8 - chemical energetics

Question 1

what does the first law of thermodynamics state?

Answer 1

during a chemical reaction, energy cannot be created or destroyed. however, one form of energy can be converted into another.
two most important energy types for a chemist - chemical and heat

Question 2

what is chemical energy made up of?

Answer 2

kinetic energy, which is a measure of the motion of the particles (atoms, molecules or ions) in a substance
potential energy, which is a measure of how strongly these particles interact with one another (i.e. both attract and repel one another)

Question 3

what is heat energy? (dk if this is worded right)

Answer 3

the portion of potential energy and kinetic energy of a substance that is responsible for the temperature of the substance.
the heat energy of a substance is directly proportional to its absolute temperature (i.e. the temp measured in Kelvin)

Question 4

what is enthalpy?

Answer 4

a measure of the total energy of the system
you cannot directly determine the enthalpy of a system, but you can measure the enthalpy change that takes place during a physical or chemical change
the enthalpy change of a process is the heat energy that is transferred between the system and the surroundings at constant pressure.

Question 5

exothermic

Answer 5

where heat energy is transferred from the system to the surroundings
e.g. freezing water, dissolving sodium hydroxide in water
recognised due to immediate increase in temp

Question 6

endothermic

Answer 6

where heat energy is transferred from the surroundings to the system
e.g. melting ice, evaporating water
often produce a decrease in temp of the reaction mixture
any reaction that has to be continually heated is endothermic

Question 7

what does the system refer to?

Answer 7

refers to the reaction mixture

Question 8

what do the surroundings refer to?

Answer 8

everything outside of the system such as the air in the room

Question 9

standard conditions

Answer 9

100kPa and 298K

Question 10

change in enthalpy equation

Answer 10

changeH = Hproducts - Hreactants

Question 11

enthalpy level diagrams

Answer 11

exo - reactants on top and products below
Hreactants > Hproducts so changeH is -
endo - reactants below and products on top
Hreactants < Hproducts so changeH is +

Question 12

method for determining enthalpy change of combustion of a liquid (copper can experiment thing)

Answer 12

• spirit burner containing the liquid is weighed
• known volume of water is added to copper can
• temp of water measured
• the burner is lit
• mixture is constantly stirred with the thermometer
• when the temp of the water has reached approx 20*C above its initial temp, the flame is extinguished and the burner is immediately reweighed
• final temp is measured

Question 13

Q =

Answer 13

mc^T
make sure to include a sign +/-

Question 14

sources of error made in the experiments

Answer 14

• some of the heat energy produced in burning is transferred to the air and not the air
• some of the ethanol may not burn completely to form carbon dioxide and water. (incomplete combustion would cause soot to form on the bottom of the copper can)
• some of the heat energy produced in burning is transferred to the copper can and not the water
• conditions not standard e.g. water vapour, not liquid water, is produced
• the experiment takes a while and not all of the heat energy transferred from the water to the surroundings is compensated for

Question 15

method for determining enthalpy change of neutralisation (polystyrene cup one)

Answer 15

• wear safety glasses and lab coat
• using a pipette fitted with a safety filler, place 25 cm3 of 1 mol dm-3 acid into an expanded polystyrene cup
• measure temp of acid
• using pipette fitted with a safety filler, place 25 cm3 of the alkali (using dilute sodium hydroxide of a concentration slightly greater than 1 mol dm-3 to make sure all the acid is neutralised0 into a beaker
• measure the temp of the alkali
• add the alkali to the acid, stir with the thermometer and measure the max temp reached

Question 16

errors with neutralisation reaction (polystyrene cup experiment)

Answer 16

• usual uncertainties of measurements involved with the use of the pipette and thermometer
• heat energy transferred to the thermometer and the polystyrene

Question 16

Hess’s Law

Answer 16

states that the enthalpy change of a reaction is independent of the path taken in converting reactants into products, provided initial and final conditions are the same in each case
Hess cycles! down combustion, up formation

Question 17

mean bond enthalpy shorthand

Answer 17

E
e.g. E(C-C) = +347kJ mol-1

Question 18

limitation of bond enthalpy calculations

Answer 18

bond enthalpies in gaseous state when during the reaction the reactants may be in different states