The shape of the lesson
By the end of the lesson, learners can
- Exothermic or endothermic: Switch between reactions in the enthalpy profile simulator.
- The standard definitions: Open the definitions tab and match each equation to its definition.
- Getting the equation right: Most marks lost on definitions are lost in the equation, not the words.
- Exothermic: ΔH negative; endothermic: positive.
- Standard: 298 K, 101 kPa, 1 mol dm−3.
- Formation: 1 mol of compound from elements; elements are 0.
- Combustion: 1 mol burned completely; neutralisation: 1 mol of water.
One picture every idea returns to
An enthalpy change, ΔH, is the heat energy exchanged with the surroundings at constant pressure. Exothermic reactions release heat, so ΔH is negative; endothermic reactions absorb heat, so ΔH is positive. Standard values (⊖) are for 298 K, 101 kPa and 1 mol dm−3 solutions, with substances in their standard states. Learn four definitions exactly: reaction (the molar quantities in the equation), formation (1 mol of compound from its elements), combustion (1 mol burned completely in oxygen) and neutralisation (1 mol of water formed).
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, Heat in, heat out.. Every reaction either releases energy to the surroundings or takes it in. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 13 min | Teach and check: Exothermic or endothermic | Switch between reactions in the enthalpy profile simulator. Project tab 1 of the lab (Chemical energetics lab, enthalpy profiles) and let a student drive it while the class predicts. | Whole class, then pairs |
| 13 to 21 min | Teach and check: The standard definitions | Open the definitions tab and match each equation to its definition. Project tab 5 of the lab (Chemical energetics lab, definitions) and let a student drive it while the class predicts. | Pairs |
| 21 to 30 min | Teach and check: Getting the equation right | Most marks lost on definitions are lost in the equation, not the words. | Quiz-Quiz-Trade |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 13 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Getting the equation right, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Quiz-Quiz-Trade
Each student holds a card with a question on the front and the answer on the back. They pair up, quiz each other, coach if needed, trade cards and find a new partner. Three rounds covers the whole set.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| The standard enthalpy change of formation of water and the standard enthalpy change of combustion of hydrogen are the same equation, H 2 (g) + ½O 2 (g) → H 2 O(l), so they have the same value, −286 kJ mol −1. | Writing 2H 2 + O 2 → 2H 2 O doubles the value and fits neither definition. |
Enthalpy changes and definitions: worksheet
6 questions, 13 marks. Written in the style of the papers; no past paper question is reproduced.
Define standard enthalpy change of formation.
Define standard enthalpy change of combustion, and write the equation for methane.
Sketch and label a reaction pathway diagram for an endothermic reaction, showing ΔH and the activation energy.
Write equations, with state symbols, for the standard enthalpy change of formation of (a) ammonia and (b) sodium chloride.
Explain why the standard enthalpy change of formation of an element is zero.
The enthalpy change of neutralisation of ethanoic acid with sodium hydroxide is −55.2 kJ mol−1, less exothermic than for hydrochloric acid (−57.1 kJ mol−1). Suggest why.
Answer key and marking notes
- The enthalpy change when one mole of a compound is formed from its elements in their standard states,
- under standard conditions (298 K and 101 kPa).
- The enthalpy change when one mole of a substance is burned completely in oxygen under standard conditions.
- CH4(g) + 2O2(g) → CO2(g) + 2H2O(l).
- Axes: enthalpy against progress of reaction; the products line is drawn above the reactants line.
- ΔH: arrow pointing up from reactants to products, labelled positive.
- Ea: arrow from the reactants up to the top of the hump.
- (a) ½N2(g) + &frac32;H2(g) → NH3(g).
- (b) Na(s) + ½Cl2(g) → NaCl(s).
- Forming an element in its standard state from itself involves no chemical change,
- so no enthalpy change occurs.
- Ethanoic acid is a weak acid and is only partly ionised.
- Some energy is used to ionise the acid fully as H+ ions are removed, so less heat is released overall.
8 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
Which statement about an exothermic reaction is correct?
- Aenergy is transferred to the surroundings, the temperature of the surroundings rises and ΔH is negative
- Benergy is transferred to the surroundings, the temperature of the surroundings rises and ΔH is positive
- Cenergy is taken in from the surroundings, the temperature of the surroundings falls and ΔH is negative
- Dthe products have more energy than the reactants, so ΔH is negative
Which set of conditions is used for standard enthalpy changes, ΔH⊖?
- A298 K, 101 kPa, and solutions at a concentration of 1 mol dm−3
- B273 K, 101 kPa, and solutions at a concentration of 1 mol dm−3
- C298 K, 1 kPa, and solutions at a concentration of 1 mol dm−3
- D298 K, 101 kPa, and solutions at a concentration of 0.1 mol dm−3
Which equation represents the standard enthalpy change of formation of ethanol, C2H5OH?
- A2C(s) + 3H2(g) + ½O2(g) → C2H5OH(l)
- B4C(s) + 6H2(g) + O2(g) → 2C2H5OH(l)
- CC2H5OH(l) + 3O2(g) → 2CO2(g) + 3H2O(l)
- D2C(g) + 6H(g) + O(g) → C2H5OH(l)
What is the standard enthalpy change of formation of oxygen, O2(g)?
- A0 kJ mol−1
- B+496 kJ mol−1, the energy needed to break the O=O bond
- C−496 kJ mol−1, the energy released when the O=O bond forms
- Dit cannot be defined, because oxygen is not a compound
Which is the definition of the standard enthalpy change of neutralisation?
- Athe enthalpy change when an acid and an alkali react to form 1 mol of water under standard conditions
- Bthe enthalpy change when 1 mol of acid reacts completely with an alkali under standard conditions
- Cthe enthalpy change when 1 mol of salt is formed from an acid and an alkali under standard conditions
- Dthe enthalpy change when 1 mol of alkali is dissolved in water under standard conditions
The reaction pathway diagram is for the reaction A → B. What are the activation energy, Ea, and the enthalpy change, ΔH, for the forward reaction?
- AEa = +80 kJ mol−1; ΔH = −70 kJ mol−1
- BEa = +180 kJ mol−1; ΔH = −70 kJ mol−1
- CEa = +80 kJ mol−1; ΔH = +70 kJ mol−1
- DEa = +150 kJ mol−1; ΔH = −70 kJ mol−1
Use the same reaction pathway diagram. What is the activation energy of the reverse reaction, B → A, and what is its enthalpy change?
- AEa = +150 kJ mol−1; ΔH = +70 kJ mol−1
- BEa = +80 kJ mol−1; ΔH = +70 kJ mol−1
- CEa = +150 kJ mol−1; ΔH = −70 kJ mol−1
- DEa = +10 kJ mol−1; ΔH = +70 kJ mol−1
Which equation has an enthalpy change equal to the standard enthalpy change of combustion of ethane, C2H6?
- AC2H6(g) + 3½O2(g) → 2CO2(g) + 3H2O(l)
- B2C2H6(g) + 7O2(g) → 4CO2(g) + 6H2O(l)
- CC2H6(g) + 3½O2(g) → 2CO2(g) + 3H2O(g)
- DC2H6(g) + 2½O2(g) → 2CO(g) + 3H2O(l)
Quiz key
In an exothermic reaction the chemicals lose energy to the surroundings, so the surroundings warm up and the enthalpy of the products is lower than that of the reactants: ΔH = H(products) − H(reactants) is negative. Saying ΔH is positive confuses the energy gained by the surroundings with the enthalpy change of the system. Taking in energy and cooling the surroundings describes an endothermic reaction, which has a positive ΔH. Products with more energy than the reactants would also be endothermic, so ΔH would be positive, not negative.
Standard conditions are a temperature of 298 K (25 °C), a pressure of 101 kPa (1 atm; 100 kPa is also used), every substance in its standard state, and any solutions at 1 mol dm−3. 273 K (0 °C) is the temperature used for s.t.p. in gas calculations, not for standard enthalpy changes. 1 kPa is about one hundredth of atmospheric pressure. The standard concentration is 1 mol dm−3, not 0.1 mol dm−3.
The standard enthalpy change of formation is the enthalpy change when 1 mol of a compound is formed from its elements in their standard states under standard conditions. Carbon is a solid (graphite), and hydrogen and oxygen are diatomic gases, so one mole of ethanol needs 2C(s), 3H2(g) and ½O2(g); a fraction is allowed so that the product is exactly 1 mol. The equation making 2C2H5OH is balanced but forms 2 mol, so its enthalpy change is twice ΔHf. The equation with oxygen on the left and CO2 and water on the right is the combustion of ethanol. Gaseous atoms are not the standard states of carbon, hydrogen or oxygen.
The formation equation for oxygen is O2(g) → O2(g): the element is already in its standard state, so there is no change and ΔHf = 0 by definition. This is true for every element in its standard state. +496 kJ mol−1 is the O=O bond energy, which refers to forming gaseous oxygen atoms, not to formation from the element. −496 kJ mol−1 would be for making O=O bonds from separate atoms, which are not the standard state of oxygen. The definition applies to elements too; their value is simply zero, which is what makes enthalpy of formation data useful as a reference level.
The standard enthalpy change of neutralisation is defined per mole of water formed, because the reaction in every strong acid/strong alkali case is H+(aq) + OH−(aq) → H2O(l). Per mole of acid would give double the value for a diprotic acid such as H2SO4, which releases 2 mol of water per mole of acid. Per mole of salt has the same problem: 1 mol of Na2SO4 comes with 2 mol of water. Dissolving an alkali in water is an enthalpy change of solution, not neutralisation.
The activation energy is the difference between the reactants (100) and the top of the curve (180): 180 − 100 = +80 kJ mol−1. ΔH = products − reactants = 30 − 100 = −70 kJ mol−1, negative because the products are lower, so the reaction is exothermic. +180 reads the peak from zero on the axis instead of from the reactants. ΔH = +70 has the right size but the wrong sign; energy falls from reactants to products. +150 is the gap from the products up to the peak, which is the activation energy of the reverse reaction.
The reverse reaction starts at the level of B (30), so it must climb to the same peak (180): Ea(reverse) = 180 − 30 = +150 kJ mol−1, which equals Ea(forward) − ΔH(forward) = 80 − (−70). It ends at A (100), so ΔH = 100 − 30 = +70 kJ mol−1: reversing a reaction reverses the sign of ΔH. +80 is the forward activation energy; the barrier is not the same from both sides unless ΔH is zero. ΔH = −70 is the forward value without the sign reversed. +10 comes from subtracting the forward ΔH the wrong way (80 − 70).
The standard enthalpy change of combustion refers to 1 mol of the substance burned completely in oxygen under standard conditions, so one ethane molecule with 3½ O2 giving CO2 and liquid water, the standard state of water at 298 K. The equation with 2C2H6 burns 2 mol, so its enthalpy change is twice ΔHc. Water as a gas is not in its standard state; the value would be less exothermic because the energy released on condensing is missing. Forming CO is incomplete combustion, which releases less energy and does not match the definition.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.