Enthalpy

Question 1

What is bond dissociation enthalpy?

Answer 1

Energy required to break a covalent bond between species in a gaseous state

Question 2

Why is mean bond enthalpy used?

Answer 2

The same bond may have different bond enthalpies in different molecules

Question 3

What are the steps in calculating mean bond enthalpies

Answer 3

1. Draw out chemical drawing
2. Circle bonds broken and bonds formed
3. Add together bonds broken and then

Question 4

Explain the difference between heat and temperature

Answer 4

Heat is mass x energy

Temperature is the average kinetic energy of particles

Question 5

What is specific heat capacity?

What units is specific heat capacity measured in?

Answer 5

The amount of heat needed to raise 1g of substance by 1k

Joules per kelvin jg-1 k-1

Question 6

What’s the specific heat capacity of water?

Answer 6

4.2 jg-1 k-1

Question 7

What is the formula for enthalpy change?

Answer 7

Enthalpy change =mass of substance x specific heat capacity x temperature change

Q=m x c x 🔺t

Question 8

What does an exothermic reaction do?

Is delta h positive or negative?

Answer 8

Gives out energy

Negative

Question 9

What does an endothermic reaction do?

Is delta h positive or negative?

Answer 9

Absorbs energy

Positive

Question 10

Are oxidation reactions usually exothermic or endothermic?

Answer 10

Exothermic

Question 11

Breaking bonds is….

Making bonds is…

Answer 11

Endothermic

Exothermic

Question 12

Mean bond enthalpies are always

Answer 12

Positive

Question 13

What is the standard enthalpy of formation.

Answer 13

When 1mol of a compound is formed from standard elements in their standard states

Question 14

?What is standard enthalpy of combustion?

Answer 14

When 1 mol of substance is completely burned in o2 under standard conditions

Question 15

Describe step by step how to an enthalpy change calculation

Answer 15

Work out enthalpy change by using q=mc🔺t formula

Divide answer in jules by 1000 to get kJ

Work out how many moles are in solution by using standard molar formula

Then divide number of kJ by moms to get answer

If question tells you the reaction is exothermic make sure to put a negative sign before your answer

Question 16

Describe how to do a naturalisation reaction step by step

Answer 16

First, enthaply change calculation using q=mc🔺t

Then divide by 1000 to give kJ

Then work out the amount of moms from the concentration triangle

Then divide kJ by moles

Ensure there is a negative symbol before if it is an exothermic reaction