energetics Flashcards
law of conservation of energy
the overall amount of energy does not change as energy is conserved in reactions- this means energy can’t be created or destroyed, only transferred
exothermic reaction
releases heat energy
endothermic reaction
takes in heat energy
neutralisation reactions always
give energy out
displacement reactions always
take energy in or give it out
combustion reactions always
give energy out
calorimetry is
a technique used to measure changes in enthalpy of chemical reactions
two types of calorimetry experiments-
enthalpy changes of reactions in solution, enthalpy changes of combustion
enthalpy changes of reactions in solution experiment
A fixed volume of one reagent is added to the calorimeter and the initial temperature taken with a thermometer
An excess amount of the second reagent is added and the solution is stirred continuously
The maximum temperature is recorded and the temperature rise calculated
enthalpy changes of combustion experiments
A fixed volume of water is measured and transferred to the copper can
The spirit burner containing, for example, an alcohol, is weighed on a 2 d.p. balance
The initial temperature of the water is measured and recorded
The alcohol is ignited and the water is stirred continuously
When the temperature has risen by about 20 degrees the flame is extinguished and the spirit burner is re-weighed
The final temperature of the water is recorded
sources of error in combustion experiment
Not all the heat produced by the combustion reaction is transferred to the water
Some heat is lost to the surroundings
Some heat is absorbed by the calorimeter
to minimise the heat loss
copper calorimeter should not be placed too far above the flame and a lid placed over the calorimeter, shielding can be used to reduce draughts
in combustion experiment the two main sources of error are
heat losses, incomplete combustion
heat transferred=
mass of water x specific heat capacity x temp change
heat energy change is calculated by
calculating the heat transferred
molar enthalpy change =
heat change for the reaction / number of moles
materials needed to investigate temperature change
Dilute hydrochloric acid Dilute sodium hydroxide solution Styrofoam (polystyrene) calorimeter & lid 25 cm3 measuring cylinder Thermometer & stirrer
method to investigate temperature change
Using a measuring cylinder, place 25 cm3 of the NaOH solution into the calorimeter
Measure and record the temperature of the solution
Add 5 cm3 of the dilute HCl and stir
Measure and record the highest temperature reached by the mixture
Repeat steps 1 – 4 increasing the amount of acid added by 5 cm3 each time
evaluation of investigating temperature change
Plot a graph of the results and draw a line of best fit, using the graph to determine what volume of acid causes the biggest change in temperatureConclusion:
The larger the difference in the temperature the more energy is absorbed or released
