Topic 5: Energetics/Thermochemistry Flashcards
Heat vs Temperature
Heat is a form of energy that flows from something at a high temperature to something at a lower temperature
Temperature is a measure of the average kinetic energy of particles
Define internal energy
Internal energy is the name given the total amount of energy (kinetic and potential) in a sample of a substance.
If a 100g block of iron at 100˚C is placed in contact with a 50g block of iron at 50˚C, heat will flow from the hotter block to the colder one until they are the same temperature. When they are at the same temperature, the 100g block of iron will have higher internal energy simply because there is more of it, but the average kinetic energy of the particles in the two blocks will be the same because they are the same temperature.
What is systems and surrounding in terms of energy flow in chemical reactions?
The ‘system’ is a chemical reaction and the ‘surrounding’ is everything else in the universe
What can chemical reactions be classified as?
Exothermic or endothermic
What happens in an exothermic reaction?
In an exothermic reaction, heat energy is transferred from a system (chemical reaction) to the surrounding – the surrounding gets hotter
Release heat into the environment
What happens in an endothermic reaction?
In an endothermic reaction, a system (chemical reaction) takes in heat energy from the surroundings - the surroundings get cooler
To absorb heat form energy
What is enthalpy change (∆H) and what does it tell us?
Enthalpy changes to heat energy exchange with the surroundings at constant pressure
It tells us when the system moves from an initial state to some final state
What does ∆ mean?
∆ means a change in a quantity
What is ∆H of a system?
The ∆H is basically the amount of heat energy taken in/ given out in a chemical reaction. Enthalpy is given the symbol H and enthalpy change is ∆H
Which is negative, which is positive ∆H? Exothermic or endothermic?
∆H for an exothermic reaction is negative (Because the enthalpy of the products is less than the reactants).
∆H for an endothermic reaction is positive
What does an enthalpy diagram for the combustion of methane look like?
What is the stability of these reactions?
The term stability is usually used to describe the relative energies of reactants and products in a chemical reaction. If the products have less energy than the reactants than they are more stable.
Less energy = More stable
In an exothermic reaction the products are at a lower energy (enthalpy) level than the reactants, and we say that the products are more stable than the reactants
How can the enthalpy change be measured with other enthalpy?
enthalpy change = enthalpy of products - enthalpy of reactants
What does this symbol mean ⦵?
It means ‘under standard condition’
What does standard enthalpy change of reaction mean? ΔHr°?
Standard enthalpy change is the change when molar amounts of reactants react together under standard condition to give products with everything else also in standard condition
What does the standard enthalpy change of combustion mean? ?ΔH°c? Is it positive of negative?
Standard enthalpy change of combustion is the enthalpy change when one mole of a substance is completely burned in oxygen under standard conditions. It is always negative because combustion is always an exothermic process.
What is the pressure at standard condition?
100kPa (1.00 x 10⁵Pa)
What does specific heat capacity mean?
Is the energy required to raise the temperature of 1g/kg of substance by 1˚/1K.
The higher the specific heat capacity the more difficult it is to heat up the substances than substances with low specific heat capacity. It also applies when a substance cools.
What is specific heat capacity represented by?
(c)
What is the formula to calculate heat energy? q? What does each component stand for?
q=mcΔT
q= heat energy
m= mass
c=specific heat capacity
ΔT= change in temperature
How do we use the basic technique calorimetry?
The idea is that the heat given out in a combustion reaction is used to heat another substance of no specific heat capacity such as water.
What is the specific heat capacity of water?
4.18J g-1˚C-1
What are some systematic errors in the experiment using a calorimetry? How can it be improved?
The most major problem is that of heat lost to the surroundings. Of the heat energy released when the ethanol burns, only some of it goes into heating the water – the rest goes into heating the copper can and the surrounding air. There can also be evaporation of the water and alcohol.
The experiment could’ve been improved by determining the specific heat capacity of the can and taking this into account when doing the calculations, insulating the can so that less he is lost through the can to the surrounding, using some sort of draught shield to reduce convection currents around experiment
What is incomplete combustion?
When there is not a sufficient supply of oxygen to the flame, some of the ethanol can burn to produce carbon monoxide and soot as well as water. It gives out less heat than complete combustion. Incomplete combustion causes the flame to be yellow/orange rather than blue because of the presence of soot particles, and the soot can also be seen on the bottom of the cooper can.
What is the enthalpy change of neutralisation ΔHn? Exo or Endo?
It is the enthalpy change when one mole of water molecules are formed when an acid (H+) reacts with an alkali (OH) under standard condition.
It is always exothermic
What is enthalpy change of solution ΔHsol? Exo or Endo?
It is the enthalpy change when one more solute is dissolved in excess solvent to form a solution of infinite dilution under standard condition.
May be exothermic or endothermic
What does infinite dilution mean?
any further dilution of the solution produces no further enthalpy change
i.e the solute particles are assumed not to interact with each other in the solution
Why do we use Hess’s Law?
Using Hess’s Law we can work out unknown enthalpy changes from ones that are known
What does Hess’s Law tell us?
It says that if you convert reactants A into products B, the overall enthalpy change will be exactly the same whether you do it in one step or two steps or however many steps
What will the enthalpy level diagram look like?
What is standard enthalpy change of formation? ΔH˚f?
It’s the enthalpy change when one mole of a substance is formed from its elements in their standard states under standard conditions
Can a standard enthalpy change of formation be exothermic or endothermic?
It can be exo or endo depending on the substance
What does standard state refer to?
to the pure substance at 100kPa and a specified temperature (assume 298K unless another temperature is specified)
What is the enthalpy change of formation for elements in its standard state?
0 zero
What is the formula of enthalpy change of reaction? ΔH˚r?
ΔH˚r = the sum of ΔHf (products) - the sum of ΔHf (reactants)
What do we need to know in order to work out and will be changes for reactions in the gas phase?
It can be worked out if we know the amount of energy required/released when bonds are broken/made
What is bond enthalpy/bond energy? When is bond enthalpies used?
Bond enthalpy is the enthalpy change when one mole of covalent bonds, in a gaseous molecule, is broken under standard condition.
They are used for reactions occurring in the gaseous state
Which process is endothermic and which process is exothermic? What is it in terms of enthalpy change? Bond breaking or bond making?
Bond breaking requires energy (Endothermic): ΔH positive
Bond making releases energy (Exothermic): ΔH negative
Energy must be supplied to break apart two atoms in a molecule against the attractive force holding them together, therefore the process of breaking bonds must be endothermic.
Energy is released when two atoms come together to form a bond, therefore the process of making bonds is exothermic
What is bond enthalpies defined in terms of?
Bond enthalpies are defined in terms of breaking bonds and therefore, have positive values
What type of values are given in a bond energies table?
Average values
Because C2H6 has a slightly different value than C6H6
Due to this bond energy calculations may include inaccuracies
What is average bond enthalpy?
Average bond enthalpy is the average amount of energy required to break one mole of covalent bonds in gaseous molecules under standard conditions.
‘Average’ refers to the fact that the bond enthalpy is different in different molecules and therefore the value quotes is the average amount of energy to break a particular bond in a range of molecules.
Why are values for enthalpy changes of reaction obtained using bond enthalpy less reliable than those calculated using experimental data such as the enthalpy change of combustion or formation?
Because enthalpy changes of formation are specific to the particular substances, whereas bond enthalpies are average values
What phase must it be to use bond enthalpy?
It is important to note that bond energies can only be used to decide whether a reaction in the gas phase is exothermic or endothermic. For reactions involving solids and liquids, other interactions, such as intermolecular forces, must also be taken into account
In an exothermic and endo reaction are the products or reactants more stable?
In an exothermic reaction, the products are at a lower energy than the reactants and are therefore said to be more stable than the reactants.
For endothermic reaction, the products are at higher energy and are less stable than the reactants
When can bond energies only be used in?
to decide whether a reaction in the gas phase is exothermic or endothermic
reactions invovling solids or liquids, other interactions like intermolecular forces must also be taken into account
What can a potential energy diagram (profile) be used to show?
to show the energy changes occuring during a chemical reaction
What is the role of ozone and oxygen in the atmosphere?
protect the surface of the Earth from the damaging effects of ultraviolet radiation
UV radiation is absorbed by the molecues of O2 and/or O3 a they undergo dissociation - breaking the bond between oxygen atoms
Does O2 or O3 absorbed higher energy UV radiation?
O2 molecules
why does O2 molecules absorb higher energy UV radiation then O3?
Because the ball between the oxygen atom and O2 is stronger than that between the oxygen atoms in O3
The UV radiation required to break the bond in O2 (double bond) is in the UV-C part of the spectrum and must have a wavelength shorter than 242nm to provide sufficient energy to break the bond
The bond in ozone is between a single bond and double bond and the absorptio of lower energy, UV-B, radiation will break the bond
