Unit 3 - Energy Changes and Rates of Reaction

Question 1

thermochemistry

Answer 1

the study of the energy changes that accompany physical or chemical changes in matter

Question 2

energy (E)

Answer 2

• the ability to do work
• unit: Joules (J)

Question 3

potential energy (E_p)

Answer 3

the energy of a body or system due to its position or composition

Question 4

kinetic energy (E_k)

Answer 4

the energy of an object due to its motion

Question 5

thermal energy

Answer 5

the total quantity of kinetic and potential energy in a substance

Question 6

heat

Answer 6

the transfer of thermal energy from a warm object to a cooler object

Question 7

temperature

Answer 7

a measure of the average kinetic energy of entities in a substance

e.g. The total quantity of thermal energy of an iceberg is greater than that of a cup of hot water. The average water molecule in the hot water has more kinetic energy (higher temperature) than in the iceberg, but the iceberg contains billions more water molecules than a cup of hot water. (more thermal energy ≠ higher temperature)

Question 8

Law of Conservation of Energy

Answer 8

Energy cannot be created or destroyed.

Question 9

chemical system

Answer 9

a group of reactants and products being studied

Question 10

surroundings

Answer 10

all the matter that is not part of the chemical system

Question 11

open system

Answer 11

a system in which both matter and energy are free to enter and leave the system

e.g. An operating propane barbeque is an open system, since propane gas flows into the burner and thermal energy, and the products of combustion flow out.

Question 12

closed system

Answer 12

a system in which energy can enter and leave the system, but matter cannot

e.g.; A glow stick is a closed system. Light energy is released from a chemical reaction, but the chemicals remain sealed inside.

Question 13

exothermic reaction

Answer 13

• releasing energy to the surroundings
• the products of an exothermic reaction have lower potential energy than the reactants
• the temperature of the surroundings increases
• e.g. CH_4(g) + 2O_2(g) → CO_2(g) + 2H₂O_(g) + energy

Question 14

endothermic reaction

Answer 14

• absorbing energy from the surroundings
• the products of an exothermic reaction have higher potential energy than the reactants
• the temperature of the surroundings decreases
• e.g. N_2(g) + O_2(g) → energy + 2NO_(g)

Question 15

enthalpy (H)

Answer 15

the total amount of thermal energy in a substance

Question 16

enthalpy change (ΔH)

Answer 16

• the energy released to or absorbed from the surroundings during a chemical or physical change
• ΔH for physical changes (overcoming intermolecular forces) < ΔH for chemical changes (overcoming chemical bonds) < ΔH for nuclear changes (overcoming the forces holding nuclei together
• ΔH > 0; endothermic reaction
• ΔH < 0; exothermic reaction

Question 17

expressing enthalpy change

Answer 17

1. thermochemical equations with energy terms (e.g. AgNO_3(s) + 22.6kJ → Ag⁺¹_(aq) + NO₃^-1_(aq))
2. thermochemical equations with ΔH values (e.g. 2SO_2(g) + O_2(g) → 2SO_3(g); ΔH = -197.8 kJ
3. molar enthalpy change (e.g. CH₃OH_(ℓ) + 3/2O_2(g) → CO_2(g) + 2H₂O_(ℓ); ΔH_comb = -726 kJ/mol
4. potential energy diagram

Question 18

specific heat capacity (c)

Answer 18

• the quantity of thermal energy required to raise the temperature of 1 g of a substance by 1°C
• unit: J/(g⋅°C)

Question 19

specific heat capacity of water

Answer 19

4.18 J/(g⋅°C)

Question 20

calorimetry

Answer 20

the experimental process of measuring the thermal energy change in a chemical or physical change

Question 21

calorimeter

Answer 21

a device that is used to measure thermal energy changes in a chemical or physical reaction, consisting of:

• a well-insulated reaction chamber to minimize energy losses
• a tight-fitting cover with insulated holes for a thermometer
• a stirring mechanism

Question 22

types of calorimeter

Answer 22

• simple calorimeter (a.k.a “coffee cup” calorimeter), for reactions in solution
• flame calorimeter (a.k.a. bomb calorimeter), for combustion reactions

Question 23

assumptions to be made when analyzing data obtained using a simple calorimeter

Answer 23

• any thermal energy transferred from the calorimeter to the outside environment is negligible
• any thermal energy absorbed by the calorimeter itself is negligible
• all dilute, aqueous solutions have the same density (1.00 g/mL) and specific heat capacity (4.18 J/(g⋅°C)) as water

Question 24

quantity of heat transferred

Answer 24

• equation: q = mcΔT
• unit: kiloJoules (kJ)
• if q is negative, it is an exotermic change (the temperature of the water increases)
• if q is positive, it is an endothermic change (the temperature of the water decreases)

Question 25

calorimetry equation

Answer 25

• ΔH_system = -q_surroundings
• ΔH_system = nΔH_r
• q_surroundings = mcΔT
• nΔH_r = -mcΔT

Question 26

Hess’ Law

Answer 26

the enthalpy change for the conversion of reactants to products is the same, whether the conversion occurs in one step or several steps

ΔH_target = ΣH_known

Question 27

rules for enthalpy changes (Hess’ Law)

Answer 27

1. If you reverse a chemical reaction, you must also reverse the sign of ΔH.
2. If the coefficients in a balanced equation are altered by multiplying or dividing by a factor, the value of ΔH must also be altered by the same factor.

Question 28

standard enthalpy of formation (ΔH_f°)

Answer 28

• the change in enthalpy that accompanies the formation of 1 mol of a compound from its elements in their standard states
• the standard enthalpy of an element in its standard state is zero, unless it exists in more than one form at SATP (e.g. diamond, graphite)

Question 29

standard state

Answer 29

the most stable form of a substance under standard conditions (25°C and 100kPA)

Question 30

formation equations

Answer 30

formation equations show…
1. One mole of compound being formed from its elements.
2. Reactant elements are in their most stable form at SATP (some elements are diatomic; H₂, O₂, F₂, Br₂, I₂, N₂, Cl₂).
3. Coefficients are balanced to yield one mole of product.

e.g. 2C_(s) + 3H_2(g) + 1/2O_2(g) → C₂H₅OH_(ℓ); ΔH_f° = -235.2kJ/mol

Question 31

equation for using standard enthalpies of formation to calculate enthalpy change

Answer 31

ΔH_r° = Σn_productsΔH_f°_products - Σn_reactantsΔH_f°_reactants

Question 32

chemical kinetics

Answer 32

the area of chemistry that deals with rates of reactions

Question 33

reaction rate

Answer 33

the change in concentration of a reactant or a product of a chemical reaction, per unit time

Question 34

physical properties used to measure reaction rates

Answer 34

1. change in volume and/or pressure of a gas
2. change in colour
3. change in transparency
4. change in mass of a solid reactant or product
5. change in pH
6. change in conductivity
7. change in temperature

Question 35

change in volume and/or pressure of a gas

Answer 35

can be measured using a eudiometer, if the number of moles of gas is different on the reactant and product side of the equation

e.g. CaCO₃ + 2H⁺_(aq) → 2Ca⁺²_(aq) + H₂O_(ℓ) + CO_2(g)

Question 36

change in colour

Answer 36

can be measured using a spectrophotometer, if the amount of coloured substance is changing during a chemical reaction,

examples of coloured species:

• Cu⁺²_(aq) = blue
• Cu⁺_(aq) = green
• MnO₄^-_(aq) = purple
• IO^-_(aq) = yellow
• Cl_2(g) = green
• I_2(g) = purple
• Br_2(g) = red-brown
• NO_2(g) = brown

e.g. 2Cu_(s) + 2H⁺_(aq) → 2Cu⁺_(aq) + H_2(g)

Question 37

change in transparency

Answer 37

can be measured using a spectrophotometer, if a fine precipitate forms during the reaction

e.g. Pb(NO₃)_(aq) + NaI_(aq) → PbI_2(s) + 2NaNO_3(aq)

Question 38

change in mass of a solid reactant or product

Answer 38

can be measured when a solid reactant is disappearing or a solid product (of large particle size, not a precipitate) is appearing

e.g. Mg_(s) + 2H⁺_(aq) → H_2(g) + Mg⁺²_(aq)

Question 39

change in pH

Answer 39

can be measured using a pH meter or pH indicator, if the moles of hydrogen ions is different on the reactant and product side of the equation

e.g. SO_3(g) + H₂O_(ℓ) → 2H⁺_(aq) + SO₄^-2_(aq)

Question 40

change in conductivity

Answer 40

can be measured using a conductivity meter or voltmeter, if the moles of ions is different on the reactant and product side of the equation

e.g. C₄H₉Br_(g) + H₂O_(ℓ) → C₄H₉OH_(aq) + H⁺_(aq) + Br^-_(aq)

Question 41

change in temperature

Answer 41

can be measured using a thermometer, as all reactions have some change in enthalpy

Question 42

average reaction rate

Answer 42

the change in reactant or product concentration over a given time interval (secant)

rate_A = Δ[A] ÷ Δt

Question 43

instantaneous reaction rate

Answer 43

the rate of a chemical reaction at a single point in time (tangent)

rate_inst = [A] ÷ t

Question 44

collision theory

Answer 44

the theory that chemical reactions can occur, only if reactants collide with proper orientation, and with enough kinetic energy to break reactant bonds and form product bonds

Question 45

activation energy (E_a)

Answer 45

the minimum energy that reactant molecules must possess for a reaction to be successful; represents the energy difference between reactants and the activated complex

Question 46

purposes of activation energy

Answer 46

1. to overcome the electrostatic repulsive forces between colliding entities
2. to weaken the bonds of the reactants

Question 47

activated complex (transition state)

Answer 47

an unstable arrangement of atoms containing partially formed and unformed bonds that represents the maximum potential energy point in the change

Question 48

factors affecting reactant rates

Answer 48

chemical nature of reactants: for any reactant, the bond type, strength, and number determine the activation energy required for successful collisions
concentration of reactants: if the concentration of a reactant is increased, the probability of collisions between reactant molecules increases and a great number of effective collisions is likely to occur
surface area: for any reaction in which the reactants are in more than one state of matter (solid and liquid), the greater the surface area of the solid, the faster the reaction rate
temperature of the reaction system: in general, the rate of a chemical reaction increases with the temperature at which it is carried out (the reaction rate of many reactions doubles for every 10°C temperature increase)
presence of a catalyst: a catalyst provides an alternative pathway for a reaction, which has a lower activation energy

Question 49

catalyst

Answer 49

a substance that alters the rate of a chemical reaction, without itself being permanently changed

Question 50

heterogenous catalyst

Answer 50

a catalyst in a reaction, in which the reactants and the catalysts are in different physical states

Question 51

homogeneous catalyst

Answer 51

a catalyst in a reaction, in which the reactants and the catalysts are in the same physical states

Question 52

elementary step

Answer 52

a step involving one-, two-, or three-entity collisions that cannot be explained by simpler reactions

Question 53

reaction mechanism

Answer 53

a series of elementary steps by which a chemical reaction occurs

Question 54

reaction intermediate

Answer 54

an entity that is neither a reactant, nor a product, but is formed and consumed during the reaction sequence

Question 55

molecularity

Answer 55

refers to the number of reactant particles that are involved in an elementary step (i.e. unimolecular, bimolecular, termolecular)

Question 56

rate law equations

Answer 56

A → products: rate = k[A]
A + A → products: rate = k[A]²
A + B → products: rate = k[A][B]