Enthalpy Changes (Delta H) and Definitions
What term is used to describe a reaction that releases heat energy to the surroundings, resulting in a negative Delta H value?
Exothermic
Which component of the equation Q = mcDelta T represents the specific heat capacity?
What is c
State the principle behind Hess's Law.
What is that the total enthalpy change for a reaction is independent of the route taken?
In terms of energy, are the processes of bond breaking and bond making exothermic or endothermic?
What are Bond breaking is Endothermic and Bond making is Exothermic?
State the two standard conditions, pressure and temperature, required for standard enthalpy measurements
What are 100kPa and 298K
A reaction causes the temperature of 50.0g of water to increase by 5.00K. Given c for H2O = 4.18Jg^-1K^-1, calculate the heat change Q in Joules.
What is 1045J
If you are using standard enthalpies of formation, do the arrows in the Hess cycle point up to the reactants/products or down to the elements?
What is Down to the elements?
Calculate the overall Delta H for a reaction where the total energy required to break bonds is +2400 kJ\mol and the total energy released when new bonds form is -2800kJ/mol.
What is Delta H = 2400 + (-2800) = -400 kJ\mol?
Define the term Standard Enthalpy of Formation
What is the enthalpy change when one mole of a compound is formed from its elements in their standard states?
When calculating Delta H from a simple solution calorimetry experiment, what is the key approximation made regarding the mass m and specific heat capacity c?
What is the assumption that the specific heat capacity and density of the solution are the same as pure water?
For a reaction A -> B, given the enthalpy of formation data, write the equation for Delta H_reaction.
What is Delta H = Sum of Enthalpy of formation of products - Sum of Enthalpy of formation of reactants
Explain why the Delta H calculated using mean bond enthalpies is often less accurate than the value calculated using standard enthalpies of formation
What is because mean bond enthalpies are average values taken from a range of different compounds, whereas standard enthalpies are specific to the exact compound and its state?
Write the balanced equation that represents the standard enthalpy of combustion for ethanol.
What is C2H5OH(l)+ 3O2(g) -> 2CO2(g) + 3H2O(l)
A student measures a temperature rise of 7.5C. If the uncertainty in each thermometer reading is +/-0.1C, what is the percentage uncertainty in the temperature change Delta T?
What is percentage uncertainty = 2.67%
Given: C(s) + O2(g) -> CO2(g), Delta H = -394 kJ\mol and CO(g) + 1/2O2(g) -> CO2(g), Delta H = -283kJ\ mol. Calculate Delta H for C(s) + 1/2 O2(g) -> CO(g).
What is -111 kJ/mol
Using the formula Delta H = sum (Bonds Broken) - sum (Bonds Formed), why must you assume all substances are in the gaseous state when using mean bond enthalpies?
What is because mean bond enthalpies do not account for the energy required to change state (e.g., vaporisation or sublimation), which is included in Delta H_f values?
Explain why the Standard Enthalpy of Atomisation for gaseous diatomic elements like H2(g) is half of the bond dissociation enthalpy for the H-H bond.
What is because the Standard enthalpy of atomisation is defined as forming one mole of gaseous atoms, so H2(g) -> 2H(g) yields two moles of atoms, requiring division by two for the one mole definition
Give two reasons why a value for the enthalpy of combustion obtained using a simple spirit burner and water calorimeter is likely to be less exothermic (less negative) than the accepted data book value.
What are Incomplete combustion (less energy released) and significant heat loss to the surroundings (not a truly isolated system)?
When determining the enthalpy of formation of methane (CH4) from C(s) and H2(g), explain why a Hess cycle using enthalpies of combustion is used instead of directly measuring the enthalpy of formation.
What is because the direct reaction of Carbon (s) and Hydrogen (g) to form CH4(g) is too slow to measure accurately (or produces many by-products/is hard to measure, whereas combustion is fast and complete)?
Using the mean bond enthalpies: C-H = 413, Cl-Cl = 243, C-Cl = 346, and H-Cl = 432 (all in kJ\mol). Calculate Delta H for the reaction: CH4(g) + Cl2(g) -> CH3Cl(g) + HCl(g).
What is -122kJ/mol