Unit 3: Energy Changes & Reaction Rates Flashcards
Law of conservation of energy
Energy cannot be created or destroyed
Energy in the universe is constant
Energy can only be transferred from one object to another or transformed into a different type of energy
Define thermodynamics
Study of energy and energy transfer
Define system
Part of the universe being studied
Chemistry: chemical particles involved with reaction
Define surroundings
Everything else in the universe
Ex. Beaker, water, you
Enthalpy (H)
Total internal energy of a substance at a constant pressure
Measured in KJ or KJ/mol
Takes into account all potential/kinetic energy the particles undergo in a system
Enthalpy change (Delta H)
Undergoing a physical change or reaction
Occurs when the type and # of bonds in your chemical system change
Whenever a new bond is formed ( including IMF) energy is released from the system creating more stability
Energy required to break bonds and energy is absorbed from surroundings
Exothermic
Amount of energy released forming bonds in products is greater than the energy required to break the bonds in reactants
Energy is released into surroundings
Energy absorbed < energy released
Endothermic
Amount of energy required to break the bonds in the reactants is greater than the nervy released forming the bonds in the products
Energy is absorbed by the system
Energy absorbed > energy released
How is an endothermic reaction communicated through a thermochemical equation?
Reactant + energy = products
Energy is a reactant because it is required for the reaction to occur.
How is an exothermic reaction communicated in a thermochemical equation?
Reactant + reactant = products + energy
Energy is released so it is a product
Endothermic Enthalpy reaction value
Positive delta h because energy is entering the system
Exothermic reactions Enthalpy reaction values
Negative delta h because energy is leaving the system
Energy diagrams for endothermic
Products on top
Reactants on bottom
Energy diagram for exothermic
Reactants on top
Products on bottom
The amount of energy released or absorbed by the system is directly related to…
The amount of chemical used or made
Refer to stoichiometry and mole ratios
How do physical changes cause Enthalpy change?
- Changing states - involves breaking the intermolecular forces between particles going from solid to liquid to gas (endothermic)
Or breaking the intermolecular forces between particles to go from gas to liquid to solid (exothermic)
- Dissolving a solute
Depending on the type and number of intermolecular forces dissolving can be either endothermic or exothermic
How do nuclear reactions cause Enthalpy change?
Elements transform into one or more different elements
New nuclei creates extremely strong forces called nuclear bonds between neutrons and protons
Forming of this attraction causes a huge release in energy
Energy is made from converting some mass of protons and neutrons to pure energy
Nuclear fission
A heavy nucleus splits into a lighter nuclei resulting in vast amounts of energy released
Nuclear fusion
Two smaller nuclei join together to form a larger nucleus
Assumptions in calculations
- The volume of the water is equal to the volume of the liquid in the calorimeter
- The water in the cup is pure so it has a c value of 4.18
- The water is the only thing that exchanges energy with the system
- That no energy is lost from the calorimeter
How to minimize error from assumptions 1&2
Only dilute solutions 1.0 M max
Minimize error from assumptions 3&4
Materials and design if calorimeter
Limitations of calorimeters
- Only do reactions that occur in water
- Too much heat will melt cup
- Only do reactions that don’t react with the cup
Flame & bomb calorimeters can be used but come with their own limitations
Hess’s law
Enthalpy change of a physical or chemical process depends only on the Enthalpy of the original reactants and final prods
When do we need to use Hess’s law?
Can’t have the reaction in the cup calorimeter
Different forms of energy besides heat
Enthalpy change without experiment
Hess’s law algebraic method
Combine chemical reactions to match target equation
1. Rewrite equations so reactants and products are on proper side. Reverse sign
2. Multiply equations by factor to equal amounts and multiply delta h by same factor
3. Add the equations if same chemical appears on opposite sides if ardor subtract amounts
If on same side, add amounts
4. Sum of chemical equation equals target and delta h should equal the Enthalpy change of target
Enthalpy change of formation method
Delta h = sum of(moles x delta h products) - sum of (moles x delta h of reactants)
How are reaction rates calculated?
What is instantaneous rate?
Ave rate = delta moles/delta time
Is the speed at a particular point in time found by using the slope of the tangent
How is the amount of chemical changes determined experimentally?
- Mass
Reaction produces a gas m = nxMM - Pressure
Reaction produces a gas and a pressure increase
PV= nR(8.314)T - Volume
Reaction produces a gas can be collected through downward displacement PV= nRT - pH use pH = -log[H3O+]
- Electrical conductivity
Different quantities of dissolved ions measured with voltmeter - Colour
Distinct colour change can be found using a soectrophmeter to track colour intensity
What must occur at the molecular level for a successful reaction?
- Reactants must collide with each other while moving vibrational, rotational, and translationally in 3 dimension space
- Reactants must collide at a particular orientation in order to form bonds in products or they will bounce of each other
- Reactants must collide with enough energy to break bonds in reactants(activation energy)
- If conditions 1-3 are met, an unstable chemical with partial bonds called activated complex which has a 50% chance of falling back into reactants or products
Define transition state
The point on the graph where the system has the most potential energy
This where the activated complex exists and the reactants transition into products
How can you speed up or slow down a reaction?
- Reactant concentration
Increasing concentration means increasing number of probable collisions - Reactant surface area
Adding solid reactants as powder increases the number of particles able to apply
Adding solid chunks slows down reactions as the inside of chunk is harder to react away - Temperature
Increasing temp will increase kinetic energy
For many reactions a 10 degrees increase will double rate - Adding catalyst
Adding a substance to speed up reaction
They provide a different reaction mechanism with lower activation energy
How does reactant concentration affect rate?
- Zero order
The reaction will go at the same speed no matter the concentration - First order
Proportional increase in concentration and rate - Second order
The rate increases by the square of the concentration increase
Ex. Triple concentration the rate increased by 9
How do you find the rate constant and exponents for a reaction?
- Determine rate exponent for reactants where concentration of one reactant changes where the rest stay the same
- Choose a trial and sub in the concentrations for the rate law. Solve for K and units
Describe the three tests a mechanism must pass in order to be reasonable
- Steps must algebraically add together to match overall equation
- Probable collisions
Unimolecular(1 reactant decomposing)
Bimolecular (2 reactants colliding) are probable where trimolecular(3 reactants colliding simultaneously) are not - The mechanism must support determined rate law
Overall reaction can only go as fast as slowest step. Sub coefficients of balanced equation in as rate exponents
Describe intermediate
A chemical made in one step and used in another and don’t show up in overall reaction
What is a reaction half life and how can it be calculated?
Time needed for the amount of chemical to decrease by half
t1/2= 0.693/k first order
How many half lifes need to occur in order x how long a half life takes
How much reactant us left at a particular time or when considered completed
