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Energetics
AQA A Level Chemistry
| Question | Answer |
|---|---|
| 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
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