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 is the ability to do work, and heat is only one type of energy.

Systems and Conservation of Energy

The system is the reacting substances inside the reaction vessel; the surroundings are everything outside.
In an open system, both matter and energy can move in and out; most chemical reactions occur in open systems.
In a closed system, energy can move in and out but matter cannot.
In an isolated system, neither matter nor energy can be exchanged with the surroundings; these are very rare.
The Law of Conservation of Energy states that energy cannot be created or destroyed, only transferred.

Exothermic and Endothermic Reactions

Enthalpy (H) is the total chemical energy inside a substance; an enthalpy change is represented by ΔH.
In an exothermic reaction, products have less enthalpy than reactants; heat is given off to the surroundings, so the surroundings' temperature increases and ΔH is negative.
In an endothermic reaction, products have more enthalpy than reactants; heat is absorbed from the surroundings, so the surroundings' temperature decreases and ΔH is positive.
Exothermic reactions are thermodynamically possible because reactants have higher enthalpy than products, but they may be too slow (kinetically controlled).
In simple terms, exothermic reactions release energy to the surroundings, while endothermic reactions take in energy, making the surroundings feel colder.

Exothermic and endothermic reactions

Energy Profiles

An energy profile shows how the energy of a system changes during a reaction, including reactants, products, and the transition state.
The transition state is the highest energy point where bonds are partially broken and formed; it is unstable and cannot be isolated.
Activation energy (Ea) is the minimum energy needed for reactant molecules to collide successfully and start the reaction.
In an exothermic reaction, reactants are higher in energy than products and closer to the transition state, so Ea is lower than for an endothermic reaction.
In an endothermic reaction, reactants are lower in energy than products and further from the transition state, so Ea is higher.
The activation energy is the energy difference from reactants to the transition state; the enthalpy change is the energy difference from reactants to products.

Reaction pathway diagrams

Standard Enthalpy Changes

Standard conditions are: pressure of 100 kPa, concentration of 1 mol dm⁻³ for solutions, and substances in their standard states; a temperature of 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 an alkali under standard conditions; always exothermic.

Calorimetry: Measuring Enthalpy Changes

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 calculations, assume the specific heat capacity and density of the solution are the same as pure water (density = 1 g cm⁻³), ignore the container's heat capacity, assume complete reaction, and assume negligible heat losses.
There is no need to convert temperature from °C to K because the change in temperature is the same in both scales.

Polystyrene-cup calorimetry

Calorimetry Experiments for Reactions in Solution

For reactions in solution, carry out the reaction with an excess of one reagent and measure the temperature change over a few minutes.
Use a polystyrene cup to reduce heat loss; a lid and shielding can further reduce heat loss.
Calculate q using q = m × c × ΔT, then calculate moles of the limiting reagent, and finally ΔH = –q ÷ n (or q ÷ n with sign determined by whether the reaction is exothermic or endothermic).
For example, excess zinc added to copper(II) sulfate solution: q = m × c × ΔT, moles of CuSO₄ = volume × concentration, ΔH = –q ÷ n.
The main sources of error are heat loss to surroundings and incomplete reaction.

Temperature Correction Graphs

For reactions that are not instantaneous, there may be a delay before maximum temperature is reached, and heat loss can occur during that delay.
A temperature correction graph plots temperature against time; the cooling section is extrapolated back to the time of mixing to find the true maximum temperature change.
Steps: record temperature before adding reactants for a few minutes, add the second reactant and continue recording, plot the graph and extrapolate the cooling part to the time of addition.
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 substances return to room temperature.

Calorimetry Experiments for Enthalpy of Combustion

The heat released by a combustion reaction is used to increase the heat content of water in a calorimeter.
A typical simple calorimeter uses a spirit burner to heat a known mass of water in a copper calorimeter.
Calculate q = m × c × ΔT for the water, then moles of fuel burned = mass ÷ molar mass, and ΔH = –q ÷ n (converted to kJ mol⁻¹).
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 calorimeter not too far above the flame, use a lid, and shield to reduce draughts.
The main sources of error are heat losses and incomplete combustion.

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1.Which statement correctly describes the difference between heat and temperature?
Easy
- ATemperature is a measure of the average kinetic energy of the particles, while heat is a measure of the total energy content of a substance.
- BTemperature is a measure of the total energy content of a substance, while heat is a measure of the average kinetic energy of the particles.
- CHeat and temperature are the same physical quantity, just expressed in different units.
- DHeat is measured in kelvin, while temperature is measured in joules.
2.In an exothermic reaction, what happens to the temperature of the surroundings?
Easy
- AIt increases because heat is released from the system to the surroundings.
- BIt decreases because heat is absorbed by the system from the surroundings.
- CIt stays the same because energy is conserved.
- DIt increases because heat is absorbed by the system from the surroundings.
3.Which of the following is the correct symbol for the standard enthalpy change of formation?
Easy
- AΔHf⦵
- BΔHc⦵
- CΔHr⦵
- DΔHneut⦵
4.Which of the following statements about standard enthalpy changes are correct? (select all that apply)
Medium
- AThe standard enthalpy of combustion is always exothermic.
- BThe standard enthalpy of formation can be either exothermic or endothermic.
- CThe standard enthalpy of neutralisation is always endothermic.
- DThe standard enthalpy of reaction can be either exothermic or endothermic.
- EThe standard enthalpy of combustion can be either exothermic or endothermic.
5.In an isolated system, neither matter nor energy can be exchanged with the surroundings.
Easy
True or false?
6.The Law of Conservation of Energy states that energy can be created and destroyed in a chemical reaction.
Easy
True or false?
7.Match each term with its correct definition.
Medium
- Enthalpy change of reaction
- Enthalpy change of formation
- Enthalpy change of combustion
- Enthalpy change of neutralisation
- The enthalpy change when one mole of a compound is formed from its elements under standard conditions.
- The enthalpy change when one mole of a substance is burnt in excess oxygen under standard conditions.
- The enthalpy change when one mole of water is formed by reacting an acid and alkali under standard conditions.
- The enthalpy change when the reactants in the stoichiometric equation react to give the products under standard conditions.
8.Place the following steps of a temperature correction graph for a calorimetry experiment in the correct order.
Medium
- Take a temperature reading before adding the reactants for a few minutes to get a steady value.
- Add the second reactant and continue recording the temperature and time.
- Plot the graph and extrapolate the cooling part of the graph until it intersects the time at which the second reactant was added.

Soal lainnya

9.A student performs a calorimetry experiment to determine the enthalpy of combustion of ethanol. The experimental value is found to be significantly less exothermic than the data booklet value. Which is the most likely reason?
Medium
- AHeat loss to the surroundings and incomplete combustion.
- BThe specific heat capacity of water was taken as 4.18 J g⁻¹ K⁻¹ instead of 4.18 kJ g⁻¹ K⁻¹.
- CThe mass of water used was too large.
- DThe ethanol was completely burnt in excess oxygen.
10.In a calorimetry experiment using a polystyrene cup, which assumption is made about the solution?
Medium
- AThe specific heat capacity and density of the solution are the same as pure water.
- BThe specific heat capacity of the solution is zero.
- CThe density of the solution is 1 g cm⁻³ and the specific heat capacity is 4.18 kJ g⁻¹ K⁻¹.
- DThe container absorbs all the heat.
11.Which of the following modifications would reduce heat loss in a calorimetry experiment using a polystyrene cup? (select all that apply)
Medium
- APlace a lid on the cup.
- BUse a vacuum flask instead of a polystyrene cup.
- CAdd the reactants quickly.
- DUse a copper calorimeter instead of a polystyrene cup.
- EStir the solution continuously.
12.In an endothermic reaction, the temperature of the surroundings decreases.
Easy
True or false?
13.Which of the following is the correct equation for calculating heat transferred in a calorimetry experiment?
Medium
- Aq = m × c × ΔT
- Bq = m × ΔT / c
- Cq = c × ΔT / m
- Dq = m × c / ΔT
14.The reaction of propane with oxygen is: C₃H₈ + 5O₂ → 3CO₂ + 4H₂O, ΔH = -2219 kJ mol⁻¹. What is the classification of this enthalpy change?
Hard
- AExothermic, because ΔH is negative.
- BEndothermic, because ΔH is negative.
- CExothermic, because ΔH is positive.
- DEndothermic, because ΔH is positive.
15.Which of the following statements about energy profiles is correct?
Medium
- AThe transition state is the highest energy point along the reaction pathway and is unstable.
- BThe transition state is the lowest energy point and can be isolated.
- CThe activation energy is the energy difference between reactants and products.
- DExothermic reactions have higher activation energies than endothermic reactions.
16.Which of the following are standard conditions for enthalpy changes? (select all that apply)
Medium
- AA pressure of 100 kPa
- BA concentration of 1 mol dm⁻³ for all solutions
- CA temperature of 298.15 K
- DEach substance in its standard state
- EA pressure of 1 atm
17.In a calorimetry experiment, a student plots temperature against time and adds the powdered reactant at a certain time. What does the point where the temperature starts to change rapidly indicate?
Medium
- AThe time at which the second reactant was added.
- BThe maximum temperature reached.
- CThe start of the cooling curve.
- DThe end of the reaction.
18.In an exothermic reaction, the products have less enthalpy than the reactants.
Easy
True or false?
19.Which of the following is the correct definition of average bond enthalpy?
Medium
- AThe average enthalpy change when one mole of a given bond is broken in the gaseous state.
- BThe enthalpy change when one mole of a compound is formed from its elements.
- CThe energy required to break one mole of a substance into its gaseous atoms.
- DThe enthalpy change when one mole of a substance is burnt in excess oxygen.
20.Which of the following statements about bond breaking and bond forming are correct? (select all that apply)
Medium
- ABond breaking is endothermic.
- BBond forming is exothermic.
- CBond breaking releases energy.
- DBond forming requires energy.
- EBond breaking requires energy.
21.An element X undergoes complete combustion: X + O₂ → XO₂. The enthalpy change, ΔH, is -520 kJ mol⁻¹ and the activation energy, Ea, is +630 kJ mol⁻¹. Which statement is correct?
Hard
- AThe reaction is exothermic because ΔH is negative.
- BThe reaction is endothermic because Ea is positive.
- CThe reaction is exothermic because Ea is positive.
- DThe reaction is endothermic because ΔH is negative.
22.Which quantity is a measure of the average kinetic energy of the particles in a substance?
Easy
- ATemperature
- BHeat
- CEnthalpy
- DActivation energy
23.In an endothermic reaction, which statement about the system and surroundings is correct?
Medium
- AThe system absorbs heat energy and the temperature of the surroundings decreases.
- BThe system releases heat energy and the temperature of the surroundings increases.
- CThe system absorbs heat energy and the temperature of the surroundings increases.
- DThe system releases heat energy and the temperature of the surroundings decreases.
24.Which of the following statements about a closed system are correct? (select all that apply)
Medium
- AEnergy can move in and out of the system.
- BMatter cannot move in or out of the system.
- CBoth energy and matter can move in and out of the system.
- DNeither energy nor matter can move in or out of the system.
- EThe total energy of the system and surroundings remains constant.
25.In an exothermic reaction, the enthalpy of the products is lower than the enthalpy of the reactants.
Easy
True or false?
26.Match each type of system with its correct description.
Medium
- Open system
- Closed system
- Isolated system
- Both matter and energy can be exchanged with the surroundings
- Energy can be exchanged but matter cannot
- Neither matter nor energy can be exchanged with the surroundings
27.Place the following steps for constructing a temperature correction graph in the correct order.
Medium
- Record the temperature of the solution for a few minutes before adding the second reactant.
- Add the second reactant and continue recording temperature and time.
- Plot the graph and extrapolate the cooling section back to the time of addition.
- Determine the maximum temperature change from the extrapolated intersection.
28.In a calorimetry experiment, 100.0 cm³ of 0.200 mol dm⁻³ copper(II) sulfate solution reacts with excess iron powder. The maximum temperature rise is 7.5 °C. Using q = mcΔT and assuming the solution has the density and specific heat capacity of water, what is the enthalpy change of reaction in kJ mol⁻¹?
Medium
- A-156.75 kJ mol⁻¹
- B-3135 kJ mol⁻¹
- C-15.68 kJ mol⁻¹
- D-313.5 kJ mol⁻¹
29.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 the same as pure water.
- CThe specific heat capacity of the container is ignored.
- DThe reaction is complete.
- EThere are significant heat losses to the surroundings.
30.In the reaction C₃H₈(g) + 5O₂(g) → 3CO₂(g) + 4H₂O(l), ΔH = -2219 kJ mol⁻¹. What is the classification of this enthalpy change?
Medium
- AExothermic
- BEndothermic
- CActivation energy
- DBond enthalpy
31.A student adds 0.05 mol of zinc to 100 cm³ of 0.5 mol dm⁻³ copper(II) sulfate solution. The temperature rises by 10.0 °C. Using q = mcΔT and assuming the solution has the density and specific heat capacity of water, what is the heat energy released, in kJ?
Easy
- A4.18 kJ
- B2.09 kJ
- C41.8 kJ
- D0.418 kJ
32.In a calorimetry experiment, 50.0 cm³ of 1.0 mol dm⁻³ copper(II) sulfate solution reacts with excess zinc powder. The temperature rises by 22.6 °C. What is the enthalpy change of the reaction, in kJ mol⁻¹? (Assume the solution has the density and specific heat capacity of water.)
Medium
- A-94.5 kJ mol⁻¹
- B-189 kJ mol⁻¹
- C-47.2 kJ mol⁻¹
- D-4.72 kJ mol⁻¹
33.A student burns 0.500 g of methanol (M = 32.04 g mol⁻¹) in a spirit burner, heating 150.0 g of water in a copper calorimeter. The temperature of the water rises by 25.0 °C. What is the experimental enthalpy of combustion of methanol, in kJ mol⁻¹?
Hard
- A-1.00 × 10³ kJ mol⁻¹
- B-104 kJ mol⁻¹
- C-52.0 kJ mol⁻¹
- D-15.7 kJ mol⁻¹
34.A student records the temperature of a solution every minute. After 3 minutes, a solid reactant is added. The temperature rises to a maximum and then slowly falls. Which of the following is the most likely reason for the initial steady temperature before the reactant is added?
Medium
- ATo establish a baseline so the temperature change can be measured accurately
- BTo allow the solution to reach its boiling point
- CTo ensure the reaction has gone to completion
- DTo measure the activation energy of the reaction
35.In a calorimetry experiment, 25.0 cm³ of 2.00 mol dm⁻³ hydrochloric acid is mixed with 25.0 cm³ of 2.00 mol dm⁻³ sodium hydroxide solution. The temperature rises by 13.5 °C. What is the enthalpy change of neutralisation, in kJ mol⁻¹? (Assume the combined solution has the density and specific heat capacity of water.)
Medium
- A-56.4 kJ mol⁻¹
- B-113 kJ mol⁻¹
- C-28.2 kJ mol⁻¹
- D-5.64 kJ mol⁻¹
36.Which of the following are valid assumptions made when using q = mcΔT to calculate the enthalpy change of a reaction in aqueous solution? (select all that apply)
Medium
- AThe specific heat capacity of the solution is the same as pure water.
- BThe density of the solution is the same as pure water.
- CThe reaction goes to completion.
- DHeat losses to the surroundings are negligible.
- EThe temperature change is measured in Kelvin.
37.In an exothermic reaction, the temperature of the surroundings increases.
Easy
True or false?
38.In an endothermic reaction, the temperature of the system decreases.
Easy
True or false?
39.In a calorimetry experiment, 50.0 cm³ of 1.0 mol dm⁻³ copper(II) sulfate solution reacts with excess zinc powder. The temperature rises by 22.6 °C. What is the amount, in moles, of copper(II) sulfate used?
Medium
- A0.050 mol
- B0.50 mol
- C0.025 mol
- D0.10 mol
40.In a calorimetry experiment using q = mcΔT, which value should be used for m?
Medium
- AThe mass of the solution
- BThe mass of the solid reactant
- CThe mass of the calorimeter
- DThe total mass of reactants and products
41.Calculate the energy transferred, q, in joules when 100 g of water is heated and the temperature rises by 10.0 °C. The specific heat capacity of water is 4.18 J g⁻¹ K⁻¹.
Medium
42.Complete the definition of activation energy by filling in the blanks.
Easy
The activation energy is the minimum amount of ____ needed for reactant molecules to have a successful ____ and start the reaction.
43.What is the name of the process where bonds are broken, requiring thermal energy?
Easy
44.In a calorimetry experiment, 50.0 cm³ of 1.0 mol dm⁻³ copper(II) sulfate solution is reacted with excess zinc. The temperature rises by 22.6 °C. Calculate the energy change, q, in kJ. Assume the specific heat capacity and density of the solution are the same as pure water (c = 4.18 J g⁻¹ K⁻¹, density = 1.0 g cm⁻³).
Medium
45.Complete the definition of standard enthalpy change of formation by filling in the blanks.
Medium
The standard enthalpy change of formation is the enthalpy change when ____ mole of a compound is formed from its ____ under standard conditions.
46.Complete the sentence about systems by filling in the blanks.
Medium
In a ____ system, energy can move in and out, but matter cannot. In an ____ system, neither matter nor energy can be exchanged with the surroundings.
47.What is the specific heat capacity of water, in J g⁻¹ K⁻¹?
Easy
48.A student burns 0.61 g of ethanol (M = 46.08 g mol⁻¹) in a spirit burner, heating 100.0 g of water in a copper calorimeter. The temperature rises by 40.0 °C. Calculate the enthalpy of combustion of ethanol in kJ mol⁻¹. Assume c = 4.18 J g⁻¹ K⁻¹.
Hard
49.Complete the definition of standard enthalpy change of neutralisation by filling in the blanks.
Easy
The standard enthalpy change of neutralisation is the enthalpy change when ____ mole of water is formed by reacting an ____ and alkali under standard conditions.
50.What is the name of the process where bonds are formed, releasing thermal energy?
Easy
51.25.0 cm³ of 0.5 mol dm⁻³ hydrochloric acid reacts with 25.0 cm³ of 0.5 mol dm⁻³ sodium hydroxide. The enthalpy change of neutralisation is -57.9 kJ mol⁻¹. Calculate the enthalpy change, in kJ, for this reaction. Give your answer to 2 decimal places.
Medium
52.A student burns 1.023 g of propan-1-ol (M = 60.11 g mol⁻¹) in a spirit burner, heating 200 g of water in a copper calorimeter. The temperature of the water rises by 30.0 °C. Calculate the enthalpy of combustion of propan-1-ol in kJ mol⁻¹. Give your answer to 3 significant figures.
Medium
53.Define the term standard enthalpy of combustion, ΔHc⦵.
Hard
54.Explain why a polystyrene cup is used as a calorimeter in school laboratory experiments to measure enthalpy changes.
Hard
55.Explain why the experimental value for the enthalpy of combustion of an alcohol is often less exothermic than the theoretical value.
Hard
56.Describe how the heat energy transferred (q) for the reaction between solid ammonium nitrate and water could be determined in a school laboratory. Include the apparatus needed and the measurements taken.
Hard
57.Explain why the experimental value for the enthalpy of combustion of an alcohol, determined using a spirit burner and copper calorimeter, is often less exothermic than the theoretical value from the data booklet.
Hard
58.Calculate the energy transferred, q, in joules when 100 g of water is heated and the temperature rises by 10.0 °C. The specific heat capacity of water is 4.18 J g⁻¹ K⁻¹.
Medium
- A4598 J
- B3762 J
- C4180 J
- D5016 J
59.In a calorimetry experiment, 50.0 cm³ of 1.0 mol dm⁻³ copper(II) sulfate solution is reacted with excess zinc. The temperature rises by 22.6 °C. Calculate the energy change, q, in kJ. Assume the specific heat capacity and density of the solution are the same as pure water (c = 4.18 J g⁻¹ K⁻¹, density = 1.0 g cm⁻³).
Medium
- A3.72 kJ
- B6.72 kJ
- C5.72 kJ
- D4.72 kJ
60.A student burns 0.61 g of ethanol (M = 46.08 g mol⁻¹) in a spirit burner, heating 100.0 g of water in a copper calorimeter. The temperature rises by 40.0 °C. Calculate the enthalpy of combustion of ethanol in kJ mol⁻¹. Assume c = 4.18 J g⁻¹ K⁻¹.
Hard
- A-1008 kJ mol⁻¹
- B-1134 kJ mol⁻¹
- C-1386 kJ mol⁻¹
- D-1260 kJ mol⁻¹
61.25.0 cm³ of 0.5 mol dm⁻³ hydrochloric acid reacts with 25.0 cm³ of 0.5 mol dm⁻³ sodium hydroxide. The enthalpy change of neutralisation is -57.9 kJ mol⁻¹. Calculate the enthalpy change, in kJ, for this reaction. Give your answer to 2 decimal places.
Medium
- A-1.72 kJ
- B-0.72 kJ
- C0.28 kJ
- D1.28 kJ
62.A student burns 1.023 g of propan-1-ol (M = 60.11 g mol⁻¹) in a spirit burner, heating 200 g of water in a copper calorimeter. The temperature of the water rises by 30.0 °C. Calculate the enthalpy of combustion of propan-1-ol in kJ mol⁻¹. Give your answer to 3 significant figures.
Medium
- A-1323 kJ mol⁻¹
- B-1617 kJ mol⁻¹
- C-1470 kJ mol⁻¹
- D-1176 kJ mol⁻¹

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