Energy Changes And Reactions

Question 0

What is ENTHAPLY?

Answer 0

H, the heat content stored in a chemical system.

Question 1

Key things to remember during calculations

Answer 1

1) When writing equations, make sure it’s one mole. 2) Don’t forget the sign. 3) Arrows when using formation ↖️↗️ 4) Arrows when using combustion ↘️↙️

Question 2

What is chemical energy?

Answer 2

A form of potential energy within chemical bonds.

Question 3

What is an important law about energy?

Answer 3

It is conserved.

Question 4

If energy is conserved, what does this mean in terms of heat?

Answer 4

Heat lost in a chemical system = heat gained to surroundings.

Question 5

What is the chemical system? What are the surroundings?

Answer 5

Chemical system = the reactants and the products. Surroundings = what is outside the chemical system.

Question 6

What is the ENTHAPLY CHANGE?

Answer 6

The heat exchange with the surroundings during a chemical reaction at a constant pressure.

Question 7

Equation for ENTHAPLY CHANGE?

Answer 7

ΔH = Hproducts - Hreactants

 (alphabet)

Question 8

What two types of heat exchange are there?

Answer 8

EXOthermic (exit) and ENDOthermic (into)

Question 9

Define EXOTHERMIC

Answer 9

A reaction in which the ENTHAPLY of the PRODUCTS is SMALLER than the ENTHAPLY of the REACTANTS, resulting in HEAT LOSS to the surroundings.

Question 10

Define ENDOTHERMIC.

Answer 10

A reaction in which the ENTHAPLY of the PRODUCTS is GREATER than the ENTHAPLY of the REACTANTS, due to HEAT GAINED from the surroundings.

Question 11

What is the sign of the enthalpy change in:

• exothermic?
• endothermic?

Answer 11

EXO is negative

ENDO is positive

Question 12

Diagram of an exothermic enthalpy change

Answer 12

Question 13

Diagram of an Endothermic enthalpy change?

Where is the Ea?

Answer 13

Question 14

What type of reaction (exothermic or endothermic) is less likely to happen naturally?

Answer 14

Endothermic You have to supply more energy, equal to produced +Ea

Question 15

Name 6 exothermic reactions?

Answer 15

▪️respiration

▪️neutralisation

▪️displacements

▪️combustion

▪️hand warmers

▪️CaO + H₂O ➡️ Ca (OH)₂

Question 16

Name endothermic reactions.

Question 17

What is a real life use of something exothermic?

Answer 17

Self heating cans / hand warmers

Question 18

What is a real life use of endothermic?

Answer 18

Ice packs

Question 19

What happens to the surrounding temperature in:

Answer 19

Exothermic: temperature of surroundings rises as heat exits into surroundings Endothermic: temperature of surroundings lowers as heat is removed from surroundings.

Question 20

Exothermic?

Question 21

ENDOTHERMIC?

Question 22

What is the activation energy? (Ea)

Answer 22

The activation energy is the minimum energy required to start a reaction by the breaking of bonds.

Question 23

What are the standard conditions that must be met for a reaction to take place?

Answer 23

• Pressure 1 atm (100kPa)
• Temperature 298K (25°C)
• Concentration 1 mol dm^-3

Question 24

What are standard states?

Answer 24

The physical state of a substance under the standard condition of temp 298K and pressure 1 atm.

Question 25

What is the standard enthalpy change of reaction? And symbol?

Answer 25

The enthalpy change that accompanies a reaction in the molar quantities expressed in a chemical equation under standard conditions, all reactants and products being in their standard states.

ΔHr^Ө

Question 26

What is the standard enthalpy change of combustion? And symbol?

Answer 26

The enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standard conditions, all reactants and products being in their standard state.

ΔHc^Ө

Question 27

What must you be extremely careful of when balancing equations?

Answer 27

That the reactant is ONE MOLE.

Question 28

What is the standard enthalpy of formation?

Answer 28

The enthalpy change that takes place when one mole of compound in its standard state is formed from its constituent elements in their standard states, all reactants and products under standard conditions.

ΔHf^Ө

Question 29

Standard enthalpy change of formation

Na + Cl₂→ NaCl

Balance

Answer 29

Na + ½Cl₂ → NaCl

Question 30

What is important to remember when doing diagrams?

Answer 30

The arrowhead in exothermic must be down

In endo arrowhead is UP

If formation, check it’s one mole.

Question 31

How can we experimentally determine standard enthalpy change?

Answer 31

Measuring the heat exchange to the surroundings.

As heat is lost by chemicals, heat is gained by surroundings..

Question 32

Equation and units for experimentally determining the standard enthalpy change?

Answer 32

Question 33

3 important things…then what

(regarding experimentally determining the standard enthalpy change)

Answer 33

1. Find out the moles that reacted (i.e. 0.02 moles)
2. Divide / multiply to get the heat lost per one mole
3. Remember sign

Question 34

Standard enthalpy change calcuations using mcΔT steps?

Answer 34

1. Write down mass of what reacted.
2. Put it into mcΔT.
3. Work out the moles that reacted.
4. Get it per one mole.

Question 35

20cm³ of 2 mol dm^-3 of nitric acid + 20cm³ of KOH solution. Temperature rise 13.8º.

Calculate enthalpy per mole

Answer 35

2 x (20 ÷ 1000) = 0.04 mol

1. Mass = 40 (1cm³ = 1g)
2. ΔH = 40 x 4.18 x 13.8 = 230.736
3. n = c x v
4. 230.74 x 25 = 57.7 kjmol^-1