2.1 - Thermochemistry

Question 1

What 2 forms of energy are there?

Answer 1

Kinetic energy and potential energy

Question 2

What is kinetic energy?

Answer 2

Energy of motion at a molecular level.

Question 3

What is potential energy?

Answer 3

Positions of the atoms relative to one another (involves bond breaking and making)

Question 4

What is the equation for an internal energy system?

Answer 4

the sum of kinetic energy + the sum of potential energy

Question 5

What is an exothermic reaction?

Answer 5

One that releases energy to the surroundings, there is a temperature rise and delta H is negative.
Examples
Acids+metals
Hand warmers
Thermite reactions

Question 6

What is a endothermic reaction?

Answer 6

One that takes in energy from its surroundings, there is a temperature drop and delta H is positive.
Examples
Melting ice
Cold packs
Thermal decomposition of group 2 carbonates

Question 7

What is enthalpy (H)?

Answer 7

Heat content of a system at constant pressure.

Question 8

What is enthalpy change? (Delta H)

Answer 8

The heat added to a system at constant pressure.

Question 9

What is the equation for enthalpy change? (Delta H)

Answer 9

Delta H = H(products)-H(reactants)

Question 10

Describe an energy profile for an exothermic reaction.

Answer 10

Reactants have higher enthalpy than products, therefore delta H is negative.

Question 11

Describe an energy profile for an endothermic reaction.

Answer 11

Products have higher enthalpy than reactants, therefore delta H is positive.

Question 12

What is the principle of conservation of energy?

Answer 12

Energy cannot be created nor destroyed, only changed from one form to another.

Question 13

What are the standard conditions of enthalpy change?

Answer 13

• all substances have to be within their standard states
• a temperature of 298K (25*c)
• a pressure of 1atm (101,000Pa)

Question 14

What is the definition for the standard enthalpy change of formation? delta fH

Answer 14

• when one mole of a substance is formed from its constituent elements in their standard states under standard conditions
• all elements in their standard state have a standard enthalpy change of formation of 0Kj/mol-1.
• per mole regards to its products

Question 15

What is the definition of the standard enthalpy change of combustion? delta cH

Answer 15

Enthalpy change when one mole of a substance is completely combusted in oxygen under standard conditions.
- per mole regards to its reactants

Question 16

What is the equation for calculating enthalpy change of a reaction?

Answer 16

Delta rH = the sum of delta formation enthalpy (products) - the sum of delta formation enthalpy (reactants)
- remember any exothermic values (making bonds) need to be changed to a negative

Question 17

What is Hess’s law?

Answer 17

The total enthalpy change for a reaction is independent of the rate taken from the reactants to the products.
delta H1 = delta H2 + delta H3
energy cycles :)

Question 18

In formation enthalpy cycles, which way do the arrows face?

Answer 18

Arrows face up (elements to the equation), meaning usually left side doesnt match cycle so thats the number to reverse.

Question 19

In combustion enthalpy cycles, which way do the arrows face?

Answer 19

Arrows face down (equation to the elements), meaning usually right side doesn’t match so thats the number to reverse.

Question 20

What is the definition of bond enthalpies?

Answer 20

Enthalpy required to break a covalent x-y bond into x atoms and y atoms, all in a gaseous state.

Question 21

Is breaking bonds endothermic or exothermic?

Answer 21

Endothermic, requires energy put in by surroundings

Question 22

Is making bonds endothermic or exothermic?

Answer 22

Exothermic, energy released to its surroundings

Question 23

What is the defintion of an average bond enthalpy?

Answer 23

The energy needed to break one mole of a specific bond in a molecule in the gaseous state.
Its an average as for example C-H has different values in methane compared to methanol, so average is created.

Question 24

Why does average bond enthalpy often not reflect standard enthalpy change?

Answer 24

Elements must be gaseous in average bond enthalpy, whilst in standard enthalpy change the elements must be in their standard state, which could be a liquid or a solid.

Question 25

What is the definition of the standard enthalpy change of a reaction?

Answer 25

The enthalpy change when a reaction takes place in the molar quantities shown by the balanced chemical equation under standard conditions with all the chemicals in their standard states.

Question 26

How do you find the temp change in a time/temp reaction graph?

Answer 26

Draw a line vertical as to where the reacting substance was added, draw a line following the decrease in temperature at the top. Where the 2 lines meet gives where the temp would have reached with no cooling. Then minus starting temp to find the temperature change.

Question 27

What is the equation for the energy change of water and what do the symbols mean?

Answer 27

q = m c deltaT
q = energy change in water (J)
m = mass of water (g)
c = specific heat capacity of water (4.18 Jg-1k-1)
deltaT = temperature change of the water