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Energetics

AQA A Level Chemistry

QuestionAnswer
What is enthalpy? A measure of the heat energy of a substance
What is enthalpy change ΔH? The change in heat energy of a substance at constant pressure
What is the unit of ΔH? KJmol-1
What is ΔHθ? The change in heat energy under standard conditions - 100kPa + 298K
Are breaking bonds exo/endothermic? Explain your answer Endo - energy is taken in to break the bonds
Are making bonds exo/endothermic? Explain your answer Exo- energy is given out when bonds are made
Draw an energy level diagram for an exothermic reaction Draw
Draw an energy level diagram for an endothermic reaction Draw
How do you calculate the overall energy change of a reaction? Energy released when bonds are made in the products- energy needed to break bonds in the reactants
In an exothermic reaction is the enthalpy change positive or negative? negative
Why is the enthalpy change in an exothermic reaction negative? The products have less energy than the reactants because energy is lost to the surroundings
In an endothermic reaction in the enthalpy change positive or negative? positive
Why is the enthalpy change in an endothermic reaction positive? The products have more energy than the reactants because energy is taken in from the surroundings
What is the standard enthalpy of formation ∆Hf? The enthalpy change when one mole of a compound is formed from its elements in their standard states under standard conditions
Write an equation to show the standard enthalpy of formation of ethanol 2C(s) + 3H2(g) + 1/2 O2(g) → C2H5OH(l)
What is the standard enthalpy of combustion ∆Hc? The enthalpy change when one mole of a compound is completely burned in oxygen in standard conditions
Write an equation to show the standard enthalpy of combustion of ethene. C2H4(g) + 3O2(g) → 2CO2(g) + 2H2O(l)
What is the mean bond enthalpy (bond dissociation enthalpy)? The average energy needed to break a certain type of bonds in a range of compounds
Why are bond enthalpies always positive? energy is required to break bonds - endothermic
Why might the bond enthalpy calculated using Hess's law be different to the mean bond enthalpy from a data book? mean bond enthalpies aren't exact, they are averaged over a range of compounds
How can you use bond enthalpies tocalculate the enthalpy change for a reaction? (∑ bond enthalpies of products)-(∑ bond enthalpies of reactants)
What is the specific heat energy of a substance © The amount of energy needed to raise the temperature 1g of substance by 1K
What is the unit of specific heat capacity Jg-1K-1
Describe the experiment you would do to calculate the enthapy change of a combustion reaction? Calorimetry-burn a known amount of reactant and record the ∆T of known mass of water
What is the equation you can use to calculate enthalpy change? q=mc∆T
Describe each of the components in the q=mc∆T with units q = heat lost/gained in J, m-mass of solution in g/water in calorimetry, ∆T change in temperature of the water Kelvin(K), c=specific heat capacity of solution/water JK-1K-1
Describe the experiment you would do to calculate the enthalpy change of an exo/endo reaction carry out the reaction in a polystyrene cup, measure the temperature every minute starting before mixing, plot time vs temp on a graph to obtain ∆T on mixing by extrapolating
Why is extrapolation used to find an accurate ∆T for an exo/endo reaction? to allow for a heat loss from the polystyrene cup
Suggest why the experimental ∆Hc of ethanol is lower than the data book value Some heat is lost to the surroundings and is not all transferred to the water
Suggest what could be done to reduce the heat loss in a calorimetry experiment use a heat shield to prevent the heat lost from the burning substance
What is Hess's law? The overall enthalpy change for a reaction is the same independent of the route taken
Draw a thermochemical cycle that you would draw to find out ∆H for the following reaction if given ∆Hf data, C2H2 + H2 → C2H6 Draw
Draw a thermochemical cycle that you would draw to find out ∆H for the following reaction if given ∆Hc data, C2H2 + H2 → C2H6 Draw
What is the ∆Hf of O2 and why? Zero, because the ∆Hf of elements in their standard states are zero
Created by: JBearfield