40S Unit 1: Chemical Kinetics Flashcards
define chemical kinetics
the study of the rate of reactions and the factors involved
what is a spontaneous reaction?
a reaction that happens without the addition of energy or outside intervention and nothing forces it to occur
define activation energy
the energy needed for a reaction to occur
how can you make a reaction occur?
- lower the activation energy by increasing the temperature
- lower the activation energy by adding a catalyst
what are the factors affecting the spontaneity of a reaction?
entropy and enthalpy
define entropy
the level of chaos/organization in a system
what is the mathematical symbol for entropy
S
define enthalpy
the total energy level of a system
what is the mathematical system for enthalpy
H
define exothermic
the loss of heat energy from a species
define endothermic
the gain/addition of heat energy in a species
which one is spontaneous, exothermic or endothermic and why?
exothermic reactions are spontaneous because they loose heat energy, reducing their enthalpy level and causing them to be spontaneous
how can an endothermic reaction be spontaneous?
endothermic reactions have a higher enthalpy level, so their entropy increase (increase in chaos) has to be higher than their enthalpy increase.
(entropy rise must be greater than enthalpy rise)
in an endothermic reaction and graph, is the change in enthalpy (ΔH) positive or negative?
positive
in an exothermic reaction and graph, is the enthalpy change (ΔH) positive or negative?
negative
state an example of an exothermic reaction
the decomposition of hydrogen peroxide
H2O2(l) → H2O(l) + O2(g)
state an example of an endothermic reaction
the reaction between barium hydroxide and ammonium thiocyanate
Ba(OH)2•8H2O(s) + 2NH4SCN(s) → Ba(SCN)2(s) + 10H2O(l) + 2NH3(g)
what is the importance of Gibbs Free Energy and what is the equation?
Gibbs Free Energy (G) is used to determine if a reaction will be spontaneous or not
ΔG = ΔH - TΔS
if G is negative, the reaction is spontaneous
state 3 ways to measure the rate of reaction
- measuring the change in gas production using an inverted graduated cylinder in water
- measuring the concentration of ion production
- measuring the production of colour using a colourometer
what is reaction rate measured in?
M/s or mol/L/s or mol/L•s
what is the formula for reaction rate?
r = Δconcentration (M or mol/L) ÷ time (s)
on a concentration vs time graph, the magnitude of the curvature if directly related to the ____________ of the species. that means in this reaction 2NO2(g) → 2NO(g) + O2(g), the curvature of O2 is _________ the curvature of NO and NO2
coefficients
half
in reactants does concentration increase or decrease overtime?
decrease
in products does reactants increase or decrease overtime?
increase
how can you find the reaction rate on a concentration vs time graph?
using the slope where m = Δconcentration ÷ Δtime
what are the 2 types of slopes and what is the difference?
tangents and secants
tangents find the instantaneous slope at one point while secants find the average slope between two points
are tangents and slopes always similar? explain
no they are not. for first order reactions the average rate is similar to the instantaneous rate at the halfway point, but for reactions with higher orders, they might be similar over short intervals of time but they differ over longer time intervals.
reactions rates for all species in a chemical reaction are _____________ ________________
stoichiometrically proportional
state the rate law equation
r = k • [A]^x • [B]^y • [C]^z
in the rate law equation, what is ‘k’ measured in?
mol^-n • L^+n / s
n = overall over - 1
what does a zero order mean?
the concentration of a species has no effect on the reaction rate
what does a first order mean?
a change in the concentration of a species causes a proportional change on the reaction rate
what does a second order mean?
a change in the concentration of a species causes a (proportional change)^2 on the reaction rate
what is the overall order of reaction
the sum of all the orders within a given reaction
state the 5 factors affecting the reaction rate and experiments to explain them
- concentration - 18M sulfuric acid + sugar is more dramatic than 4M of sulfuric acid + sugar
- temperature - Alka seltzer lab
- surface area (only for different physical states)- metal rod vs steel wool
- nature of reactants - alkali metals + water gets more reactive down the group
- catalyst - decomposition of H2O2 is faster with MnO2 which produces ‘elephants toothpaste’
what is the formula for proportional change?
porportional Δ = final ÷ initial
how do you find ‘k’ in an experiment?
use Trial 1 values and substitute all known values into rate law equation
how is the rate law related to the stoichiometry of the chemical reaction in question?
the stoichiometric proportions within a reaction are equal to the rat proportions between the same species at the exact same time
