Why do reactions take in or give out energy, and how do the groups of the periodic table behave?
Exothermic and endothermic reactions, energy level (reaction profile) diagrams, activation energy, and the trends in reactivity of group 1, group 7 and group 0 elements.
A focused answer to Edexcel GCSE Combined Science Topics 6 and 7 (CC6 to CC7), covering exothermic and endothermic reactions, reaction profile diagrams and activation energy, and the trends in reactivity of group 1, group 7 and group 0 elements.
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What this dot point is asking
Edexcel wants you to distinguish exothermic and endothermic reactions, draw and interpret reaction profile (energy level) diagrams including activation energy, and describe the trends in reactivity of group 1, group 7 and group 0 of the periodic table.
Exothermic and endothermic reactions
Reaction profiles and activation energy
A reaction profile (energy level diagram) shows the energy of the reactants and products:
- Exothermic: the products are lower in energy than the reactants (energy is released).
- Endothermic: the products are higher in energy than the reactants (energy is taken in).
The activation energy is the minimum energy needed to start the reaction, shown as a hump between the reactants and products. A catalyst lowers this hump.
Group 1: the alkali metals
Group 7: the halogens
The halogens (fluorine, chlorine, bromine, iodine) are non-metals that form coloured molecules with two atoms (). Reactivity decreases down the group, because the atoms get larger and gain an electron less easily. A more reactive halogen will displace a less reactive one from a solution of its salt.
Group 0: the noble gases
Group 0 (helium, neon, argon) are the noble gases: they are very unreactive because they have full outer electron shells, so they have no tendency to gain or lose electrons. Their boiling points increase down the group, and they exist as single atoms rather than molecules.
The reason these group trends exist all comes back to electron arrangement. Group 1 metals each have one outer electron to lose, group 7 non-metals each need to gain one electron, and group 0 already have a full shell. As you go down a group, the outer shell is further from the nucleus and more shielded by inner electrons, so the attraction between the nucleus and the outer electrons weakens. This makes electrons easier to lose (so group 1 gets more reactive) but harder to gain (so group 7 gets less reactive). Linking each trend back to the outer electrons is what earns the explanation marks.
Try this
Q1. State whether a reaction that makes the temperature rise is exothermic or endothermic. [1 mark]
- Cue. Exothermic.
Q2. Describe the trend in reactivity down group 7. [1 mark]
- Cue. Reactivity decreases down the group.
Exam-style practice questions
Practice questions written in the style of Pearson Edexcel exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Edexcel 20204 marksA reaction causes the temperature of a solution to fall. State whether the reaction is exothermic or endothermic, and explain your answer in terms of energy transfer. Sketch the shape of its reaction profile.Show worked answer →
A 4-mark question on energy changes.
The reaction is endothermic, because the temperature falls (1 mark). It is endothermic because it takes in (absorbs) energy from the surroundings, so the surroundings (the solution) lose thermal energy and cool down (1 mark). On a reaction profile, the products are at a higher energy level than the reactants (1 mark), and the curve rises over an activation-energy hump before reaching the higher product level (1 mark).
Markers reward identifying it as endothermic from the temperature fall, the energy-taken-in explanation, and a profile with products above reactants.
Edexcel 20223 marksDescribe and explain the trend in reactivity going down group 1 (the alkali metals) of the periodic table.Show worked answer →
A 3-mark question on group 1 trends.
Reactivity increases going down group 1 (1 mark). This is because the atoms get larger and the outer electron is further from the nucleus, so it is less strongly attracted (1 mark). The outer electron is therefore lost more easily, so the metal reacts more readily, which is why potassium is more reactive than sodium, which is more reactive than lithium (1 mark).
Markers reward the increasing-reactivity trend and the explanation in terms of the outer electron being further from the nucleus and lost more easily.
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Sources & how we know this
- Edexcel GCSE (9-1) Combined Science (1SC0) specification — Pearson (2016)