C5 - Chemical Changes Flashcards
Oxidation
• adding oxygen to an element
What is reduction?
Removing oxygen from an element
Electrons in terms of oxidation and reduction
When an element is oxidised, it loses electrons
When an element gains electrons it is reduced
What kind of ions do metals form?
Positive ions
What are metal ores?
• naturally occurring mineral from which a metal can be extracted economically
• metals are present as positive ions in a compound
• need to be reduced
• chemically (displacement with a more reactive metal) or electrically (electrolysis)
What is considered before metal ores are extracted?
• whether it is economically viable
• how much metal it contains
• demand for metal
• how easy it is
Metals are present as positive ions in a compound.
Need to be reduced.
Chemically (displacement) or electrically (electrolysis)
The pH scale
• how acidic or alkaline a solution is
• pH scale goes from 0 to 14
• less than 7 = acid - lower, the more acidic
• more than 7 = alkali
• neutral substances = pH 7
Testing pH
• one way to test pH is using an indicator
• an indicator is a dye that changes colour depending on whether it’s above or below a certain pH
• some indicators contain a mixture of dyes that mean they gradually change colour over a broad range of pHs - these are a wide range of indicators
• they’re useful for estimating the pH of a solution
• universal indicator
Another method is to use a pH probe attached to a pH meter which can measure pH electronically
A probe
Acids
• substances that produce H+ ions when in aqueous solution - must contain hydrogen
• bases = anything which is OH- ions - must contain hydroxide
• bases are substances which dissolve acids
• all alkalis = bases
Salts from metals
• A salt is a compound formed when the hydrogen in an acid is wholly or partially replaced by metal or ammonium ions.
Salts can be made by reacting a suitable metal with an acid. The metal must be above hydrogen in the reactivity series, but not
dafigerously redcuve
• The reaction between a metal and an acid produces hydrogen gas as well as a salt. A sample of the salt made can then be crystallised out of solution by evaporating off the water.
• The reaction between a metal and an acid is an example of a
redox reaction. The metal atoms lose electrons and are oxidised, and hydrogen ions from the acid gain electrons and are reduced.
Salt from insoluble bases
• When an acid reacts with a base, a
neutralisation reaction occurs.
• The reaction between an acid and a
base produces a sait and water.
• The sum of the charges on the ions in a salt add up to zero. This enables you to work out the formula of salts, knowing the charges on the ions present.
• A pure, dry sample of the salt made in an acid-base reaction can be crystallised out of solution by evaporating off most of the water, and drying with filter papers if necessary
Formulae of salt
Salts are made up of positive metal ions (or ammonium ions, NH_*) and a negative ion from an acid. The positive ions can come from a metal, a base, or a carbonate. A carbonate reacts with an acid to form a salt, water, and carbon dioxide gas.
Like all ionic compounds, salts have no overall charge, as the sum of the charges on their ions equals zero. So once you know the charges on the ions that make up a salt, you can work out its formula.
Making more salt
• An indicator is needed when a soluble salt is prepared by reacting an alkali with an acid.
• The experiment can be repeated without the indicator to make a salt, then a pure, dry sample of its crystals prepared.
• A carbonate reacts with an acid to produce a salt, water, and carbon dioxide gas.
Acid strength
Strength of an acid tells you about the proportion of acid particles that will dissociate to produce H+ ions in solution
Strong acids
Strong acids, such as sulfuric, hydrochloric and nitric acid ionise completely in water - all the acid particles dissociate to release H+ ions
Weak acids
• weak acids only partially ionise in water - if you put a sample of weak acid in water, only some of the acid molecules will ionise and release H+ ions
• carboxylic acids are weak acids (they don’t ionise completely when dissolved in water) as are citric and carbonic acids
• ionisation of a weak acid is a reversible reaction, which sets up an equilibrium between the undissociated and ans dissociated acid
• since only a few of the acid particles release H+ ions, the position of equilibrium lies well to the left
Effect of acid strength on reactivity
Reactions of acids involve the H+ ions reacting with other substances. If the concentration of H+ ions are higher, the rate of reaction will be faster. Since strong acids dissociate more than weak acids, a strong acid will be more reactive than a weak acid of the same concentration
Effect of acid strength on pH
• the pH is a measure of the concentration of H+ ions
