Chemical Changes
Chemical changes is the GCSE chemistry topic covering the reactivity series of metals, oxidation and reduction, the reactions of acids with metals and bases, electrolysis, and pH. It includes required-practical skills such as preparing a soluble salt from an insoluble base, carrying out a titration, and electrolysing solutions.
Before you start
Make sure you're comfortable with these topics first:
Method
- Learn the reactivity series (potassium, sodium, lithium, calcium, magnesium, zinc, iron, copper, in decreasing reactivity) and use it to predict displacement reactions and how a metal is extracted: metals less reactive than carbon can be extracted by reduction with carbon, while more reactive metals need electrolysis.
- Learn the general word equations for reactions of acids: acid + metal -> salt + hydrogen; acid + metal oxide/metal hydroxide -> salt + water; acid + metal carbonate -> salt + water + carbon dioxide.
- For salt-preparation practical questions, describe adding an insoluble base to a warm acid until it is in excess, filtering off the excess solid, then evaporating and crystallising the filtrate to obtain the salt.
- For titration calculations, use moles = concentration x volume (in dm3) to find the unknown moles or concentration, remembering to convert volumes from cm3 to dm3 by dividing by 1000.
- Learn that a strong acid ionises completely in water while a weak acid only partially ionises, and that a lower pH means a higher concentration of hydrogen ions (H+).
- For electrolysis questions, identify that positive ions move to the cathode (negative electrode) and are reduced, and negative ions move to the anode (positive electrode) and are oxidised; for electrolysis of a solution, remember that if a metal is more reactive than hydrogen, hydrogen gas forms at the cathode instead.
Worked example
In a titration, 20.0 cm3 of hydrochloric acid was exactly neutralised by 25.0 cm3 of 0.200 mol/dm3 sodium hydroxide solution. Calculate the concentration, in mol/dm3, of the hydrochloric acid. NaOH + HCl -> NaCl + H2O.
- Calculate the moles of sodium hydroxide used: moles = concentration x volume (dm3) = 0.200 x (25.0 / 1000).
- Calculate: 0.200 x 0.0250 = 0.00500 mol.
- Use the balanced equation, a 1:1 ratio, to find the moles of HCl: moles of HCl = moles of NaOH = 0.00500 mol.
- Convert the volume of HCl to dm3: 20.0 / 1000 = 0.0200 dm3.
- Calculate the concentration: concentration = moles / volume = 0.00500 / 0.0200.
- Final answer: the concentration of the hydrochloric acid is 0.250 mol/dm3.
Practice questions
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Q1State the name given to a reaction in which a substance loses electrons.Show answer
Answer: Oxidation.
Q2Write the word equation for the reaction between zinc and hydrochloric acid.Show answer
Answer: Zinc + hydrochloric acid -> zinc chloride + hydrogen
Q3State the pH value of a neutral solution.Show answer
Answer: 7
Q4Calculate the number of moles of sodium hydroxide in 200 cm3 of a 0.500 mol/dm3 solution.Show answer
Answer: 0.1 mol (0.500 x 200/1000 = 0.500 x 0.2)
Q5Explain why aluminium cannot be extracted from aluminium oxide by heating with carbon.Show answer
Answer: Aluminium is more reactive than carbon, so carbon cannot displace/reduce it; carbon can only reduce metals less reactive than itself, so aluminium must be extracted by electrolysis instead.
Q6A student reacts 2.4 g of magnesium with excess dilute hydrochloric acid: Mg + 2HCl -> MgCl2 + H2. Relative atomic mass of Mg = 24. Calculate the number of moles of hydrogen gas produced.Show answer
Answer: 0.1 mol (moles Mg = 2.4/24 = 0.1; 1:1 ratio with H2)
Exam-style questions
Written in the style of a GCSE Science exam paper, with a full mark scheme.
Write the half equation for the reduction reaction that occurs when copper ions, Cu2+, gain electrons to form copper atoms during the displacement of copper by a more reactive metal.
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A student reacts 4.8 g of magnesium ribbon with excess dilute sulfuric acid: Mg + H2SO4 -> MgSO4 + H2. Relative atomic masses: Mg = 24, S = 32, O = 16. Calculate the mass of magnesium sulfate, MgSO4, produced.
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In a titration, 30.0 cm3 of sodium hydroxide solution was exactly neutralised by 18.0 cm3 of 0.150 mol/dm3 hydrochloric acid. NaOH + HCl -> NaCl + H2O. (a) Calculate the number of moles of hydrochloric acid used. (b) Calculate the concentration, in mol/dm3, of the sodium hydroxide solution.
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Free printable worksheet
Want more practice on paper? Download the chemical changes worksheet pack - 17 pages of exam-style questions with a full mark scheme. One email opens every download in this browser for 14 days - no account, no card. Print it for personal and classroom use.
This topic is chapter 3 of GCSE Chemistry Workbook, the whole course as one free printable PDF.
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