Measuring Enthalpy Change

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Heat vs Temperature

Temperature is a measure of the average kinetic energy of the particles in a substance.
Heat is a measure of the total energy content of a substance.
A beaker of boiling water has a much higher heat content than a single drop at the same temperature, so it can cause a more severe burn.
Particles have kinetic energy because they are moving; the faster they move, the higher the temperature.
Energy cannot be created or destroyed, only transferred — this is the Law of Conservation of Energy.
In a chemical reaction, the system is the reacting substances and the surroundings is everything else.
An open system allows both matter and energy to move in and out; a closed system allows energy but not matter; an isolated system allows neither.

Exothermic and Endothermic Reactions

The total chemical energy inside a substance is called its enthalpy (or heat content).
An enthalpy change is represented by ΔH (Δ = change, H = enthalpy) and can be positive or negative.
In an exothermic reaction, the products have less enthalpy than the reactants; heat is given off to the surroundings, so the surroundings warm up and ΔH is negative.
In an endothermic reaction, the products have more enthalpy than the reactants; heat is absorbed from the surroundings, so the surroundings cool down and ΔH is positive.
Exothermic reactions are thermodynamically possible because the reactants have higher enthalpy than the products, but they may not occur if the rate is too slow (kinetically controlled).
A decrease in temperature of the surroundings indicates an endothermic reaction.
When the energy of the system decreases, the reaction is exothermic.

Exothermic and endothermic reactions

Energy Profiles

An energy profile shows how the energy of a chemical system changes during a reaction, including reactants, products, and the transition state.
The transition state is a stage where bonds are partially broken and formed; it is unstable, cannot be isolated, and is the highest energy point on the pathway.
Activation energy (Ea) is the minimum energy needed for reactant molecules to collide successfully and start the reaction.
In an exothermic reaction, the reactants are higher in energy than the products, so the activation energy is relatively low.
In an endothermic reaction, the reactants are lower in energy than the products, so the activation energy is relatively high.
On an energy level diagram, Ea is the energy difference from reactants to the transition state, while ΔH is the energy difference from reactants to products.
Always label the axis of energy level diagrams.

Reaction pathway diagrams

Standard Enthalpy Change

Standard enthalpy change, ΔH⦵, refers to the heat transferred at constant pressure under standard conditions and states.
Standard conditions are: pressure of 100 kPa, concentration of 1 mol dm⁻³ for solutions, and each substance in its standard state.
Temperature is not part of the definition of standard state, but 298.15 K is usually specified.
The symbol ⦵ indicates standard conditions, e.g. ΔH⦵.
Standard enthalpy of reaction, ΔHr⦵: enthalpy change when reactants in the stoichiometric equation react to give products under standard conditions; can be exothermic or endothermic.
Standard enthalpy of formation, ΔHf⦵: enthalpy change when one mole of a compound is formed from its elements under standard conditions; can be exothermic or endothermic.
Standard enthalpy of combustion, ΔHc⦵: enthalpy change when one mole of a substance is burnt in excess oxygen under standard conditions; always exothermic.
Standard enthalpy of neutralisation, ΔHneut⦵: enthalpy change when one mole of water is formed by reacting an acid and alkali under standard conditions; always exothermic.

Calorimetry Experiments

Calorimetry is a technique used to measure enthalpy changes of chemical reactions.
A simple calorimeter can be made from a polystyrene cup, a vacuum flask, or a metal can.
The energy transferred as heat is calculated using q = m × c × ΔT, where q is heat transferred (J), m is mass of water (g), c is specific heat capacity (J g⁻¹ K⁻¹), and ΔT is temperature change (K).
The specific heat capacity of water is 4.18 J g⁻¹ K⁻¹.
For reactions in solution, carry out the reaction with an excess of one reagent and measure the temperature change over a few minutes.
Assumptions made in calorimetry: the specific heat capacity and density of the solution are the same as pure water, the container's heat capacity is ignored, the reaction is complete, and heat losses are negligible.
There is no need to convert temperature units between °C and K because the change in temperature is the same.

Polystyrene-cup calorimetry

Temperature Correction Graphs

For reactions that are not instantaneous, heat may be lost to the surroundings before the maximum temperature is reached.
A temperature correction graph is used to find the true maximum temperature change.
Steps: record temperature before adding reactants for a few minutes, add the second reactant and continue recording, then plot the graph.
Extrapolate the cooling section of the graph back to the time when the second reactant was added to find the maximum temperature change.
This method assumes the rate of cooling is constant.
The same analysis can be used for endothermic reactions, but there will be a 'warming' section as the substances return to room temperature.

Enthalpy of Combustion Experiments

In a combustion calorimetry experiment, the heat released by burning a fuel is used to increase the temperature of a known mass of water.
A typical simple calorimeter uses a spirit burner and a copper calorimeter containing water.
The heat released by combustion is calculated using q = m × c × ΔT, where m is the mass of water.
The amount of fuel burned is found from the mass lost by the spirit burner and its molar mass.
The enthalpy of combustion is calculated as ΔH = q ÷ n, where n is the moles of fuel burned.
Not all heat produced is transferred to the water; some is lost to the surroundings and some is absorbed by the calorimeter.
To minimise heat losses, place the copper calorimeter not too far above the flame, use a lid, and shield from draughts.
Main sources of error in combustion experiments are heat losses and incomplete combustion.

Worked Example: Calculating Enthalpy Change from Calorimetry Data

Example: 0.01 mol of propan-1-ol heated 250 g of water, raising the temperature from 298 K to 310 K.
Calculate q = m × c × ΔT = 250 g × 4.18 J g⁻¹ K⁻¹ × 12 K = 12 540 J.
This is the energy released by 0.01 mol of propan-1-ol.
Energy per mole = q ÷ n = 12 540 J ÷ 0.01 mol = 1 254 000 J mol⁻¹.
Convert to kJ mol⁻¹ by dividing by 1000: ΔH = -1254 kJ mol⁻¹ (negative because combustion is exothermic).

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1.Which statement best describes the difference between heat and temperature?
Easy
- ATemperature is a measure of the average kinetic energy of particles, while heat is a measure of the total energy content of a substance.
- BHeat is a measure of the average kinetic energy of particles, while temperature is a measure of the total energy content of a substance.
- CHeat and temperature are both measures of the total energy content of a substance.
- DTemperature is a measure of the total energy content of a substance, while heat is a measure of the average kinetic energy of particles.
2.In an exothermic reaction, the temperature of the surroundings increases.
Easy
True or false?
3.The reaction of propane with oxygen is: C₃H₈ + 5O₂ → 3CO₂ + 4H₂O, ΔH = -2219 kJ mol⁻¹. What is the classification of this enthalpy change?
Easy
- AEnthalpy of combustion
- BEnthalpy of formation
- CEnthalpy of neutralisation
- DEnthalpy of reaction
4.Which of the following are standard conditions for enthalpy changes? (select all that apply)
Medium
- APressure of 100 kPa
- BConcentration of 1 mol dm⁻³ for solutions
- CTemperature of 298.15 K
- DEach substance in its standard state
- EPressure of 1 atm
5.Match each enthalpy change with its symbol.
Medium
- Standard enthalpy of reaction
- Standard enthalpy of formation
- Standard enthalpy of combustion
- Standard enthalpy of neutralisation
- ΔHr
- ΔHf
- ΔHc
- ΔHneut
6.Place the steps of a calorimetry experiment to determine the enthalpy change of a reaction in solution in the correct order.
Medium
- Measure initial temperature of the solution
- Add the second reactant
- Record temperature over time
- Extrapolate the cooling curve to find maximum temperature change
7.In an endothermic reaction, the products have less enthalpy than the reactants.
Easy
True or false?
8.Which of the following is the correct definition of average bond enthalpy?
Medium
- AThe energy required to break one mole of a specific bond in a gaseous molecule, averaged over a range of compounds.
- BThe energy released when one mole of a bond is formed in the gaseous state.
- CThe energy required to break one mole of bonds in a solid compound.
- DThe energy change when one mole of a compound is formed from its elements.

Soal lainnya

9.A student reacts 25 cm³ of 0.5 mol dm⁻³ HCl with 25 cm³ of 0.5 mol dm⁻³ NaOH. The enthalpy of neutralisation is -57.9 kJ mol⁻¹. What is the enthalpy change for this reaction?
Medium
- A-0.72 kJ
- B-1.45 kJ
- C-57.9 kJ
- D-0.36 kJ
10.Which of the following statements about exothermic reactions are correct? (select all that apply)
Medium
- AThe products have less enthalpy than the reactants.
- BΔH is negative.
- CThe temperature of the surroundings increases.
- DThe reaction absorbs heat from the surroundings.
- EThe activation energy is always higher than for endothermic reactions.
11.Match each term with its correct description.
Medium
- System
- Surroundings
- Open system
- Closed system
- The substances reacting in a chemical reaction
- Everything outside the reaction vessel
- Both matter and energy can be exchanged
- Energy can be exchanged but matter cannot
12.Which of the following equations represents a standard enthalpy of formation?
Easy
- AC(s) + O₂(g) → CO₂(g)
- BCO₂(g) → C(s) + O₂(g)
- CHCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)
- DCaCO₃(s) → CaO(s) + CO₂(g)
13.Arrange the steps for drawing a temperature correction graph in the correct order.
Medium
- Record temperature before adding reactants for a few minutes
- Add the second reactant and continue recording temperature
- Plot the graph and extrapolate the cooling part
- Determine the maximum temperature change at the intersection
14.Match each term with its correct definition.
Easy
- Temperature
- Heat
- Enthalpy change
- Activation energy
- A measure of the average kinetic energy of the particles
- A measure of the energy content of a substance
- The heat transferred at constant pressure, symbol ΔH
- The minimum energy needed for a successful collision
15.Which of the following statements about an exothermic reaction are correct? (select all that apply)
Medium
- AThe products have less enthalpy than the reactants.
- BΔH is negative.
- CThe temperature of the surroundings increases.
- DThe system absorbs heat from the surroundings.
- EThe activation energy is always higher than for an endothermic reaction.
16.Place the steps for making a temperature correction graph in the correct order.
Medium
- Record the temperature before adding the second reactant for a few minutes to get a steady value.
- Add the second reactant and continue recording temperature and time.
- Plot the graph and extrapolate the cooling part back to the time of addition.
- Determine the maximum temperature change from the extrapolated line.
17.Which equation correctly represents the standard enthalpy of combustion of ethanol?
Medium
- AC₂H₅OH(l) + 3O₂(g) → 2CO₂(g) + 3H₂O(l)
- B2C(s) + 3H₂(g) + ½O₂(g) → C₂H₅OH(l)
- CC₂H₅OH(l) + 3O₂(g) → 2CO₂(g) + 3H₂O(g)
- DC₂H₅OH(l) → C₂H₄(g) + H₂O(l)
18.Which of the following are assumptions made in calorimetry calculations using q = mcΔT? (select all that apply)
Medium
- AThe specific heat capacity of the solution is the same as pure water.
- BThe density of the solution is 1 g cm⁻³.
- CThe specific heat capacity of the container is ignored.
- DThe reaction goes to completion.
- EThere are significant heat losses to the surroundings.
19.Match each standard enthalpy change with its correct symbol.
Medium
- Standard enthalpy of reaction
- Standard enthalpy of formation
- Standard enthalpy of combustion
- Standard enthalpy of neutralisation
- ΔHr
- ΔHf
- ΔHc
- ΔHneut
20.Place the stages of an energy profile diagram from left to right in order of increasing energy.
Medium
- Reactants
- Transition state
- Products
21.A student adds 50.00 cm³ of 1.50 mol dm⁻³ copper(II) sulfate solution to a polystyrene cup. After recording the temperature for 3 minutes, 6.00 g of powdered zinc is added. The maximum temperature change is 29.0 °C. Using the data booklet, which is the limiting reagent? (M(Zn) = 65.4 g mol⁻¹)
Medium
- ACopper(II) sulfate
- BZinc
- CNeither, they are in stoichiometric amounts
- DWater
22.In an isolated system, neither matter nor energy can be exchanged with the surroundings.
Easy
True or false?
23.A student adds 100.0 cm³ of 0.200 mol dm⁻³ copper(II) sulfate solution to a polystyrene cup and records the temperature for 3 minutes. They then add excess iron powder and record the temperature for a further 7 minutes. The maximum temperature rise is 7.5 °C. Use q = mcΔT with c = 4.18 J g⁻¹ K⁻¹ and density = 1 g cm⁻³. What is the heat energy transferred, q, in joules?
Medium
- A3135 J
- B313.5 J
- C31 350 J
- D750 J
24.In the same experiment, 100.0 cm³ of 0.200 mol dm⁻³ CuSO₄ reacts with excess iron. The heat energy transferred is 3135 J. What is the enthalpy change of reaction, ΔH, in kJ mol⁻¹?
Medium
- A-156.75 kJ mol⁻¹
- B-15.68 kJ mol⁻¹
- C-313.5 kJ mol⁻¹
- D-0.627 kJ mol⁻¹
25.A student uses a spirit burner containing methanol to heat water in a copper calorimeter. The experimental enthalpy of combustion is less exothermic than the data booklet value. Which of the following are valid reasons for this difference? (select all that apply)
Medium
- AHeat is lost to the surroundings.
- BSome methanol undergoes incomplete combustion.
- CThe water absorbs all the heat released by the burner.
- DThe copper calorimeter absorbs some of the heat released.
- EThe specific heat capacity of water is 4.18 J g⁻¹ K⁻¹.
26.In a calorimetry experiment, the temperature of the solution is recorded for a few minutes before adding the second reactant in order to establish a steady initial temperature.
Easy
True or false?
27.A student determines the enthalpy of combustion of methanol using a spirit burner and copper calorimeter. The experimental value is −500 kJ mol⁻¹, while the data booklet value is −726 kJ mol⁻¹. Which improvement would most directly reduce the difference between the experimental and data booklet values?
Hard
- APlace a lid on the calorimeter and shield it from draughts.
- BUse a larger mass of water in the calorimeter.
- CIncrease the initial temperature of the water.
- DUse a smaller mass of methanol in the burner.
28.The equation for the combustion of methanol is CH₃OH(l) + 1½O₂(g) → CO₂(g) + 2H₂O(g). Using the mean bond enthalpies below, what is the theoretical enthalpy of combustion of methanol? Bond enthalpies (kJ mol⁻¹): C–O 358, O–H 463, C–H 414, O=O 498, C=O 804.
Hard
- A-422 kJ mol⁻¹
- B-1046 kJ mol⁻¹
- C+422 kJ mol⁻¹
- D-726 kJ mol⁻¹
29.Which of the following statements about temperature correction graphs are correct? (select all that apply)
Medium
- AThe cooling section is extrapolated back to the time the second reactant was added.
- BThe graph assumes a constant rate of cooling.
- CThe maximum temperature change is read directly from the highest point on the graph.
- DThe method can be adapted for endothermic reactions by extrapolating a warming section.
- EThe graph is only valid if the reaction is instantaneous.
30.In a calorimetry experiment, the mass of the solution is measured in grams and the specific heat capacity of water is 4.18 J g⁻¹ K⁻¹. Which expression gives the heat energy transferred, q, in joules?
Medium
- Aq = m × c × ΔT
- Bq = m × c ÷ ΔT
- Cq = m × ΔT ÷ c
- Dq = c × ΔT ÷ m
31.A student reacts 50.0 cm³ of 1.00 mol dm⁻³ copper(II) sulfate solution with excess zinc powder in a polystyrene cup. The temperature rises by 10.0 °C. Assuming the solution has the same density and specific heat capacity as water (4.18 J g⁻¹ K⁻¹), what is the heat energy released, in J?
Medium
- A2090 J
- B4180 J
- C209 J
- D418 J
32.In a calorimetry experiment, 0.020 mol of copper(II) sulfate reacts with excess zinc and releases 2090 J of heat. What is the enthalpy change of reaction, ΔH, in kJ mol⁻¹?
Medium
- A-105 kJ mol⁻¹
- B-104.5 kJ mol⁻¹
- C-41.8 kJ mol⁻¹
- D-209 kJ mol⁻¹
33.In a temperature correction graph for an exothermic reaction, the cooling section is extrapolated back to the time of mixing to estimate the maximum temperature change.
Easy
True or false?
34.Match each stage of a temperature correction graph with its correct description.
Medium
- Initial steady temperature
- Temperature rise
- Cooling section
- Extrapolation
- Recorded before adding the second reactant
- Occurs immediately after mixing the reactants
- Shows gradual heat loss to surroundings
- Extended back to the time of mixing to find maximum temperature change
35.Place the steps for determining the enthalpy change of a reaction in solution using a polystyrene cup calorimeter in the correct order.
Medium
- Measure initial temperature of the solution for a few minutes.
- Add the second reactant and stir.
- Record temperature at regular intervals until the reaction is complete.
- Plot a temperature-time graph and extrapolate to find the maximum temperature change.
- Calculate q using q = mcΔT and then ΔH = -q/n.
36.A student determines the enthalpy of combustion of methanol using a spirit burner and copper calorimeter. The experimental value is -500 kJ mol⁻¹, while the data booklet value is -726 kJ mol⁻¹. Which of the following is the most significant reason for this difference?
Hard
- AHeat loss to the surroundings and incomplete combustion
- BThe water used was not pure
- CThe thermometer was not calibrated
- DThe methanol was not completely vaporised
37.In an open system, both matter and energy can move in and out.
Easy
True or false?
38.Which statement correctly describes the difference between heat and temperature?
Easy
- AHeat is a measure of the average kinetic energy of particles, while temperature is a measure of the total energy content.
- BTemperature is a measure of the average kinetic energy of particles, while heat is a measure of the energy content of a substance.
- CHeat and temperature are both measures of the total energy content of a substance.
- DTemperature is a measure of the total energy content, while heat is a measure of the average kinetic energy of particles.
39.Which of the following is the correct definition of the standard enthalpy of formation?
Medium
- AThe enthalpy change when one mole of a compound is formed from its elements under standard conditions.
- BThe enthalpy change when one mole of a substance is burnt in excess oxygen under standard conditions.
- CThe enthalpy change when one mole of water is formed by reacting an acid and an alkali under standard conditions.
- DThe enthalpy change when the reactants in a stoichiometric equation react to give the products under standard conditions.
40.Which of the following is the correct definition of the standard enthalpy of combustion?
Medium
- AThe enthalpy change when one mole of a compound is formed from its elements under standard conditions.
- BThe enthalpy change when one mole of a substance is burnt in excess oxygen under standard conditions.
- CThe enthalpy change when one mole of water is formed by reacting an acid and an alkali under standard conditions.
- DThe enthalpy change when the reactants in a stoichiometric equation react to give the products under standard conditions.
41.What is the term for the energy needed to reach the transition state in a reaction?
Easy
42.Complete the definition of standard enthalpy of combustion.
Easy
The standard enthalpy of combustion is the enthalpy change when one mole of a substance is burnt in excess ____ under standard conditions.
43.Calculate the energy absorbed by 100.0 g of water when its temperature rises by 40.0 °C. The specific heat capacity of water is 4.18 J g⁻¹ K⁻¹. Give your answer in kJ to 2 decimal places.
Medium
44.Complete the statement about energy profiles.
Easy
The ____ is the highest energy point along the reaction pathway.
45.What is the symbol for the standard enthalpy change of neutralisation?
Easy
46.A spirit burner containing ethanol is burned, heating 100.0 g of water. The temperature rises by 40.0 °C. The specific heat capacity of water is 4.18 J g⁻¹ K⁻¹. Calculate the heat energy transferred to the water in kJ. Give your answer to 2 decimal places.
Hard
47.Complete the definition of enthalpy of neutralisation.
Easy
The standard enthalpy of neutralisation is the enthalpy change when one mole of ____ is formed by reacting an acid and alkali under standard conditions.
48.A student burns 0.61 g of ethanol (M = 46.0 g mol⁻¹) in a spirit burner, heating 100.0 g of water. The temperature rises by 40.0 °C. Calculate the enthalpy of combustion of ethanol in kJ mol⁻¹. Use c = 4.18 J g⁻¹ K⁻¹ and give your answer to 3 significant figures.
Medium
49.Complete the definition of standard enthalpy of formation.
Medium
The standard enthalpy of formation is the enthalpy change when one mole of a ____ is formed from its ____ under standard conditions.
50.A student burns 0.61 g of ethanol (M = 46.0 g mol⁻¹) in a spirit burner, heating 100.0 g of water. The temperature rises by 40.0 °C. Calculate the energy absorbed by the water, q, in kJ. Use c = 4.18 J g⁻¹ K⁻¹. Give your answer to 2 decimal places.
Medium
51.Using the data from the previous question, calculate the enthalpy of combustion of ethanol in kJ mol⁻¹. The mass of ethanol burned is 0.61 g and M = 46.0 g mol⁻¹. Give your answer to 3 significant figures.
Medium
52.Complete the equation used in calorimetry calculations.
Medium
q = m × ____ × ΔT
53.Define the term enthalpy of formation, ΔHf, and state the standard conditions.
Medium
54.Explain why a polystyrene cup is used in calorimetry experiments.
Hard
55.Explain why the experimental value for the enthalpy of combustion of ethanol is often less exothermic than the data booklet value.
Medium
56.Explain why the experimental value for the enthalpy of combustion of methanol is often less exothermic than the data booklet value.
Hard
57.Explain why the experimental value for the enthalpy of combustion of an alcohol is often less exothermic than the data booklet value.
Hard
58.Calculate the energy absorbed by 100.0 g of water when its temperature rises by 40.0 °C. The specific heat capacity of water is 4.18 J g⁻¹ K⁻¹. Give your answer in kJ to 2 decimal places.
Medium
- A17.72 kJ
- B16.72 kJ
- C18.72 kJ
- D15.72 kJ
59.A spirit burner containing ethanol is burned, heating 100.0 g of water. The temperature rises by 40.0 °C. The specific heat capacity of water is 4.18 J g⁻¹ K⁻¹. Calculate the heat energy transferred to the water in kJ. Give your answer to 2 decimal places.
Hard
- A15.72 kJ
- B17.72 kJ
- C18.72 kJ
- D16.72 kJ
60.A student burns 0.61 g of ethanol (M = 46.0 g mol⁻¹) in a spirit burner, heating 100.0 g of water. The temperature rises by 40.0 °C. Calculate the enthalpy of combustion of ethanol in kJ mol⁻¹. Use c = 4.18 J g⁻¹ K⁻¹ and give your answer to 3 significant figures.
Medium
- A-1260 kJ mol⁻¹
- B-1008 kJ mol⁻¹
- C-1134 kJ mol⁻¹
- D-1386 kJ mol⁻¹
61.A student burns 0.61 g of ethanol (M = 46.0 g mol⁻¹) in a spirit burner, heating 100.0 g of water. The temperature rises by 40.0 °C. Calculate the energy absorbed by the water, q, in kJ. Use c = 4.18 J g⁻¹ K⁻¹. Give your answer to 2 decimal places.
Medium
- A17.72 kJ
- B15.72 kJ
- C16.72 kJ
- D18.72 kJ
62.Using the data from the previous question, calculate the enthalpy of combustion of ethanol in kJ mol⁻¹. The mass of ethanol burned is 0.61 g and M = 46.0 g mol⁻¹. Give your answer to 3 significant figures.
Medium
- A-1009 kJ mol⁻¹
- B-1387 kJ mol⁻¹
- C-1261 kJ mol⁻¹
- D-1135 kJ mol⁻¹

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