Relate Enthalpy to Reactions - Chemistry

1. Choose the correct expression for the enthalpy change of neutralization for strong acid and strong base.

   Approximately $-57\ \mathrm{kJ\,mol^{-1}}$ per mol $\mathrm{H_2O}$ formed. Standard neutralization enthalpy for strong acids and bases.

2. What happens to $\Delta H$ when all coefficients in a reaction are multiplied by a factor $k$?

   $\Delta H$ is multiplied by $k$. Scaling coefficients scales the enthalpy proportionally.

3. State the meaning of stoichiometric coefficients in thermochemical equations with respect to $\Delta H$.

   $\Delta H$ applies to the reaction as written with those coefficients. Enthalpy corresponds to exact stoichiometric amounts shown.

4. What is $\Delta H$ for a reaction that releases $250\ \mathrm{kJ}$ of heat at constant pressure?

   $\Delta H=-250\ \mathrm{kJ}$. Heat released means negative enthalpy change.

5. Calculate $\Delta H_{rxn}^\circ$ given $\sum n\Delta H_f^\circ(\text{prod})=-120\ \mathrm{kJ}$ and $\sum n\Delta H_f^\circ(\text{react})=-300\ \mathrm{kJ}$.

   $\Delta H_{rxn}^\circ=+180\ \mathrm{kJ}$. Products minus reactants: $-120 - (-300) = +180$.

6. State the relationship between heat at constant pressure and enthalpy change for a process.

   $q_p=\Delta H$. At constant pressure, heat equals enthalpy change.

7. What is the sign of $\Delta H_{vap}$ for boiling a liquid under typical conditions?

   $\Delta H_{vap}>0$. Vaporization requires energy input, so positive.

8. Which is more exothermic: a reaction with $\Delta H=-50\ \mathrm{kJ}$ or $\Delta H=-10\ \mathrm{kJ}$?

   $\Delta H=-50\ \mathrm{kJ}$. Larger negative value means more heat released.

9. Estimate $\Delta H$ given bonds broken total $800\ \mathrm{kJ}$ and bonds formed total $650\ \mathrm{kJ}$.

   $\Delta H\approx+150\ \mathrm{kJ}$. Using bond formula: $800 - 650 = +150$.

10. Identify the enthalpy change name for converting $1$ mol of a solid to a liquid at its melting point.

    Enthalpy of fusion, $\Delta H_{fus}$. Energy required to convert solid to liquid.

11. What does a thermochemical equation include that a regular balanced equation may omit?

    A stated $\Delta H$ value for the balanced reaction. Includes the enthalpy change alongside the chemical equation.

12. Identify the enthalpy change name for forming $1$ mol of a compound from its elements in standard states.

    Standard enthalpy of formation, $\Delta H_f^\circ$. Enthalpy to form one mole from constituent elements.

13. State the formula relating heat, mass, and temperature change for a substance with specific heat $c$.

    $q=mc\Delta T$. Basic calorimetry equation relating heat to temperature change.

14. Calculate $q$ if $n=2.0\ \mathrm{mol}$ and $\Delta H_{vap}=+40\ \mathrm{kJ\,mol^{-1}}$.

    $q=+80\ \mathrm{kJ}$. Using $q = n\Delta H_{vap} = 2.0 \times 40$.

15. Calculate $\Delta H$ for a target reaction made by adding two steps with $\Delta H_1=-90\ \mathrm{kJ}$ and $\Delta H_2=-10\ \mathrm{kJ}$.

    $\Delta H=-100\ \mathrm{kJ}$. Adding individual step enthalpies: $-90 + (-10) = -100$.

16. What is the enthalpy change name for heating $1$ mol of a substance by $1\ \mathrm{K}$ at constant pressure?

    Molar heat capacity at constant pressure, $C_{p,m}$. Heat capacity measures energy per degree temperature change.

17. What is $\Delta H$ for a reaction that absorbs $18\ \mathrm{kJ}$ of heat at constant pressure?

    $\Delta H=+18\ \mathrm{kJ}$. Heat absorbed means positive enthalpy change.

18. Identify the enthalpy change name for converting $1$ mol of a liquid to a gas at its boiling point.

    Enthalpy of vaporization, $\Delta H_{vap}$. Energy required to convert liquid to gas.

19. Identify the enthalpy change name for dissolving $1$ mol of a solute in a solvent to infinite dilution.

    Enthalpy of solution, $\Delta H_{soln}$. Energy change when dissolving solute completely.

20. Calculate molar $\Delta H$ if $q_p=-45\ \mathrm{kJ}$ for $n=1.5\ \mathrm{mol}$ of reaction as written.

    $\Delta H=-30\ \mathrm{kJ\,mol^{-1}}$. Molar enthalpy equals total heat divided by moles.

21. What is the unit most commonly used for molar enthalpy changes in chemistry, such as $\Delta H$?

    $\mathrm{kJ\,mol^{-1}}$. Standard unit for enthalpy per mole in chemical reactions.

22. What is the enthalpy change name for complete combustion of $1$ mol of a substance in $\mathrm{O_2(g)}$?

    Enthalpy of combustion, $\Delta H_{comb}$. Energy released when substance burns completely.

23. What is the enthalpy change name for breaking $1$ mol of a specific bond in the gas phase?

    Bond enthalpy (bond dissociation enthalpy). Energy required to break one mole of bonds.

24. Calculate $\Delta H_{rxn}$ if $H_{reactants}=120\ \mathrm{kJ}$ and $H_{products}=80\ \mathrm{kJ}$.

    $\Delta H_{rxn}=-40\ \mathrm{kJ}$. Using $\Delta H = H_{products} - H_{reactants} = 80 - 120$.

25. Which process is endothermic: breaking a chemical bond or forming a chemical bond?

    Breaking a bond is endothermic. Breaking bonds requires energy input.

26. Which sign of $\Delta H$ indicates an endothermic reaction process?

    $\Delta H>0$. Positive $\Delta H$ means heat is absorbed from surroundings.

27. Calculate $\Delta H$ for a target reaction made by adding two steps with $\Delta H_1=-20\ \mathrm{kJ}$ and $\Delta H_2=+55\ \mathrm{kJ}$.

    $\Delta H=+35\ \mathrm{kJ}$. Adding individual step enthalpies: $-20 + 55 = +35$.

28. What is the sign of $\Delta H_{fus}$ for melting a solid under typical conditions?

    $\Delta H_{fus}>0$. Melting requires energy input, so positive.

29. Estimate $\Delta H$ given bonds broken total $1200\ \mathrm{kJ}$ and bonds formed total $1500\ \mathrm{kJ}$.

    $\Delta H\approx-300\ \mathrm{kJ}$. Using bond formula: $1200 - 1500 = -300$.

30. Identify the process change that makes $\Delta H$ change sign: reversing, scaling, or balancing only.

    Reversing the reaction changes the sign of $\Delta H$. Only reversing changes the sign, not scaling or balancing.

31. Find $\Delta H$ for the reverse reaction if the forward reaction has $\Delta H=+125\ \mathrm{kJ}$.

    $\Delta H=-125\ \mathrm{kJ}$. Reversing a reaction changes the sign of $\Delta H$.

32. What is the standard temperature commonly assumed for tabulated $\Delta H_f^\circ$ values?

    $298\ \mathrm{K}$. Standard temperature for thermodynamic data tables.

33. Identify the sign of $\Delta H$ if products are at higher enthalpy than reactants on an energy diagram.

    $\Delta H>0$. Higher energy products indicate endothermic reaction.

34. Identify the sign of $\Delta H$ if products are at lower enthalpy than reactants on an energy diagram.

    $\Delta H<0$. Lower energy products indicate exothermic reaction.

35. Which process is exothermic: breaking a chemical bond or forming a chemical bond?

    Forming a bond is exothermic. Forming bonds releases energy.

36. What is the value of $\Delta H_f^\circ$ for any element in its standard state?

    $\Delta H_f^\circ=0$. Elements in standard state have zero formation enthalpy.

37. What is the relationship between $\Delta H$ and heat flow if the system releases heat to the surroundings?

    Heat released means $\Delta H<0$. Releasing heat corresponds to negative enthalpy change.

38. Calculate $\Delta H_{rxn}$ if $H_{reactants}=55\ \mathrm{kJ}$ and $H_{products}=90\ \mathrm{kJ}$.

    $\Delta H_{rxn}=+35\ \mathrm{kJ}$. Using $\Delta H = H_{products} - H_{reactants} = 90 - 55$.

39. What happens to $\Delta H$ when a chemical equation is reversed?

    The sign of $\Delta H$ changes. Reversing makes products become reactants.

40. What is the enthalpy change name for dissolving $1$ mol of an ionic solid into aqueous ions?

    Enthalpy of hydration/solution (context-dependent). Energy change when ionic solid dissolves in water.

41. What is the typical sign of $\Delta H_{comb}$ for a fuel undergoing complete combustion?

    $\Delta H_{comb}<0$. Combustion reactions typically release energy.

42. Which sign of $\Delta H$ indicates an exothermic reaction process?

    $\Delta H<0$. Negative $\Delta H$ means heat is released to surroundings.

43. Find the new $\Delta H$ if a reaction with $\Delta H=-80\ \mathrm{kJ}$ is multiplied by $3$.

    $\Delta H=-240\ \mathrm{kJ}$. Multiplying coefficients multiplies the enthalpy change.

44. Find the new $\Delta H$ if a reaction with $\Delta H=+60\ \mathrm{kJ}$ is multiplied by $\frac{1}{2}$.

    $\Delta H=+30\ \mathrm{kJ}$. Dividing coefficients divides the enthalpy change proportionally.

45. What is the enthalpy change name for forming $1$ mol of gaseous ions from $1$ mol of an ionic solid?

    Lattice enthalpy of dissociation (lattice enthalpy). Energy to separate ionic solid into gaseous ions.

46. State the bond enthalpy estimate formula for reaction enthalpy using bonds broken and formed.

    $\Delta H\approx\sum D(\text{broken})-\sum D(\text{formed})$. Energy to break bonds minus energy released forming bonds.

47. State the formula for $\Delta H_{rxn}^\circ$ using standard enthalpies of formation.

    $\Delta H_{rxn}^\circ=\sum n\Delta H_f^\circ(\text{prod})-\sum n\Delta H_f^\circ(\text{react})$. Products minus reactants, weighted by stoichiometric coefficients.

48. Identify the correct interpretation of $\Delta H=-100\ \mathrm{kJ}$ written under a balanced equation.

    The reaction as written releases $100\ \mathrm{kJ}$ per stoichiometric reaction. Enthalpy refers to the complete balanced equation as written.

49. Using Hess's law, find $\Delta H$ for $A\to C$ if $A\to B$ is $+40\ \mathrm{kJ}$ and $B\to C$ is $-15\ \mathrm{kJ}$.

    $\Delta H_{A\to C}=+25\ \mathrm{kJ}$. Using Hess's law: $40 + (-15) = +25$.

50. State Hess's law in one sentence for reaction enthalpy changes.

    $\Delta H$ depends on initial and final states, not the path. Enthalpy is a state function, path-independent.

51. Calculate $q$ if $n=0.50\ \mathrm{mol}$ and $\Delta H_{fus}=+12\ \mathrm{kJ\,mol^{-1}}$.

    $q=+6.0\ \mathrm{kJ}$. Using $q = n\Delta H_{fus} = 0.50 \times 12$.

52. What is the relationship between $\Delta H$ and heat flow if the system absorbs heat from the surroundings?

    Heat absorbed means $\Delta H>0$. Absorbing heat corresponds to positive enthalpy change.

53. What does the symbol $\circ$ mean in $\Delta H^\circ$?

    Standard-state conditions (typically $1\ \mathrm{bar}$). Standard state means reference conditions for measurements.

54. Calculate $\Delta H_{rxn}^\circ$ given $\sum n\Delta H_f^\circ(\text{prod})=-500\ \mathrm{kJ}$ and $\sum n\Delta H_f^\circ(\text{react})=-200\ \mathrm{kJ}$.

    $\Delta H_{rxn}^\circ=-300\ \mathrm{kJ}$. Products minus reactants: $-500 - (-200) = -300$.

55. What is the definition of the enthalpy change of reaction, $\Delta H_{rxn}$, at constant pressure?

    $\Delta H_{rxn}=H_{products}-H_{reactants}$ at constant $P$. Enthalpy change equals final enthalpy minus initial enthalpy.

56. Using Hess's law, find $\Delta H$ for $A\to C$ if $A\to B$ is $-10\ \mathrm{kJ}$ and $B\to C$ is $-35\ \mathrm{kJ}$.

    $\Delta H_{A\to C}=-45\ \mathrm{kJ}$. Using Hess's law: $(-10) + (-35) = -45$.

57. State the formula for heat absorbed or released during a phase change using molar enthalpy $\Delta H_{phase}$.

    $q=n\Delta H_{phase}$. Heat equals moles times molar enthalpy of phase change.

Study these flashcards