Unit 3: Energy Changes & Reaction Rates

Question 0

Law of conservation of energy

Answer 0

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

Question 1

Define thermodynamics

Answer 1

Study of energy and energy transfer

Question 2

Define system

Answer 2

Part of the universe being studied

Chemistry: chemical particles involved with reaction

Question 3

Define surroundings

Answer 3

Everything else in the universe

Ex. Beaker, water, you

Question 4

Enthalpy (H)

Answer 4

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

Question 5

Enthalpy change 
(Delta H)

Answer 5

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

Question 6

Exothermic

Answer 6

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

Question 7

Endothermic

Answer 7

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

Question 8

How is an endothermic reaction communicated through a thermochemical equation?

Answer 8

Reactant + energy = products

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

Question 9

How is an exothermic reaction communicated in a thermochemical equation?

Answer 9

Reactant + reactant = products + energy

Energy is released so it is a product

Question 10

Endothermic Enthalpy reaction value

Answer 10

Positive delta h because energy is entering the system

Question 11

Exothermic reactions Enthalpy reaction values

Answer 11

Negative delta h because energy is leaving the system

Question 12

Energy diagrams for endothermic

Answer 12

Products on top

Reactants on bottom

Question 13

Energy diagram for exothermic

Answer 13

Reactants on top

Products on bottom

Question 14

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

Answer 14

The amount of chemical used or made

Refer to stoichiometry and mole ratios

Question 15

How do physical changes cause Enthalpy change?

Answer 15

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)

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

Question 16

How do nuclear reactions cause Enthalpy change?

Answer 16

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

Question 17

Nuclear fission

Answer 17

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

Question 18

Nuclear fusion

Answer 18

Two smaller nuclei join together to form a larger nucleus

Question 19

Assumptions in calculations

Answer 19

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

Question 20

How to minimize error from assumptions 1&2

Answer 20

Only dilute solutions 1.0 M max

Question 21

Minimize error from assumptions 3&4

Answer 21

Materials and design if calorimeter

Question 22

Limitations of calorimeters

Answer 22

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

Question 23

Hess’s law

Answer 23

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

Question 24

When do we need to use Hess's law?

Answer 24

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

Question 25

Hess's law algebraic method

Answer 25

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

Question 26

Enthalpy change of formation method

Answer 26

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

Question 27

How are reaction rates calculated? | What is instantaneous rate?

Answer 27

Ave rate = delta moles/delta time | Is the speed at a particular point in time found by using the slope of the tangent

Question 28

How is the amount of chemical changes determined experimentally?

Answer 28

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

Question 29

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

Answer 29

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

Question 30

Define transition state

Answer 30

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

Question 31

How can you speed up or slow down a reaction?

Answer 31

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

Question 32

How does reactant concentration affect rate?

Answer 32

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

Question 33

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

Answer 33

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

Question 34

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

Answer 34

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

Question 35

Describe intermediate

Answer 35

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

Question 36

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

Answer 36

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