test: Flashcards
(31 cards)
chemical energy:
kinetic energy + potential energy
-chemical energy is made up of two components:
1. kinetic energy: the motion of the particles (atoms, molecules, ions, electrons…) in a substance
2. potential energy: the energy stored by particles because of their position relative to each other, for example
-if two particles that attract each other are close, they have low potential energy
-if two particles that repel each other are close, they have high potential energy (that they’ll use to get far apart, turning it into kinetic energy)
heat energy:
the chemical energy responsible for temperature
-not all the chemical energy in a substance
-for example, a sample of molten iron releases light, which means some energy is lost as light energy
-therefore, heat energy is the portion of chemical energy that is responsible for temperature
system and surroundings:
two chemical concepts to keep in mind in energetics
-when considering s chemicsl reaction, you need to consider too entities:
1. the system is the reaction mixture
2. the surroundings is everything else
enthalpy (H):
the total energy of a system
-enthalpy is the total energy of a system
-it is represented with the symbol H, and measured in joules (J), although the unit kilojoule (kJ) is usually more appropiate
-we cannot measure the total enthalpy change in a reaction
enthalpy change (ΔH):
the heat energy released or absorbed in a reaction
-evert reaction will either release heat or absorb heat
-this enthalpy change is measured at constant volume and pressure
-since this heat depends on the amount of reactants or products (for example, not the same amount of heat is released when burning a stick than when a house burns), we measure enthalpy change relative to this amount
-therefore, enthalpy change is measured in kJ/mol
exothermic and endothermic:
reactions can be of these two types
-when a reaction releases heat to the surroundings, we call it exothermic, for example: combustion is always an exothermic process as it releases enthalpy
-when a reaction asborbs heat from the surroundings, we call it endothermic, for example: melting is always endothermic process as it needs to absorb heat
exothermic reactions:
-since exothermic reactions lose heat from the system to the environment, we indicate that the enthalpy is negative (ΔH<0)
-the temperature rises after the reaction
-reaction between hydrochloric acid and sodium hydroxide:
Hcl (aq) + NaOH (aq) -> NaCl (aq) + H2O (l)
ΔH = -57.1 kJmol^-1 (negative, as it’s exothermic)
endothermic reactions:
-since endothermic reactions absorb heat from the environemtn to the system, we indicate that the enthalpy change is positive (ΔH > 0)
-the temperature decreases right after the reaction
-reaction of thermal decomposition:
ΔH = +178 kJmol^-1 (positive, as it’s endothermic)
enthalpy and conditions:
enthalpy depends on conditions
-the enthalpy change of a reaction depends on the conditions around it:
-temperature: although it affects lightly, the enthalpy change of some reactions are affected by temperature changes as bonds are weakened or strengthened by temperature
-pressure: reactions involving gases are affected by changes in pressure
-amount of reactant: the enthalpy change of a reaction is directly proportional to the amount of reactant
-therefore, when a given enthalpy change of a reaction is given, the conditions must be stated
standard conditions:
298K, 101kPa
-conditions affect enthalpy changes
-therefore, we use standard conditions: 298K and 100kPa
-when an enthalpy change us given under standard conditions, we use the symbol Ꝋ: ΔH Ꝋ means standard enthalpy change
standard enthalpy change of reaction:
ΔfHꝊ
-the enthalpy change when a reaction happens in standard state (100kPa, usually 298K), happens completely and with an amount of reactants equal to the molar quantities, in its simplest whole numbers
-it happens in standard state (100kPa, usually 298K)
-it happens completely (all the reactants react, so percentage yield = 100%)
-the amount of reactants is equal to the molar quantities, in its simplest whole numbers
enthalpy level diagrams:
-enthalpy changes can be represented using a type of graph called enthalpy level diagrams
-they y axis repsents the enthalpy, H, of the species in the reaction mixture
-the x-axis represents time, t, however, its called ‘progress of reaction’
-in exothermic reactions Hproducts < Hreactants
-in endothermic reactions, Hproducts > Hreactants
standard enthalpy change of combustion:
-combustion reactions are always exothermic
-ΔcHꝊ
-the enthalpy change under standard conditions (100kPa and usually 298K) when one mole of a substance burns completely in oxygen under standard conditions
-an incomplete combustion will never have a standard enthalpy change of combustion
determining standard enthalpy change of combustion:
-standard enthalpy changes of combustion can be determines experimentally
-the amount of heat released in a reaction (or absorbed) is directly proportional to the change in temperature
-this technique is called calorimetry
calorimetry:
-in this procedure, a known mass of the liquid is burnt using a spirit burner
-the heat released in the flame is used to heat up the water while it’s continuously stirred with the thermometer
-the temperature of the water is measured, stopping the experiment when it increases by 20ºC
-the mass of the liquid is remeasured, to see how much has burnt
-this technique allows us to calculate the temperature rise and the amount of liquid burnt
-we will use a mathematical relationship to calculate the amount of heat released
-after that, we will calculate the standard enthalpy change of combustion by dividing the heat by the number of moles of liquid burnt
Calorimetry: equations
Q = mcΔT (standard enthalpy change)
ΔH°c = Q/n (standard enthalpy change of combustion)
Specific heat capacity:
-the energy needed to heat up a substance
-not all substances heat at the same rate, water takes longer to heat up than oil
-this is because water needs more energy to increase its temperature
-this is represented by the specific heat capacity
-which is the thermal energy needed to give to a kilogram (or gram) of a substance in order to increase its temperature by 1 degree
-water’s specific heat capacity is 4.18 J
Calc 1:
mira solved example
Sources of error in calorimetry of combustion:
-some of the heat is used to heat up the air and the copper can, not only water
-some incomplete combustion of ethanol may have taken place, which leads to a lower value of enthalpy change calculated
-conditions may no be standard
-the experiment takes a long time, so water is able to give too much heat to the surroundings
Standard enthalpy change of neutralisation:
-the enthalpy change measured under standard conditions when one mole of water is produced by the neutralisation of an acid with an alkali
-the amount to which this value of enthalpy change it relative is water
Many neutralisation reactions are essentially the same:
H+ + OH- -> H2O
-if we compare the standard enthalpy changes of neutralisation, we’ll see that some of them are extremely similar, despite having different reactants
-when adding HCl and NaOH to water, these are the species present: H+, Cl-, Na+, OH-
-when adding HBr and KOH, these are the species: H+, Br-, K+, OH-
-on both cases, it’s only the H+ ion and the OH- ion that react in the following reaction:
H+ + OH- -> H2O
Another calorimetry:
—standard enthalpy changes of neutralisation can be determined experimentally with calorimetry
-the equipment used is simpler because we don’t depend on flame heating the water as in combustion reactions, on this case, the solution containing the acid and alkali is the one that heats up (or down)
-we’ll calculate ΔNHº using again the formula Q = mcΔT
Sources of error in calorimetry for neutralisation:
-some of the heat is used up to heat up the cup and the thermometer, not only the solution
-some error and uncertainty is introduced by using a pipette and the thermometer
-we’ll assume the specific heat capacity of the solution to be equal to the one for water (4.18 J), but that shouldn’t be the case as it’s not pure water
Standard enthalpy change of formation:
-it is the enthalpy change measured under standards conditions (100kPa and usually 298K) when one mole of a compound is produced from its elements in their standard state
-sometimes the reaction does not occur naturally, e.g: glucose
-1 mol of the substance has to be formed
-the substance formed and the elements forming it must be in their standard state
-C has to be solid and forming graphite
-H, O, N, F, Cl, Br and I have to be gaseous diatomic molecules: H2, O2
-metals have to be solid
-formation of CO2 is both a formation and a combustion reaction (because its the combustion of graphite)
C(s,graphite) + O2(g) -> CO2(g)
-enthalpy change of combustion of graphite = enthalpy change of formation
