Enthalpy Change (ΔH) Calculation

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Questions and Answers

In a calorimetry experiment, 15.00 grams of a substance are dissolved in 750.0 mL of water. The temperature changes from 25.00°C to 29.00°C. What is the first step you must take to calculate the enthalpy of solution (ΔH)?

  • Calculate the molar mass of the substance.
  • Calculate the moles of the substance. (correct)
  • Determine the specific heat capacity of the solution.
  • Convert the volume of water to liters.

A reaction occurs in a solution with a total mass of 120 g and a specific heat capacity of 4.0 J/g·°C. If the temperature increases by 5.0°C, how much heat is absorbed by the solution?

  • 4800 J
  • 2400 J (correct)
  • 960 J
  • 240 J

When calculating the enthalpy change (ΔH) for a reaction where a solid is dissolved in water, and the temperature of the solution decreases, how should the calculated heat (q) be adjusted before dividing by the number of moles?

  • The heat (q) should be converted to kilojoules and assigned a negative sign.
  • The heat (q) should be converted to kilojoules and assigned a positive sign. (correct)
  • The heat (q) should be divided by the specific heat capacity of the solution.
  • The heat (q) should be multiplied by the molar mass of the solute.

A chemist performs a reaction in a calorimeter and determines that the reaction releases 45.0 kJ of heat. If 0.5 moles of reactant are used, what is the enthalpy change (ΔH) for the reaction?

<p>-90.0 kJ/mol (C)</p>
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In an experiment, 50.0 mL of 1.0 M HCl is mixed with 50.0 mL of 1.0 M NaOH in a calorimeter. The temperature increases. Which of the following should be included in the calculation of the heat transferred (q)?

<p>The density of the resulting solution. (D)</p>
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Given the reaction: Ag⁺(aq) + Cl⁻(aq) → AgCl(s), if 0.020 moles of AgCl are formed in a solution with a total volume of 80 mL, what additional information is needed to calculate the heat transferred (q)?

<p>The temperature change (ΔT) and the final volume of the solution. (A)</p>
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A student dissolves ammonium nitrate ($NH_4NO_3$) in water and observes that the temperature of the solution decreases. What can be concluded about the enthalpy change (ΔH) for this process?

<p>ΔH is positive and the process is endothermic. (C)</p>
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A reaction between an acid and a base is performed in a calorimeter. What is the significance of calculating ΔH° (ΔH under standard conditions) instead of just ΔH?

<p>ΔH° allows for comparison to reactions under other conditions. (A)</p>
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If 25.0 mL of 0.4 M lead(II) nitrate ($Pb(NO_3)_2$) is mixed with 25.0 mL of 0.4 M potassium iodide (KI), forming a precipitate of lead(II) iodide ($PbI_2$), how do you determine the moles of lead(II) iodide formed?

<p>Use the balanced net ionic equation to determine the limiting reactant and its corresponding mole ratio to $PbI_2$. (D)</p>
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A student measures the temperature change when mixing two solutions in a coffee cup calorimeter and wants to improve the accuracy of the experiment. Which action would best reduce systematic errors?

<p>Use a metal calorimeter instead of a coffee cup. (B)</p>
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What is the most accurate interpretation of ΔH° = -100 kJ/mol for a chemical reaction?

<p>The reaction releases 100 kJ of heat for each mole of reactant under standard conditions. (C)</p>
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When mixing two solutions, the reaction is known to produce a gas, what modification to the experimental setup is most important when performing calorimetry?

<p>Seal the calorimeter to prevent gas from escaping. (D)</p>
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In a calorimetry experiment, if the calorimeter itself absorbs a significant amount of heat, how does this affect the calculation of ΔH for the reaction?

<p>The energy absorbed by the calorimeter must be added to the calculated heat (q) of the reaction. (C)</p>
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How does increasing the concentration of reactants generally affect the magnitude of the enthalpy change (ΔH) measured in a calorimetry experiment, assuming the volume of solution remains constant?

<p>It increases the magnitude of heat transferred (q) which may lead to a larger calculated ΔH but ΔH, being an intensive property, should remain the same as long as it is normalized per mole. (B)</p>
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For a reaction where the change in temperature is very small and difficult to measure accurately, which adjustment to the calorimetry procedure would likely improve the reliability of the results?

<p>Use smaller volumes of more concentrated reactants. (D)</p>
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A reaction is carried out in a calorimeter that is not perfectly insulated, allowing some heat to escape. What effect will this heat loss have on the calculated value of ΔH if the reaction is exothermic?

<p>The calculated ΔH will be less negative than the true value. (D)</p>
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In a bomb calorimeter experiment, a small amount of water is present inside the bomb along with the reactants. What role does this water play in the experiment?

<p>It helps to distribute the heat evenly within the bomb. (A)</p>
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How does the presence of a spectator ion in a chemical reaction affect the determination of ΔH using calorimetry?

<p>Spectator ions do not affect the determination of ΔH because they do not participate in the reaction. (B)</p>
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What is the effect of an incomplete reaction on the calculated $\Delta H$?

<p>Incomplete reaction will result in a lower (less positive or more negative) $\Delta H$ value than the actual value. (B)</p>
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In calorimetry, what does 'q' represent, and in what units is it typically measured?

<p>Heat transferred, measured in joules. (A)</p>
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When calculating the heat transferred (q) in a calorimetry experiment using $q = mc\Delta T$, what does 'm' represent?

<p>The total mass of the solution in grams. (A)</p>
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For an endothermic reaction in a calorimeter, what happens to the temperature of the solution, and what is the sign of 'q'?

<p>The temperature decreases, and 'q' is positive. (A)</p>
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In the context of calorimetry, what are 'standard conditions'?

<p>25°C, 1 atmosphere pressure for gases, and 1 mole per liter concentration for solutions. (A)</p>
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If a reaction is exothermic, what is the sign of ΔH, and what does this indicate about the energy of the products compared to the reactants?

<p>ΔH is negative, indicating the products have less energy than the reactants. (B)</p>
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When 5.0 g of a salt is dissolved in 100.0 mL of water, the temperature decreases from 25.0°C to 22.5°C. What can be concluded about the solution process?

<p>The solution process is endothermic. (C)</p>
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What is the purpose of determining the net ionic equation in a calorimetry experiment involving aqueous solutions?

<p>To identify the actual chemical species that are reacting. (C)</p>
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Given that $\Delta H = -q/n$, where 'n' is the number of moles, what does the negative sign indicate, and why is it important in calorimetry?

<p>It indicates an exothermic reaction and ensures ΔH reflects the system's change in energy. (B)</p>
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What is the relationship between 'q' and ΔH, and when is it most appropriate to use ΔH instead of 'q'?

<p>ΔH is 'q' adjusted for the number of moles; it is used to standardize energy changes per mole. (A)</p>
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If 50 mL of 1.0 M HCl is mixed with 50 mL of 1.0 M NaOH in a calorimeter, and the temperature increases, what can be concluded about the reaction?

<p>The reaction is exothermic with a negative ΔH. (C)</p>
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How does the heat capacity of the calorimeter itself affect the accuracy of ΔH calculations, and what adjustments can be made to account for this?

<p>The heat absorbed by the calorimeter must be added to the heat measured in the solution because the calorimeter is part of the system. (C)</p>
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In a bomb calorimeter, why is it crucial to have a complete combustion reaction when determining ΔH?

<p>Incomplete combustion can produce different products, leading to an inaccurate measurement of ΔH. (A)</p>
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If a student forgets to account for the heat absorbed by the calorimeter itself during an exothermic reaction, how will this affect the calculated value of ΔH?

<p>The calculated ΔH will be more positive than the true value. (D)</p>
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When determining ΔH for a reaction that produces a gas, what is the most important consideration regarding the experimental setup?

<p>Measuring the pressure change to account for PV work. (A)</p>
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A student performs a calorimetry experiment and finds that the heat transferred (q) is very small. To improve the accuracy of the calculated enthalpy change (ΔH), which adjustment should be made to the experimental procedure?

<p>Use a more precise thermometer with smaller degree increments. (B)</p>
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For the reaction $A(aq) + B(aq) \rightarrow C(s)$, if only 90% of A reacts due to equilibrium limitations, how would this impact the calculated (\Delta H)?

<p>The magnitude of $\Delta H$ would be smaller than if the reaction had gone to completion because less heat would be evolved or absorbed. (B)</p>
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When using calorimetry to determine ΔH, which of the following is most critical for minimizing heat loss to the surroundings?

<p>Using a well-insulated calorimeter. (B)</p>
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In a reaction where gas is produced and escapes from the calorimeter, how does this affect the calorimetry measurements?

<p>The escaping gas carries away heat, causing an underestimation of the reaction's enthalpy change if it is exothermic. (B)</p>
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Considering a reaction where the enthalpy change is determined by measuring the temperature change in a solution, how does using a more concentrated solution of reactants typically affect the magnitude of the observed temperature change?',

<p>It increases the magnitude of the temperature change because more reactant is available to react. (C)</p>
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Which of the following is NOT one of the four methods that can be used to calculate (\Delta H)?

<p>Quantum tunneling (C)</p>
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Flashcards

Enthalpy Change (ΔH)

Heat transferred during a chemical reaction or process at constant pressure.

Calorimetry

A technique used to measure the heat change (q) of a reaction. Allows determination of ΔH of a chemical process.

Heat Transferred (q)

The heat transferred from a system or reaction to its surroundings, measured in joules.

ΔH°

The change in enthalpy under standard conditions (25°C and 1 atm).

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Enthalpy of Solution

The enthalpy change when a solute dissolves in a solvent.

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Spectator Ions

Ions that remain unchanged in solution during a reaction and do not participate in the formation of a precipitate.

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Net Ionic Equation

An equation that shows only the ions that participate in the reaction and form a solid or precipitate.

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q = mcΔT

ΔH = (mass of solution) * (specific heat capacity) * (temperature change).

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Negative ΔH

The heat change is negative, indicating heat is released, and the reaction is exothermic.

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Positive ΔH

Heat is absorbed, and the reaction is endothermic

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Calculating ΔH from q

Divide the heat transferred (q) by the number of moles of the limiting reactant.

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Molar mass

The molar mass is the sum of the atomic masses of each element in the compound.

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What is Enthalpy of Solution?

Heat change at constant pressure in kJ/mol.

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What is calorimetry?

Technique to measure heat transfer (q).

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How is enthalpy of solution calculated?

Determined using q=mcΔT and moles.

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How to calculate heat transferred (q)?

Mass of the solution multiplied by specific heat capacity and temperature change.

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What is molar mass used for?

The total mass of each element's atomic mass in a compound.

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Different methods to calculate ΔH?

Using bond enthalpies, enthalpies of formation, Hess’s Law, and calorimetry.

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Study Notes

Calculating Enthalpy Change (ΔH) for Chemical Reactions

  • Focus is on using q = mcΔT to calculate the change in enthalpy (ΔH) for chemical reactions or processes.

Example 1: Calcium Chloride Dissolution

  • A chemist dissolves 10.00 grams of calcium chloride (CaCl₂) in 500.0 mL of water at 23.50°C.
  • The temperature rises to 27.00°C after the calcium chloride dissolves completely.
  • Goal: Calculate the enthalpy of solution (ΔH) of CaCl₂ in kilojoules per mole (kJ/mol).
  • Assumption: The solution's density is 1.00 g/mL, and its specific heat capacity is 4.18 J/g·°C
  • Need to calculate heat transferred (q) in kilojoules and moles of CaCl₂.
  • Enthalpy of solution is expressed in kilojoules per mole (kJ/mol).
  • To calculate enthalpy of solution, calculate the heat transferred (q in kilojoules) and the number of moles involved.

Step 1: Calculating Heat Transferred (q)

  • Use the formula q = mcΔT.
  • Determine the mass (m) of the solution: 10 g (CaCl₂) + 500 g (water) = 510 g.
  • Specific heat capacity (c) is 4.18 J/g·°C.
  • Calculate the temperature change (ΔT): 27.00°C - 23.50°C = 3.50°C.
  • Substitute values into the formula: q = (510 g) * (4.18 J/g·°C) * (3.50°C).
  • Result: q = 7461 joules.

Step 2: Calculating Moles of Calcium Chloride

  • Convert 10.00 grams of CaCl₂ to moles using its molar mass.
  • The molar mass of CaCl₂ is approximately 110.98 g/mol (from the periodic table).
  • Use the conversion factor: (1 mol / 110.98 g).
  • Calculation: 10.00 g * (1 mol / 110.98 g) = 0.09011 moles of CaCl₂.

Step 3: Calculating Enthalpy of Solution (ΔH)

  • Divide the heat transferred (q) by the number of moles of CaCl₂.
  • Account for the sign change: The system (CaCl₂) loses heat, thus q is negative.
  • Think of it this way: Temperature increased, so it is exothermic, therefore the sign is negative
  • q = -7461 joules.
  • Calculation: ΔH = (-7461 J) / (0.09011 mol) = -82,800 J/mol.
  • Convert to kilojoules per mole: ΔH = -82.8 kJ/mol.

Key Concepts

  • Calorimetry can determine the ΔH of a chemical process.
  • ΔH can be calculated from a simple activity in a chemistry lab.
  • q is the heat transferred (in joules).
  • ΔH is the change in enthalpy (in joules/mole or kilojoules/mole).
  • ΔH° indicates standard conditions (25°C, 1 atm pressure for gases, 1 M concentration for solutions).
  • q and ΔH are related and can be converted.
  • q represents the heat transferred in a process, measured in joules (J).
  • ΔH represents the change in enthalpy for a chemical reaction, measured in joules per mole (J/mol) or kilojoules per mole (kJ/mol).

Example 2: Silver Nitrate and Potassium Chloride Reaction

  • A chemist mixes 30.0 mL of 0.5 M silver nitrate (AgNO₃) with 30.0 mL of 0.5 M potassium chloride (KCl).
  • A white precipitate forms, and the temperature rises from 23.00°C to 26.80°C.
  • Task: Write the net ionic equation for the reaction and determine ΔH in kilojoules per mole.
  • Assumption: Solution density is 1 g/mL, and specific heat capacity is 4.18 J/g·°C.

Step 1: Net Ionic Equation

  • Identify ions in solution: Ag⁺ and NO₃⁻ from AgNO₃, K⁺ and Cl⁻ from KCl.
  • Determine which ions form a precipitate.
  • Silver ions (Ag⁺) and chloride ions (Cl⁻) combine to form silver chloride (AgCl), a solid precipitate.
  • Potassium (K⁺) and nitrate (NO₃⁻) ions are spectator ions (they remain in solution).
  • Net ionic equation: Ag⁺(aq) + Cl⁻(aq) → AgCl(s).

Step 2: Calculating Moles

  • Calculate moles of silver (Ag⁺): 0.030 L * 0.5 mol/L = 0.015 moles.
  • Calculate moles of chloride (Cl⁻): 0.030 L * 0.5 mol/L = 0.015 moles.
  • The reaction has a 1:1:1 mole ratio (Ag⁺ : Cl⁻ : AgCl).
  • Therefore, 0.015 moles of silver chloride (AgCl) are formed.

Step 3: Calculating Heat Transferred (q)

  • Use the formula q = mcΔT.
  • Determine the mass (m) of the solution: 30 mL + 30 mL = 60 mL = 60 g (since density is 1 g/mL).
  • Specific heat capacity (c) is 4.18 J/g·°C.
  • Calculate the temperature change (ΔT): 26.80°C - 23.00°C = 3.80°C.
  • Substitute values: q = (60 g) * (4.18 J/g·°C) * (3.80°C) = 953 joules.
  • The reaction is exothermic because the temperature increased so q = -953 J = -0.953 kJ

Step 4: Calculating Enthalpy Change (ΔH)

  • ΔH is kilojoules divided by moles (kJ/mol).
  • Calculation: ΔH = (-0.953 kJ) / (0.015 moles) = -63.5 kJ/mol.
  • The ΔH for the reaction Ag⁺(aq) + Cl⁻(aq) → AgCl(s) is -63.5 kJ/mol.

Summary of ΔH Calculation Methods

  • Bond enthalpies.
  • Enthalpies of formation (products minus reactants).
  • Hess's Law.
  • Calorimetry.
  • By this point in the course, there are four methods to calculate ΔH:
    • Bond Enthalpies
    • Enthalpies of Formation (products minus reactants)
    • Hess's Law
    • Calorimetry

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