Energetics

Question 1

What is enthalpy?

Answer 1

The measure of the heat content of a substance.

Question 2

What are standard conditions? What is the symbol for standard enthalpy change? What are the units?

Answer 2

100kPa pressure, 1mol dm^3 concentration, standard state at a stated temperature (usually 298K/25 degrees).

ΔH°

units = kJmol^-1

Question 3

What is enthalpy change? What is the symbol for this?

Answer 3

The change in heat content at constant pressure.

ΔH

Question 4

What are the symbols for Activation Energy and Catalysts?

Answer 4

Ea and Ec

Question 5

What is the standard enthalpy of reaction (“enthalpy of reaction”)? What is the symbol?

Answer 5

The enthalpy change for a reaction with the molar quantities shown in the chemical equation.

ΔH°r

Question 6

What happens to the standard enthalpy change of reaction when the molar quantities in an equation is halved? And doubled?

Answer 6

It halves

It doubles

Question 7

What is standard enthalpy change of formation (“enthalpy of formation”)? What is the symbol?

Answer 7

The enthalpy change that takes place when 1 mol of a compound is formed from it’s elements under standard conditions.

ΔH°f

e.g. for CH4(g):
C(s) + 2H2(g) –> CH4(g)

Question 8

What is standard enthalpy change of combustion (“enthalpy of combustion”)? What is the symbol?

Answer 8

The enthalpy change that takes place when 1 mol of a substance reacts completely with O2 under standard conditions.

ΔH°c

e.g. for CH4(g):
CH4(g) + 2O2 –> CO2(g) + 2H2O(l)

Question 9

What is standard enthalpy change of neutralisation (“enthalpy of neutralisation”)? What is the symbol?

Answer 9

The enthalpy change when 1 mol of water is formed in a reaction between an acid and an alkali under standard conditions.

ΔneutH°

e.g.
1/2 H2SO4(aq) + NaOH(aq) –> H2O(l) + 1/2 Na2SO4

Question 10

What is the equation for heat energy change?

Answer 10

q = mcΔT

q (heat energy in J)
m (mass in g)
c (specific heat capacity in J g^-1 K^-1)
ΔT ( temperature rise in K)

Question 11

What is the equation for enthalpy change (per mol)?

Answer 11

q (heat energy change) / number of moles reacting

Question 12

what is the average bond enthalpy?

Answer 12

average enthalpy change when 1 mole of bonds in gaseous molecules are broken.

Question 13

Bond enthalpies are always ______. Average bond enthalpies are not always accurate because…

Answer 13

positive

they are only averages - actual enthalpies change depending on the molecule.

Question 14

Are exothermic and endothermic making or breaking bonds?

Answer 14

MEXO BENDO - or think of magnets

Bond making = exothermic
Bond breaking = endothermic

When more energy goes into making bonds than breaking bonds, the reaction is exothermic (and vise-versa).

Question 15

how do you calculate enthalpy change from average bond enthalpies?

Answer 15

Sum of bonds broken - Sum of bonds made

Question 16

What is Hess’s Law? (do not need to remember this definition)

Answer 16

The enthalpy change for a reaction is independent of the route taken.

Question 17

What is the diagram used to calculate enthalpy change when given the enthalpies of formation?

Answer 17

Reactants —∆H–> products
^ ^
| ∆fH | ∆fH
elements

Question 18

What is the diagram used to calculate enthalpy change when given the enthalpies of combustion?

Answer 18

Reactants —∆H–> products
↓ ∆cH ↓ ∆cH

oxides

Question 19

Why can the enthalpy changes of some reactions not be measured directly?

Answer 19

Because elements can react to form many different products.

Question 20

What is needed for a successful collision to occur?

Answer 20

Correct orientation and Activation energy

Question 21

What 5 factors impact the rate of reaction?

Answer 21

• concentration
• pressure of gases
• temperature
• surface area
• catalyst

Question 22

What effect does concentration have on rate of reaction and why?

Answer 22

Increased concentration increases rate of reaction, because…

• more particles per unit volume
• so more collisions per second (more frequent collisions)

Question 23

What effect does temperature have on rate of reaction and why?

Answer 23

The higher the temperature the faster the rate of reaction because…

• particles have more kinetic energy
• more particles have activation energy
• so more successful collisions
• also move faster, colliding more often
• so more frequent collisions

Question 24

What size of particle allows for large SA? What effect does Surface Area have on rate of reaction and why?

Answer 24

The smaller the particle the larger the SA and the faster the reaction because…
- larger SA allows more collisions per second (more frequent collisions) at surface

Question 25

What effect does Pressure have on rate of reaction and why?

Answer 25

Increasing pressure increases rate of reaction because... - more particles per unit volume - more collisions per second (more frequent collisions) If no gases reacting, pressure has no impact on rate.

Question 26

What effect does catalyst have on rate of reaction and why?

Answer 26

Catalyst increases rate of reaction by lowering activation energy. So... - a larger proportion of molecules will have sufficient energy for a successful collision - so more frequent successful collisions

Question 27

What does the Boltzmann distribution curve look like? And with a catalyst?

Answer 27

Note: Line starts at (0,0) and does not touch the x-axis at highest energy - because there is no maximum energy.

Question 28

How does the Boltzmann Distribution curve change with change in temperature?

Answer 28

Question 29

Catalysts lower ________ energy by allowing the reaction to take place by a different \_\_\_\_\_. catalysts speed up reactions without being ____ \_\_.

Answer 29

activation route used up

Question 30

What is a homogeneous catalyst? Give an example.

Answer 30

A catalyst that is the same physical state as the reactants. e.g. Ozone breakdown by Cl radicals (both gases)

Question 31

What is a heterogeneous catalyst? Give an example.

Answer 31

A catalyst that is in a different physical state to the reactants. e.g. Haber process - reactants = H2 and N2 (gases), iron catalyst (solid)

Question 32

What are the advantages of using a catalyst in industry?

Answer 32

Reactions can occur at a lower temperature. Less energy so less electricity / fossil fuels needed. So lower CO2 emissions. Allow reactions with higher atom economies / less toxic by-products to be used in industry. Makes reactions more specific.

Question 33

What happens at dynamic equilibrium?

Answer 33

Rate of forward reaction is equal to rate of backwards reaction so concentrations of reactants and products remain constant.

Question 34

Dynamic equilibrium is a _____ system. A catalyst _____ the rate of both the forwards and reverse reactions \_\_\_\_\_\_\_. It does not change the ______ of equilibrium, but reaches equilibrium \_\_\_\_\_.

Answer 34

closed increases equally position faster

Question 35

How does equilibrium position shift with change in temperature?

Answer 35

If temperature is increased, equilibirum shifts in the endothermic direction in order to oppose the increase in temperature. If temperature is decreased, equilibirum shifts in the exothermic direction in order to oppose the decrease in temperature.

Question 36

What is Le Chatelier's Principle?

Answer 36

If the conditions of a reaction at equilibrium are changed, then the position of equilibrium moves to oppose that change.

Question 37

How does equilibrium position shift with change in pressure?

Answer 37

If pressure is increased, equilibirum shifts to the side with fewer gas moles in order to oppose the pressure increase. If pressure is decreased, equilibirum shifts to the side with more moles of gas in order to oppose thee pressure decrease.

Question 38

How does equilibrium position shift with change in concentration?

Answer 38

If concentration of a substance is increased, equilibrium shifts to the opposite side to oppose this increase. If concentration of a substance is decreased, equilibrium shifts to that side, to oppose the derease.

Question 39

What is the equilibrium constant (Kc) expression / Eqm law?

Answer 39

Note: Solids do not appear in the Kc expression (as you cannot have a concentration)

Question 40

What does it mean when: - Kc = 1 - Kc \> 1 - Kc \< 1 ?

Answer 40

Kc = 1 Equilibrium is exactly in the middle Kc \>1 Equilibrium lies to the right (top \> bottom) Kc \< 1 Equilibrium lies to the left (top \< bottom)

Question 41

What is activation energy?

Answer 41

The minimum amount of energy required for a reaction to take place.