2O

What is neutralisation?

-In neutralisation of an acid, H+(aq) ions react with a base to form a salt and neutral water. -The H+ ions from the acid are replaced by metal or ammonium ions from the base.

Neutralisation of acids with carbonates.

-Like metal oxides, carbonates neutralise acids to form a salt and water. -There is also a third product, carbon dioxide gas.

What is a base?

-Metal oxides, metal hydroxides, metal carbonates, and ammonia are classified as bases. -A base neutralises an acid to form a salt. -e.g. MgO, Na2CO3, CuCO3.

What is oxidation?

-Oxidation is the loss of electrons. -An increase in oxidation number.

What is the oxidation number?

-Oxidation number is based on a set of rules that apply to atoms, and can be thought of as the number of electrons involved in bonding to a different element.

What are redox reactions?

-Redox reactions involve reduction and oxidation. If one process happens, so must the other.

What is reduction?

-Reduction is the gain of electrons. -A decrease in oxidation number.

What are the rules for oxidation numbers?

-The oxidation number is always zero for elements. -Each atom in a compound has an oxidation number. -An oxidation number has a sign, which is placed before the number. -Sum of oxidation numbers = total charge

What is a strong acid?

A strong acid, such as HCl, releases all its hydrogen atoms into a solution as H+ ions and completely dissociates in aqueous solution.

How many times should you repeat a titration?

Repeat until two accurate titres are concordant - agreeing to within 0.10cm^3.