Buscar
Estás en modo de exploración. debe iniciar sesión para usar MEMORY

   Inicia sesión para empezar

level: Chemical Energy

Questions and Answers List

level questions: Chemical Energy

QuestionAnswer
what is enthalpya measure of the chemical energy in a substance (measured in kJ)
is bond breaking an exothermic or endothermic reactionendothermic
is bond making an exothermic or endothermic reactionexothermic
what is meant by enthalpy changethe difference in energy between the products and reactants in a reaction
do exothermic reactions have a positive or negative enthalpynegative
do endothermic reactions have a positive or negative enthalpypositive
what is meant by enthalpy of combustionthe energy change when one mole of the substance burns completely in oxygen
what must be done when writing chemical equations for enthalpy of combustionthey must be balanced for one mole of the substance that is burning and they must show the correct state symbols
what formula do you use to calculate enthalpy of combustionEh = cmΔT where Eh is the heat energy gained by water in kJ c is specific heat capacity of water (4.18 kJ kg-1˚C-1) m is mass of water in kg ∆T is change in temperature of the water in degrees celcius
describe an experiment used to find the enthalpy of combustion of fuel, such as ethanolset up the following apparatus take the following measurements: mass of water, mass of ethanol + burner before burning, temp of water before burning, mass of ethanol + burner after burning, highest temperature of water after burning use Eh=cm∆T to calculate enthalpy of combustion
is enthalpy of combustion exothermic or endothermicexothermic enthalpy of combustion always has a negative sign
name some sources of error in this experiment of calculating enthalpy of combustionenthalpy of combustion by practical methods in the lab is often much lower than the databook value because of heat loss to surroundings loss of ethanol by evaporation incomplete combustion
name improvements that could be made to this experimentloose fitting lid could have been placed on the copper beaker to reduce heat loss to surroundings insulation of the copper beaker to reduce heat loss to surroundings use of a bomb calorimeter would give a more accurate value for enthalpy combustion
describe how a bomb calorimeter reduces heat loss to surroundingsinsulation and lids are used water surrounds the calorimeter ignition of the fuel is done wihout removing the ish burner from the appartatus the combustion occurs in a closed container reducing any evaporation of ethanol the reaction is carried out in pure oxygen (under pressure) to reduce any incomplete combustion
what is hess' lawstates that the enthalpy change for a chemical reaction is independent of route
how do you use hess's law to calculate enthalpy change of a reactionwrite down the equation for which you want to find the enthalpy write out the equations and values of the enthalpies of combustion of the other reactants in the equation compare these with the target equation and rearrange when necessary (swap equation round and change sign of enthalpy of combustion value) cancel out and calculate enthalpy
what is meant by molar bond enthalpythe energy required to break one mole of a particular bond between a pair of atoms in the gaseous state OR the energy required to break one mole of bonds in a diatomic molecule
what is mean bond enthalpythe energy required to break one mole of bonds that occur in compounds (not diatomic) the mean is used for certain bonds as the energy required differs depending on where the bond is found (position in compound, other atoms around it, etc)
how do you use bond enthalpies to calculate enthalpy change of a reactionfind the different types of bonds broken find the values for bond enthalpies in the data booklet find the different types of bonds made find the different values for bond enthalpies in the data booklet (remember bond breaking requires energy and bond making releases energy) total the enthalpies