Energetics Flashcards

Energetics

A-Level AQA Chemistry

This flashcard pack provides a comprehensive overview of thermodynamics and enthalpy changes, detailing concepts like exothermic and endothermic reactions, standard enthalpy changes of formation and combustion, calorimetry, Hess's law, mean bond energies, and calculations involving enthalpy changes for various chemical reactions.

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Define mean bond energy.	Mean bond energy is the average energy required to break a covalent bond into gaseous atoms, averaged over different molecules.
What are Hess's law cycles used for?	Hess's law cycles are used to measure the enthalpy change for a reaction that cannot be measured directly by experiments. Instead, alternative reactions are carried out that can be measured experimentally.
How is Hess's law used to determine the enthalpy change for forming a hydrated salt from an anhydrous salt?	Hess's law is used to work out the enthalpy change to form a hydrated salt from an anhydrous salt by using a series of reactions and their corresponding enthalpy changes.
Describe Hess's law.	Hess's law is a principle in thermodynamics that states that the enthalpy change of a reaction is independent of the pathway taken to reach the final products, as long as the initial and final conditions are the same.
What is the formula for calculating the enthalpy change of a reaction using Hess's law?	The formula is AH reaction = H products - H reactants.
Define enthalpy of formation.	Enthalpy of formation is the enthalpy change that occurs when one mole of a compound is formed from its constituent elements in their standard states.
How can you calculate the enthalpy change of a reaction using enthalpy of formation values?	You can calculate it by subtracting the sum of the enthalpies of formation of the reactants from the sum of the enthalpies of formation of the products.
Describe the process of using Hess's law to determine enthalpy changes from enthalpy changes of combustion.	To determine enthalpy changes from enthalpy changes of combustion, you need to use the enthalpy changes of combustion of the reactants and products and apply the principle of Hess's law to calculate the enthalpy change of the reaction.
Describe the enthalpy of formation of propene.	The enthalpy of formation of propene is the energy change when one mole of propene is formed from its constituent elements in their standard states.
How are mean bond energies used in calculations?	Mean bond energies are used to estimate the enthalpy change in reactions by subtracting the sum of bond energies broken from the sum of bond energies made.
Describe the difference between an exothermic and endothermic reaction.	In an exothermic reaction, the energy released by forming new bonds is greater than the energy required to break existing bonds. In an endothermic reaction, the energy required to break existing bonds is greater than the energy released by forming new bonds.
Describe how to calculate the enthalpy of combustion of propene using bond enthalpy data.	The enthalpy of combustion of propene can be calculated by subtracting the sum of bond energies made from the sum of bond energies broken.
How can the enthalpy of formation of NH3 be calculated using bond enthalpy data?	The enthalpy of formation of NH3 can be calculated by subtracting the sum of bond energies made from the sum of bond energies broken.
Describe the process of calculating the enthalpy of a compound using bond enthalpies and other data.	To calculate the enthalpy of a compound, bond enthalpies and other data such as atomisation energies or standard enthalpies of formation can be used. The bond energies of the bonds broken are subtracted from the bond energies of the bonds formed.
Describe how to calculate the enthalpy of propane, C3H8(g), using bond enthalpy data.	To calculate the enthalpy of propane, the sum of the bond energies of the bonds formed is subtracted from the sum of the bond energies of the bonds broken.
Describe the trend in enthalpies of combustion for successive members of a homologous series.	There is a constant rise in the size of the enthalpies of combustion as the number of carbon atoms increases.
What type of bonds are broken during the combustion of alkanes or alcohols?	C-C, C-H, and O=O bonds are broken.
How many C=O and O-H bonds are made during the combustion of ethanol?	4 C=O and 6 O-H bonds are made.
Define incomplete combustion.	It is a type of combustion where there is not enough oxygen present to completely oxidize the fuel, resulting in less energy being released and the formation of products like carbon monoxide (CO) or soot.
What assumption is made about the heat capacity of water in reactions where the reactants are dissolved in water?	The assumption is that the heat capacity is the same as pure water.
Describe an exothermic change.	In an exothermic change, energy is transferred from the system (chemicals) to the surroundings.
What is the standard enthalpy change of formation?	The standard enthalpy change of formation of a compound is the enthalpy change when 1 mole of the compound is formed from its elements under standard conditions (298K and 100kPa), all reactants and products being in their standard states.
Describe the standard enthalpy change of combustion.	The standard enthalpy of combustion of a substance is defined as the enthalpy change that occurs when one mole of a substance is combusted completely in oxygen under standard conditions (298K and 100kPa), all reactants and products being in their standard states.
What is the equation used to calculate the energy change for a reaction in solution?	energy change = mass of solution X heat capacity X temperature change
Describe the general method for measuring the enthalpy change for a reaction experimentally.	Mix substances in an insulated container, measure temperature rise
What are some errors in the calorimetric method of measuring enthalpy change?	Energy transfer from surroundings, approximation in specific heat capacity of solution, neglecting specific heat capacity of calorimeter
How can the temperature curve be extrapolated back to the time the reactants were added together?	By taking readings at regular time intervals and extrapolating the temperature curve/line
What is the purpose of using an insulated cup in the calorimetric method?	To minimize heat loss to the surroundings
Describe the general method for calculating the enthalpy change of reaction from experimental data.	Using the equation Q = mcΔT, calculate the energy change for the quantities used. Then, work out the moles of the reactants used and divide Q by the number of moles of the reactant not in excess to give AH. Finally, add a sign and unit to represent the energy change.
Define enthalpy change.	Enthalpy change is the amount of heat energy taken in or given out during any change in a system provided the pressure is constant.
How do you calculate the energy change for the amount of reactants in the calorimeter?	Multiply the mass of the solution by the specific heat capacity of water and the change in temperature (Q = mcΔT).
Define enthalpy change of reaction.	Enthalpy change of reaction is the energy change per mole of a reaction, often represented as AH.
Describe how to determine the enthalpy change per mole of a reaction.	Divide the energy change (Q) by the number of moles of the reactant not in excess (AH = Q/no of moles).
Describe how enthalpies of combustion can be calculated using calorimetry.	Enthalpies of combustion can be calculated using calorimetry by burning the fuel and using the flame to heat up water in a metal cup.
What is the purpose of using calorimetry in measuring enthalpies of combustion?	The purpose of using calorimetry is to measure the energy change produced by the combustion of a fuel.
Define enthalpy change of combustion.	Enthalpy change of combustion is the energy change that occurs when one mole of a substance is completely combusted.
How can the energy change used to heat up water be calculated in calorimetry?	The energy change used to heat up water can be calculated by multiplying the mass of water, the specific heat capacity of water, and the change in temperature.
Describe the steps involved in calculating the enthalpy change of combustion using calorimetry.	The steps involved are: 1) Calculate the energy change used to heat up the water. 2) Calculate the number of moles of alcohol combusted. 3) Calculate the enthalpy change per mole. 4) Add in the sign to represent the energy change.

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