GCSE AQA Chemistry 'Unit 2b' Revision Guide

Revision guide for the second half of the GCSE AQA Chemistry 'Unit 2' / 'C2' exam paper.

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GCSE Chemistry
`Unit 2b' written by Applequestria
Table of Contents
Table of Contents..................................................................................................................................... 1
Rates of reaction ...................................................................................................................................... 3
Different reactions happen at different rates ..................................................................................... 3
The rate of reaction depends on four factors: .................................................................................... 3
The rate of reaction can be increased by changing any one of the four factors: ................................ 3
You can draw graphs to show the rate of a reaction ........................................................................... 3
Rates of reaction (continued)................................................................................................................... 4
Precipitation reactions: ....................................................................................................................... 4
Gas reactions ....................................................................................................................................... 4
Rates of reaction (continued)................................................................................................................... 5
Reaction between sodium thiosulphate and dilute hydrochloric acid (for temperature): .................. 5
Reaction between `magnesium strips' and dilute hydrochloric acid (for concentration): ................... 5
Reaction between `marble chips' and dilute hydrochloric acid (for surface area): ............................. 5
Decomposition of hydrogen peroxide: ................................................................................................ 5
Rates of reaction (continued) ................................................................................................................... 6
Higher temperature increases collisions ............................................................................................. 6
Higher concentration (or pressure in gasses) increases collisions....................................................... 6
Higher surface area increases collisions .............................................................................................. 6
Adding a catalyst speeds up a reaction ............................................................................................... 6
Advantages .......................................................................................................................................... 6
Disadvantages ..................................................................................................................................... 6
Exothermic and endothermic reactions ................................................................................................... 7
Acids and bases........................................................................................................................................ 8
The state symbols................................................................................................................................ 8
The pH scale ........................................................................................................................................ 8
Acids and bases (continued)..................................................................................................................... 9
Acids and bases (continued)................................................................................................................... 10
Making salts ........................................................................................................................................... 11
Making soluble salts using a metal or insoluble base ........................................................................ 11
Making soluble salts using a soluble base (an alkali) ......................................................................... 11
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Making salts (continued) ........................................................................................................................ 12
Making insoluble salts by precipitation reactions ............................................................................. 12
Electrolysis ............................................................................................................................................. 13
Electrolysis (continued) .......................................................................................................................... 14
Electrolysis (continued) ..........................................................................................................................…read more

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Rates of reaction
Different reactions happen at different rates
A slow reaction could be the reaction between oxygen and iron (rusting).
A medium reaction could be the reaction between magnesium and hydrochloric acid.
A fast reaction would be an explosion.…read more

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Rates of reaction (continued)
The rate of a reaction can be observed by measuring how quickly the reactants are used or how
quickly the products are made. It is usually easier to measure how quickly the products are made. The
rate of a reaction can be calculated by the formula: `rate of reaction is equal to the amount of
reactants used or the amount of products made over the time taken'.…read more

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Rates of reaction (continued)
Reaction between sodium thiosulphate and dilute hydrochloric acid (for temperature):
Both reactant solutions are clear and the product precipitate of yellow sulphur clouds the
solution.
Add the sodium thiosulfate into the hydrochloric acid. The reaction produces a yellow
precipitate of sulphur. Observe a mark through the solution. Measure the time taken for the
mark to `disappear'. Take readings at regular intervals.…read more

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Rates of reaction (continued)
Reaction rates are explained by collision theory: the rate of a reaction depends on the frequency and
energy of particle collisions. The effects of temperature, concentration and surface area on the rate of
reaction can be explained by the frequency of particle collisions.
Higher temperature increases collisions
Increasing the temperature increases the speed of the reacting particles so that they collide
more frequently and more energetically. This increases the rate of reaction.
Increasing the temperature causes faster collisions.…read more

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Exothermic and endothermic reactions
When chemical reactions occur, energy is transferred to or from the surroundings. An exothermic
reaction is one which transfers energy to the surroundings, usually in the form of heat and usually
shown by a rise in temperature. This means that heat is given out. Examples of exothermic reactions
include: combustion reactions, neutralisation reactions and oxidation reactions (most are exothermic
such as the oxidation of sodium in water).…read more

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Acids and bases
The state symbols
(s) means solid
(l) means liquid
(g) means gas
(aq) means aqueous (dissolved in water)
The pH scale
The pH scale is used to measure the acidity or alkalinity of a solution and is measured using an
indicator such as universal indicator, which is a combination of dyes.
Substances with a pH of less than seven are acids. The more strongly acidic the solution, the
lower its pH number.…read more

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Acids and bases (continued)
Acid + Metal Salt + Hydrogen Metals react with acids to produce a salt and
hydrogen.
Metals react with acids to produce salts. For example, magnesium reacts with hydrochloric acid to
produce magnesium chloride and releases hydrogen. However, copper does not react with
hydrochloric acid and does not release hydrogen. This is because copper is less reactive than
hydrogen.
The more reactive the metal, the faster the reaction. For example, the reaction between
sodium and hydrochloric acid is fast.…read more

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Acids and bases (continued)
Acid + Metal oxide Salt + Water Metal oxides react with acids to produce a salt
and water. This is a neutralisation reaction.
Acid + Metal hydroxide Salt + Water Metal hydroxides react with acids to produce a
salt and water. This is a neutralisation reaction.
Metal oxides react with acids to produce salts. For example, magnesium oxide reacts with
hydrochloric acid to produce magnesium chloride and water. Metal hydroxides also react with acids
to produce salts.…read more

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