Module 4 - Drivers of reaction Flashcards
Temperature, heat and specific heat capacity
Heat is the total energy possessed by a body of matter due to the movement of all its particles.
Temperature is a measure of how hot an object is and it is related to the average speed of its particles.
Two objects can be at the same temperature but contain very different amounts of heat. (Hot pin vs Red hot horse shoe)
A small container of water at 100°c will contain less heat than a much larger container of water at 50°c
Enthalpy
Every substance has energy within it, every chemical species also has energy stored within its chemical bonds (bond energy) - chemical potential energy.
The sum of the internal energies within a substance is Enthalpy (H) = total internal energy.
Change of enthalpy
We cannot measure enthalpy directly, the internal energy of matter contains components that are unknown and/or not easily accessible. But we can measure the change in enthalpy (ΔH) during a reaction
ΔH = enthalpy of the products - enthalpy of reactants
E.g Textbook pg 326 Q1
Zn +2HCl -> ZnCl2 +H2
MM.Zn = 65.38 gmol
Moles of Zn Burnt = 1.165.38
= 0.017 mol
Heat released by 0.017mol = 2.6 KJ
Heat released per mol = 2.6/0.017
= - 150KJmol (2 sigfig)
Calorimetry
If a reaction occurs in an aqueous solution, the heat energy given out by an exothermic reaction can be assumed to be absorbed totally by the water. Conversely, heat absorbed by an endothermic reaction can be assumed to be provided by the water.
We can measure the heat energy transferred to (exothermic) or away from (endothermic) a body of water.
Step 1: q = m x c x ΔT
Step 2: Q = q in kJ
Step 3: ΔrH=Q/n (KJ mol)
Step4: +ve = Endo -ve = Exo
Specific heat capacity
The specific heat capacity (c) of a substance is the amount of heat required to raise the temperature of a ‘unit’ mass of the substance through 1 Kelvin.
It is measured in joules per gram per Kelvin
Water = 4.18 JgK
Calculating quantities of heat
We can use the concept of specific heat capacity to calculate the quatities of heat that flow from one object or substance to another.
When a sample of a substance undergoes a change in temperature when heat flows away/to, the quantity of heat involved (q) is calculated via: q = mcΔT J
If the temperature of the object or substance has increase, ΔT is positive and therefore q is positive.
If the temperature of the object or substance has decreased, ΔT is negative and therefore q is negative.
Enthalpy of reactions
Thermochemical equations specifically include a sign and a numerical value of the energy charge that occurs in a reaction, typically to the right of the equation.
The energy change is formally called the molar heat (enthalpy) of reactions and is commonly referred to as heat of reaction
It is abbreviated to ΔrH with units kJmol
Activation energy
The activation energy of a reaction (Ea) is the minimum amount of energy reactant molecules must process in order to break apart and move into a transitional phase on their way to from products.
The reason why many reactions do not occur spontaneously but need an ‘energy prod’ to get going is that there is often an energy barrier (activation energy) between reactants and products, therefore have to give the reactant molecules sufficient energy to scale the barrier.
Once exothermic reactions get going, they release sufficient energy to self-sustain
Endothermic reactions have to be supplied with energy continuously
Standard Enthalpy Change
/// THE ° SIGN IS THE STANDARD STATE SYMBOL///
To compare reactions we should use the Standard Enthalpy Change, ΔH°.
For a pure substance, the standard state is the stable form of the substance (s/l/g) at a pressure of 100.0kPa and the specified temperature (usually 25°)
For a substance in solution, the standard state is that substance present at the concentration of exactly 1.000 mol L
For a substance in a gaseous mixture, the standard state is that substance present at a partial pressure (concentration) of 100.0 kPa.
Hess’ Law
the amount of heat involved in producing one chemical from another is always the same, no matter how many stages are taken to obtain the desired product
C(s) + O2(g) -> CO2(g) ΔH° = -393 kJ mol
2C(s) + 2O2(g) -> 2CO2(g) ΔH° = (2 x -393) = -786 kJ mol
Calculating Entropy (S)
We can calculate entropy, unlike enthalpy, and subsequently changes in enthalpy (ΔS).
Entropy is calculated as the amount/distribution of molecular energies at a particular absolute temperature, thus its units are Joules per Kelvin per mole (JKmol).
Standard Molar Entropy (S°): the entropy of one mole of a substance in its standard state (usually 298 K and 100.0 kPa).
Chemicals in a chemical system have a tendancy to move from a state of lower entropy to one of higher entropy
Generalisations in predicting qualitative changes in entropy
- As the temperature of a substance increases, its entropy increases.
- A substance in its liquid state has more entropy than the same amount of a substance in a solid state.
- A substance in a gaseous state has more entropy than the same a
