Enthalpy

Question 1

What happens in exothermic reactions?

Answer 1

Products have less enthalpy than reactants
Chemical energy is transferred into thermal energy
The chemicals lose energy
Energy that is lost is gained by surroundings increasing temp
As a result enthalpy change (∆H) is negative

Question 2

What happens in endothermic reactions?

Answer 2

Products have more enthalpy than reactants
The chemicals gain energy
Energy that is gained by chemicals is lost by surroundings decreasing temp
As a result enthalpy change (∆H) is positive.

Question 3

What is enthalpy?

Answer 3

Is the heat energy contained in chemicals e.g reactants or products

Question 4

What is enthalpy of reaction?

Answer 4

When reactants react in their molar ratio according to the equation

Question 5

What is enthalpy change?

Answer 5

The heat energy change in a chemical reaction at a constant pressure

Question 6

What is an example of an equation for empathy of reaction?

Answer 6

2CO + O2 →2CO2

Question 7

What is enthalpy of combustion?

Answer 7

When one mole of substance is burnt completely in excess oxygen

Question 8

What is an equation for enthalpy of combustion?

Answer 8

C + O2→CO2

Question 9

What is enthalpy of formation?

Answer 9

When one mole of a compound is formed from its elements in their standard states

Question 10

What is the equation for enthalpy of formation?

Answer 10

C + 1/2O2 →CO

Question 11

What are ‘standard conditions’ for enthalpy?

Answer 11

101KPa (1 atmosphere)

25°c

Question 12

How to calculate energy change?

Answer 12

q= mc∆T

Question 13

What does q stand for and what are its units?

Answer 13

energy change

joules

Question 14

What does m stand for and what are its units?

Answer 14

Total mass of solution (what ever thermometer is touching)

grams

Question 15

What does c stand for and what are its units?

Answer 15

specific heat capacity (water mostly unless told otherwise)

4.18 Jg-1 K-1

Question 16

What does ∆T stand for and what are its units?

Answer 16

Change in temperature

degrees c

Question 17

What sign do you need to add at the end?

Answer 17

+ or -

for endo or exo

Question 18

What is the next step?

equartion?

Answer 18

∆H= Q(KJ)/ N moles

you might need to use n=cv first if solution to work out moles

Question 19

How to work out moles in limiting reagent ?

Answer 19

Mass of limiting reagent/ mr of limiting reagent

Question 20

What are problems of calculating enthalpy changes experimentally?

Answer 20

reaction rate is too slow ti allow measurement of enthalpy change
reaction may produce unwanted products (isomers)
and heat is lost to surroundings

Question 21

Ways to reduce heat being lost to surroundings?

Answer 21

add lid to polystyrene cup

use insulator around cup

Question 22

A reaction is exothermic when?

Answer 22

more energy is released during bond forming than energy taken in during bond breaking.

Question 23

A reaction is endothermic when?

Answer 23

more energy is absorbed during bond breaking than energy is released during bond breaking

Question 24

How to work out enthalpy change of combustion? ∆Hc

Answer 24

use q= mc∆T
then calculate number of moles
calculate energy transfer ∆H= Q (KJ)/ n

Question 25

What are errors of calculating ∆Hc?

Answer 25

Heat is lost to surroundings
incomplete combustion
evaporation from wick
non-standard conditions being used

Question 26

Methods for reducing heat loss?

Answer 26

adding lid to beaker

using draft shields around the sides

Question 27

How to work out bond enthalpy?

Answer 27

1- if in structural draw out in displayed
2- list bonds that are broken in reactants then do same for in products.
3- reactants subtract products

Question 28

What is the definition of bond enthalpy?

Answer 28

enthalpy change to break one mole of a covalent bond in the gaseous phase

Question 29

Limitations of using average bond enthalpys?

Answer 29

doesn’t take in to account specific atoms that surround the bond.
its only an average value.

Question 30

Enthalpy change with an acid and base definition?

Answer 30

when an acid reacts with a base in their standard states to produce 1 mole of water

Question 31

Enthalpy change of neutrallisation equations?

Answer 31

NaOH + HNO3 –> NaNO3 + H20