Unit 2.1 Thermochemistry

Question 1

Exothermic?

Answer 1

more energy released when bonds are made than taken in when bonds are broken

Question 2

Endothermic?

Answer 2

more energy taken in when bonds are broken in comparison to energy released when bonds are made

Question 3

what is an example of exothermic reactions?

Answer 3

burning of a fuel to give us heat energy. we also know that fuel contains chemical energy

Question 4

what is a chemical system?

Answer 4

When chemicals react with one another, bonds have to be broken in the reactants and new ones
made in the products reactants + products

Question 5

what is enthalpy?

Answer 5

energy contained within a substance including chemical, heat, etc

Question 6

what are the units?

Answer 6

KJ mol -1

Question 7

what is the formula for enthalpy change?

Answer 7

ΔH = Hproducts - Hreactants

Question 8

what does this apply to exothermic reactions?

Answer 8

heat is
given out to the surroundings, so the enthalpy of the reactants is greater than the enthalpy of the
products, so ΔH is negative.

Question 9

what does this apply to endothermic reactions?

Answer 9

heat is absorbed from the surroundings, so the enthalpy of the
products is greater than the enthalpy of the reactants, so ΔH is positive. This can be represented
by energy profile diagrams

Question 10

what are the standard conditions for enthalpy change?

Answer 10

all substances in their standard states
- a temperature of 298K (25°C)
- a pressure of 1 atmosphere (101,000 Pa)
The symbol for a standard enthalpy change is ΔHθ

Question 11

what is the standard enthalpy change of formation?

Answer 11

enthalpy change when one mole of a substance is formed from its elements with all standard states under standard conditions.

Question 12

what is the formula for the formation of sulfur dioxide?

Answer 12

S(s) + O2(g) → SO2(g) ΔfH = -296.4 kJ mol

Question 13

what is the standard enthalpy change of combustion?

Answer 13

the enthalpy change when 1 mole of substance is burned completely in Oxygen to produce combustion to produce combustion products with all the substances at standard conditions

Question 14

what is the enthalpy change of a reaction?

Answer 14

ΔrHθ

Question 15

what is θ?

Answer 15

symbol representing standard conditions
298k
1 atm or 101kPa

Question 16

Hess’s law?

Answer 16

The enthalpy change of a reaction is independent of the route of the reaction

Question 17

Bond disassociation enthalpy

Answer 17

the enthalpy change when 1 mole of gaseous covalent bonds are broken

Question 18

what is so special about bond disassociation?

Answer 18

is always given as a positive

Question 19

what about bond formation?

Answer 19

it will be the exact opposite - negative

Question 20

what is bond enthalpy?

Answer 20

the amount of energy needed to break a covalent bond during a chemical
reaction.

Question 21

what is the average bond enthalpy?

Answer 21

he average value of the enthalpy required to break a given type
of covalent bond in the molecules of a gaseous species. ie, calculated using a wide range of
compounds. These are used in calculations and won’t be as accurate as using actual bond
enthalpies, but they give a good indication, so are extensively used.

Question 22

what is breaking bonds?

Answer 22

requires energy
endthermic
always positive

Question 22

what is breaking bonds?

Answer 22

requires energy
endthermic
always positive

Question 23

what is making bonds?

Answer 23

releases energy
exothermic
always negative

Question 24

calorimetry equation?

Answer 24

q = mcΔT

Question 25

what are the components of the equation?

Answer 25

q = energy in joules
m= mass of the substance being heated (typically water)
water = 1cm3 = 1g
c= specific heat capacity for water . 418jk-1g-1
t= change in temperature (k)

Question 26

what do we assume?

Answer 26

-all of the heat is exchanged with just the solution
- the solution has the same specific heat capacity as water (you will be given this, 4.18 Jg-1K-1)
- the density of the solution is 1 g cm-3 (so the mass is always the same as the final volume in the
colorimeter)