Thermochemistry

SCH4U Thermochemistry and Rates of Reactions unit

thermochemistry

study of energy changes during physical, chemical, and nuclear changes

energy

ability to do work (J or kJ)

thermal energy

total kinetic and potential energy of substance. can be transferred to surroundings as heat

heat

transfer of thermal energy from one substance to another

system

the chemical process whose heat we’re examining

surroundings

anything outside the system (e.g. water)

this gets measured

3 types of thermodynamic systems

open, closed, isolated

open system

energy and matter (reactants/products) enter and leave freely

closed system

energy enters and leaves freely but matter (reactants/products) does not

example of closed system

glowstick, ice cooler

example of open system

barbeque

isolated system

neither energy nor matter can leave or enter the system. ideal theoretical state

2 types of thermochemical processes

exothermic and endothermic

exothermic reaction

heat released to surroundings from the reactant bonds/IMFs, potential energy converted to kinetic energy

exothermic reaction example

fire, combustion

endothermic reaction

heat absorbed from surroundings (requires energy input), kinetic energy converted to potential energy

\

exothermic

\

endothermic

temperature

average kinetic energy of particles in a substance

calorimetry

measuring energy changes in a chemical system

parts of calorimeter

well-insulated container (ideally isolated system), water, thermometer

general heat equation (q of surroundings)

q = mcΔT

enthalpy

thermal energy in a chemical substance

types of thermochemical kinetic energy

translational, rotational, vibrational

types of thermochemical potential energy

bonds, IMFs

how does the enthalpy change of the system compare to the heat of the system and surroundings?

ΔH of sys = q of sys = -q of surr

molar enthalpy

change in enthalpy per mol

specific heat capacity of water

4\.18 J/(g)(C)

molar enthalpy equation

ΔH per mool = (q of sys) / n (moles)

three ways to show enthalpy changes

potential energy diagrams, adding energy as reactant/product, stating the ΔH separately

who won the 1918 nobel prize in chemistry for his work in ammonia synthesis?

haber

name the fertilizer made by Haber using his namesake proccess

Zyklon A

hess’s law of summation

The change in molar enthalpy is the same regardless of whether the change happens in one step or multiple steps.

reasons to use Hess’s law

1. reaction is too high/low energy
2. reaction is too fast/slow

what is standard enthalpy of formation

energy needed to make one mol of a substance at SATP (25°C, 100 kPa)

what should be noted regarding cancelling out common compounds using Hess’s law?

they must be the same state (i.e. enthalpy of state changes not accounted for)

equation to find enthalpy change using enthalpy of formation

three things required for reactions to happen

1. reactant particles must collide
2. they must collide with enough kinetic energy
3. they must collide with the right orientation

transition state

state of reactants during the reaction where the chemical changes happen

transition state theory

1. bonds are broken and formed in chemical reactions
2. particles need enough kinetic energy to overcome the electrostatic forces between them

activation energy

minimum energy input needed to start reaction

when is the peak of potential energy in a reaction

during the transition state

5 ways to change rate of reactions

1. temperature
2. add a catalyst
3. change reactant concentrations
4. change reactants surface area
5. nature of reactants (reactivity differences in different substances)

how does temperature affect reaction rate

Temperature ∝ kinetic energy of particles ∝ # of collisions with enough energy

and

Temperature ∝ kinetic energy of particles ∝ # of total collisions ∝ # of successful collisions

how do catalysts affect reaction rate

makes reactions faster since it provides reaction with an alternative reaction mechanism that happens occurs with a lower Activation Energy (same amount of energy = more successful collisions = faster reaction)

how does concentration affect reaction rate

Concentration ∝ # of particles ∝ # of collisions ∝ # of successful collisions 

how does reactant surface area affect reaction rate

Surface area ∝ space for collisions ∝ # of collisions ∝ # of successful collisions

unit for reaction rate

mol/L/s or mol/L\*s

what do maxwell-boltzman distributions show

what proportion of molecules will collide with enough energy

describe a maxwell-boltzmnn distribution graph

\# of molecules within a range of kinetic energy, with a set x value for the Activation Energy (particles with more activation energy can collide with enough energy)

describe a maxwell-boltzmann distribution where energy has been increased

the curve is shifted right but the activation energy remains the same (the activation energy line stays in place)

describe a maxwell-boltzmann distribution where a catalyst has been added

the activation energy line is shifted left while the curve stays in place

how do you find the average rate of a reaction?

r = Δ\[\]/Δt

what is the rate law?

r = k(\[A\]^n)(\[B\]^m)

rate symbol

r

properties of constant k

* determined experimentally and unique for each reaction
* units vary based on order (degree) of reaction
* 2nd derivative of reaction curve (mol of reactant vs time for reaction)

what is order (degree) of reaction?

sum of reactant exponents in rate law

how to find rate law?

1. find the reaction rate ratio and conc. ratio in two trials where the conc. of only 1 reactant changed
2. find the exponential relationship between the conc. ratio and reaction rate ratio
3. repeat 1-2 for each reactant (this is the order of the reaction with respect to each reactant)
4. find k by subbing in the values for any trial and isolating k (**remember to include units in the calculations and how they change**)