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Grade 12 Chemistry > Unit 3: Energy Changes & Reaction Rates > Flashcards

Unit 3: Energy Changes & Reaction Rates Flashcards

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0
Q

Law of conservation of energy

A

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

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1
Q

Define thermodynamics

A

Study of energy and energy transfer

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2
Q

Define system

A

Part of the universe being studied

Chemistry: chemical particles involved with reaction

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3
Q

Define surroundings

A

Everything else in the universe

Ex. Beaker, water, you

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4
Q

Enthalpy (H)

A

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

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5
Q 
Enthalpy change 
(Delta H)
A

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

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6
Q

Exothermic

A

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

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7
Q

Endothermic

A

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

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8
Q

How is an endothermic reaction communicated through a thermochemical equation?

A

Reactant + energy = products

Energy is a reactant because it is required for the reaction to occur.

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9
Q

How is an exothermic reaction communicated in a thermochemical equation?

A

Reactant + reactant = products + energy

Energy is released so it is a product

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10
Q

Endothermic Enthalpy reaction value

A

Positive delta h because energy is entering the system

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11
Q

Exothermic reactions Enthalpy reaction values

A

Negative delta h because energy is leaving the system

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12
Q

Energy diagrams for endothermic

A

Products on top

Reactants on bottom

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13
Q

Energy diagram for exothermic

A

Reactants on top

Products on bottom

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14
Q

The amount of energy released or absorbed by the system is directly related to…

A

The amount of chemical used or made

Refer to stoichiometry and mole ratios

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15
Q

How do physical changes cause Enthalpy change?

A
  1. 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)

  1. Dissolving a solute
    Depending on the type and number of intermolecular forces dissolving can be either endothermic or exothermic
16
Q

How do nuclear reactions cause Enthalpy change?

A

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

17
Q

Nuclear fission

A

A heavy nucleus splits into a lighter nuclei resulting in vast amounts of energy released

18
Q

Nuclear fusion

A

Two smaller nuclei join together to form a larger nucleus

19
Q

Assumptions in calculations

A
  1. The volume of the water is equal to the volume of the liquid in the calorimeter
  2. The water in the cup is pure so it has a c value of 4.18
  3. The water is the only thing that exchanges energy with the system
  4. That no energy is lost from the calorimeter
20
Q

How to minimize error from assumptions 1&2

A

Only dilute solutions 1.0 M max

21
Q

Minimize error from assumptions 3&4

A

Materials and design if calorimeter

22
Q

Limitations of calorimeters

A
  1. Only do reactions that occur in water
  2. Too much heat will melt cup
  3. Only do reactions that don’t react with the cup

Flame & bomb calorimeters can be used but come with their own limitations

23
Q

Hess’s law

A

Enthalpy change of a physical or chemical process depends only on the Enthalpy of the original reactants and final prods

24
Q

When do we need to use Hess’s law?

A

Can’t have the reaction in the cup calorimeter
Different forms of energy besides heat
Enthalpy change without experiment

25
Q

Hess’s law algebraic method

A

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

26
Q

Enthalpy change of formation method

A

Delta h = sum of(moles x delta h products) - sum of (moles x delta h of reactants)

27
Q

How are reaction rates calculated?

What is instantaneous rate?

A

Ave rate = delta moles/delta time

Is the speed at a particular point in time found by using the slope of the tangent

28
Q

How is the amount of chemical changes determined experimentally?

A
  1. Mass
    Reaction produces a gas m = nxMM
  2. Pressure
    Reaction produces a gas and a pressure increase
    PV= nR(8.314)T
  3. Volume
    Reaction produces a gas can be collected through downward displacement PV= nRT
  4. pH use pH = -log[H3O+]
  5. Electrical conductivity
    Different quantities of dissolved ions measured with voltmeter
  6. Colour
    Distinct colour change can be found using a soectrophmeter to track colour intensity
29
Q

What must occur at the molecular level for a successful reaction?

A
  1. Reactants must collide with each other while moving vibrational, rotational, and translationally in 3 dimension space
  2. Reactants must collide at a particular orientation in order to form bonds in products or they will bounce of each other
  3. Reactants must collide with enough energy to break bonds in reactants(activation energy)
  4. 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
30
Q

Define transition state

A

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

31
Q

How can you speed up or slow down a reaction?

A
  1. Reactant concentration
    Increasing concentration means increasing number of probable collisions
  2. 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
  3. Temperature
    Increasing temp will increase kinetic energy
    For many reactions a 10 degrees increase will double rate
  4. Adding catalyst
    Adding a substance to speed up reaction
    They provide a different reaction mechanism with lower activation energy
32
Q

How does reactant concentration affect rate?

A
  1. Zero order
    The reaction will go at the same speed no matter the concentration
  2. First order
    Proportional increase in concentration and rate
  3. Second order
    The rate increases by the square of the concentration increase
    Ex. Triple concentration the rate increased by 9
33
Q

How do you find the rate constant and exponents for a reaction?

A
  1. Determine rate exponent for reactants where concentration of one reactant changes where the rest stay the same
  2. Choose a trial and sub in the concentrations for the rate law. Solve for K and units
34
Q

Describe the three tests a mechanism must pass in order to be reasonable

A
  1. Steps must algebraically add together to match overall equation
  2. Probable collisions
    Unimolecular(1 reactant decomposing)
    Bimolecular (2 reactants colliding) are probable where trimolecular(3 reactants colliding simultaneously) are not
  3. 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
35
Q

Describe intermediate

A

A chemical made in one step and used in another and don’t show up in overall reaction

36
Q

What is a reaction half life and how can it be calculated?

A

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

Grade 12 Chemistry (5 decks)

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