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Rates of Reaction / Energetics & Equilibrium :black_flag:, :green…
Rates of Reaction / Energetics & Equilibrium
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:green_cross:
Energetics
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Exothermic :fire:
Energy from reacting chemicals is transferred to
surroundings
Examples of reaction:
Combustion, Neutralisation, Oxidation
IRL Examples - Hand Warmers
Temp Increase
Endothermic :silhouette:
Energy from the surroundings is transferred into the
reacting chemicals
Temp Decrease
Examples of reaction: Thermal Decomposition, Citric acid + Sodium hydrogen carbonate IRL Example - Sports Injury packs
Energy is
CONSERVED
ALCOHOL COMBUSTION
Measure amount of energy released by measuring temp change in water
The mass of the alcohol and burner is measured before and after combusting the fuel.
The temperature of the water can be measured before and after to find the temperature change.
If we divide the temperature change by the mass of fuel used, we can compare to the mass needed to heat the water up by 1 degree celsius.
Practical considerations
Use draft insulators to minimise heat loss to atmosphere
Heat energy is used to heat glass instead of water
Some water can
evaporate
HEAT ENERGY TRANSFERRED
/
Enthalp Change
Q or E=m×c×ΔT
Q or E = Energy Transferred (J)
m = mass of liquid (g) - water is 4.2 always
c = specific heat capacity (J kg−1 K−1)
ΔT = temperature change
From this we can figure out
Molar Enthalpy Change
Step 1
: Q = m×c×ΔT to work out enthalpy change
Step 2
: Calculate moles using either n = c x v or n = m / mr
Step 3
: ΔH = Q ÷ mol (Divide heat energy transferred by moles)
ΔH = Change in enthalpy between the two reactions
NOTE
: The Q/E (Energy Transferred) must have either a '+' or '-' sign to indicate whether the reaction is endothermic or exothermic respectivley.
REACTION PROFILES
Exothermic Example
Usage of a
Catalyst
THEY ARE NOT USED UP
. They only lower the
activation energy
Contains info on the: Relative energies of reactants and products, Activation Energy, Overall energy Change
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Endothermic Example
Activation Energy:
The
minimum
amount of energy needed for a reaction to take place
Bond Energies
Exothermic
Making bonds releases energy
Overall energy change is < 0
Endothermic
Breaks bonds because it
requires
energy
Overall energy change is > 0
All chemical bonds have a
bond energy
that measures the strength of the bond. By subtracting the total bond energy of the
products
from the total bond energy of the
reactants
we can identify whether the reaction is exo or endothermic overall. (KJ/mol)
EC =
T
r
-
T
p
The reaction is
endothermic
if the energy required to break bonds is
greater
than the energy released by making them.
The reaction is
exothermic
if the energy released by making bonds is
greater
than energy taken in to break bonds.
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Rates of Reaction
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Reaction Practicals
Measuring Gas Production with marble chips
(Surface Area)
Set up equipment as shown in the diagram. Make sure the measuring cylinder is full with water.
Place in a known mass of marble chips. The surface area is the independent variable.
The Rate of Reaction
increases
as the surface area increases (Powdered chips)
Objective
: Observe how the differing surface areas of the marble chips reacting with hydrochloric acid affect the rate of reaction by measuring how much gas is produced.
Observing Colour Change
Conical flask and cross experiment
Objective
: Observe how the concentration affects the rate of reaction by observing the colour changes in the reaction between
sodium thiosulfate
and
hydrochloric acid
Factors affecting rates of reaction
Temperature
Increases energy of collisions (more activation energy). Leads to more frequent collisions and successful
Collision Theory
Particles must collide for chemical reactions to occur. Critically, these collisions must have enough energy.
Concentration
Increasing the number of particles increases the collision frequency.
More particles in a given volume so higher likely hood to collide with others to create successful collisions
Pressure (for gases)
Similar to increase in
concentration
, increases collision frequency
Surface Area
Increasing surface area increases collision frequency (by breaking object into smaller pieces)
Catalysts
Lowers Activation Energy
Alternate energy pathway
Start the timer.
Pour in a known concentration and excess volume of acid.
Place in solutions (except acid) in beakers.
Set up equipment as shown. Make sure the x can be seen on a sheet of paper under the reaction vessel.
Pour in acid with a known concentration (this is the independent variable) and press start on the stop clock.
Stop the clock when the cross cannot be seen. The time taken is the dependent variable.
Repeat for different concentrations of acid.
Plot a graph of time against concentration of acid.
Calculate the gradient to find how the rate changes per increase in concentration.
Draw line of best fit.
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Dynamic Equilibrium
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At
Dynamic Equilibrium
both forward and backward reactions are equal and are happening simultaneously.
Reversible Reactions
A reaction that can go forwards and backwards
La Chatelier's Principle:
States that if any conditions of the reversible reaction is changed, the closed system will adapt to counteract the change by doing the opposite.
Temperature
If the temperature is high (exothermic), the reaction will favour the endothermic direction [Vice Versa]
Concentration
If conc. is high the reaction will favour the side with the lower conc. [Vice Versa]
Concentration is always
constant
Pressure
If the pressure is high (Many particles), the reaction will favour the side with the least particles [Vice Versa]
CATALYSTS DO NOT AFFECT POSITION: ONLY SPEED UP HOW MUCH PRODUCT OR REACTANT
All this is done to counter the current conditions of the reaction
Occurs only in a
closed system
Draw a curve of best fit.
Calculate the gradient to find rate at different times.
Plot a graph of time against volume of gas.
Record volume of gas produced every 5 seconds. This is the dependent variable.
