enthalpy changes and calorimetry

not including equations

define energetics:

energy changes during chemical reactions

what is an exothermic reaction? describe the diagram for an exothermic reaction

a reaction in which heat energy is given off

what is an endothermic reaction? describe the diagram for an endothermic reaction and state whether ΔH is +ve or -ve:

a reaction in which heat energy is taken in

what is enthalpy? what symbol is it represented by?

• a measure of the heat content of a substance

• represented by the symbol H

what are the units for enthalpy?

kJ mol^-1

what is enthalpy change? what symbol(s) is it represented by?

• heat energy change at constant pressure

• represented by ΔH

what are standard conditions for a reaction?

• 100 kPa

• stated temp (usually 298K)

what is the standard enthalpy change of formation of a compound? how is it represented?

enthalpy change which occurs when:

• 1 mole of the compound is formed from its elements

• under standard conditions and w/ everything in its standard state

• represented by ∆_f H ^Ɵ

what is the standard enthalpy change of combustion of a compound? how is it represented?

• the enthalpy change which occurs when ONE MOLE of the compound is BURNED COMPLETELY in oxygen under STANDARD CONDITIONS and w/ everything in its STANDARD STATE

• represented by ΔH^Ɵc

what is the specific heat capacity equation?

q = mcΔT

what is the equation for ΔH?

ΔH = q/moles

what is calorimetry?

the process of measuring the amount of heat energy released/absorbed during a chemical reaction

suggest why the enthalpy change for the thermal decomposition of solid silver nitrate (III) is difficult to determine experimentally (1)

the silver nitrate must be heated so we cannot measure the temp change caused by the reaction

the table in the image shows values, obtained by different methods, for the enthalpy of combustion of a liquid hydrocarbon - suggest reasons for the differences between the values obtained by each of Methods 2 and 3, and the value obtained by Method 1 in the table (5)

any 5 from:

• value from calorimetry less exo than method 1

• because of heat loss

• because of incomplete combustion

• because some liquid hydrocarbon could have evaporated

• value from bond enthalpies less exo than method 1

• mean bond enthalpies values use enthalpies taken across a range of compounds

• value from bond enthalpy data ignores E changes in vaporisation of the fuel or condensing the water

Table 1 shows the enthalpies of combustion of some alcohols.

the enthalpy of combustion calculated from calorimetry is -2060 kJ mol^-1 - explain how this suggests the alcohol is butan-1-ol (2)

• value measured not accurate due to heat loss/incomplete combustion

• result from experiment must be less exo than true value ∴ cannot be propan-1-ol/others