Enthalpy Changes

Question 1

If a reaction is exothermic what is the enthalpy change, where does energy go

Answer 1

Negative, surroundings gain energy and the temperature increases

Question 2

If a reaction is endothermic what is the enthalpy change, where does energy go

Answer 2

Positive, surroundings lose energy and temperature decreases

Question 3

What is the standard enthalpy change of reaction

Answer 3

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

Question 4

What is enthalpy change of formation

Answer 4

The enthalpy change that takes place when 1 mole of a compound is formed from its elements under standard conditions, with all reactants and products in their standard states

Question 5

How do you work out the enthalpy change

Answer 5

H(products)- H(reactants)

Question 6

What is activation energy

Answer 6

The minimum energy required for a reaction to take place

Question 7

What is enthalpy change of combustion

Answer 7

The enthalpy change that takes place when 1 mole of a substance reacts completely with oxygen under standard conditions, with all reactants and products in their standard states

Question 8

Definition of average bond enthalpy

Answer 8

Energy required to break 1 mold of a specified type of bond in a gaseous molecule

Question 9

Draw an exothermic energy change

Answer 9

Reactants has a higher enthalpy then products
Surroundings gain energy and temp increases

Question 10

Draw an endothermic energy change

Answer 10

Enthalpy of products is above the reactants
Surrounding lose energy and get colder

Question 11

What are standard conditions

Answer 11

Pressure=100kPa
Temperature=25 degrees
Concentration=1moldm-3
State= physical state of a substance under standard conditions

Question 12

What’s is the standard enthalpy change of reaction

Answer 12

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

Question 13

What is the standard enthalpy change of reaction

Answer 13

The enthalpy change that accompanies a reaction

Question 14

What is the standard enthalpy change of neutralization

Answer 14

The energy change that accompanies the reaction of an acid by a base to form one mole of h20(l) under standard conditions, with all reactants and products in their standard state

Question 15

The energy change of surroundings is calculated from 3 quantities, what are they and what are the units

Answer 15

Mass= grams

Specific heat capacity is energy require to raise temp of 1g of substance by 1 degree, water is 4.18Jg-1K-1

Temp of surroundings is determined by thermometer 🔺T= T(final)-T(initial)

Question 16

What is the equation for calculating energy change

Answer 16

q (Joules)= mc🔺T

Question 17

What aren’t the experimental enthalpy change of combustion not very accurate

Answer 17

Heat loss to surrounding other than the water (includes beaker but mainly air)

Incomplete combustion of methanol (co and c instead of co2)

Evaporation of methanol from the wick (burner be weighed as soon as possible after extinguishing fire)

Non-standard conditions

All lead to less exothermic then expected

Question 18

How would you determine the enthalpy change between a solid and a solution or 2 solutions

Answer 18

Use polystyrene cup and a thermometer

Question 19

Are bond enthalpies always endo or exothermic

Answer 19

Endothermic, always have a positive enthalpy value

Question 20

How do you find the enthalpy change of reaction

Answer 20

Calculating bond enthalpies of bonds in reactants and products

For gaseous molecules of covalent substances:
£(bond enthalpies in reactants)-£(bond enthalpies in products)

£= reverse E

Question 21

What does Hess law state

Answer 21

If a reaction can take place by 2 routes, starting and finishing conditions are the same, the total enthalpy change is the same for each route

Eg A= breaking bonds (+110)
B= making bonds (-150)
C= reactants to products =40

A+B=C

Question 22

Fe2O3= -824
CaO=-635
Construct an enthalpy cycle for enthalpy change of formation

Answer 22

Elements (Fe, Ca, O2)
Reactants B= Fe2O3+3Ca
Products C= 2Fe+3CaO
A= fe2O3+3Ca——->2Fe+3CaO

Route 1: B+A
Route 2:C
Hess law= B+A=C so, A=C-B

A=(3X-635)-(-824)
= -1081kJmol-1

Question 23

For the enthalpy change of formation and combustion which way are the reactants and products in the equation

Answer 23

Formulation: products-reactants

Combustion:reactants-products