Enthalpy Changes

Question 1

What is enthalpy?

Answer 1

The total chemical energy inside a substance

Question 2

What is average bond enthaly?

Answer 2

The amount of energy it takes to break one mole of a specified bond in gaseous molecules.

Question 3

What does breaking and making bonds do?

Answer 3

A change in the enthalpy contained in the bond. Heat is either given out into the surroundings or absorbed into the bond, decreasing surrounding temperature.

Question 4

In an endotherminc reaction, what is the enthalpy change?

Answer 4

The enthalpy of the products is lower than the enthalpy of the reactants so the enthalpy change is negative.

Question 5

In an endothermic reaction what is the enthalpy change?

Answer 5

The enthalpy of the products is higher than the enthalpy of the reactants so the enthalpy change is positive.

Question 6

On an enthalpy change diagram where does the Ea arrow and delta H arrow go?

Answer 6

Ea goes from reactants to peak enthalpy and delta H goes from reactants to products enthalpy.

Question 7

What are standard states?

Answer 7

The state that a chemical would be in at standard conditions (room temperature and pressure).

Question 8

What is standard enthalpy change of reaction?

Answer 8

The enthalpy change of a reaction according to the molar quantities in the equation under standard conditions

Question 9

What is standard enthalpy change of neutralisation?

Answer 9

Enthalpy change when an acid and alkali react to form 1 mole of water, under standard conditions.

Question 10

What is standard enthalpy change of combustion?

Answer 10

The enthalpy change when 1 mole of a substance is completely burned in oxygen to make CO2 and H2O under standard conditions.

Question 11

What is standard enthalpy change of formation?

Answer 11

The enthalpy change when 1 mole of a compound is formed from its elements in their standard states under standard conditions.

Question 12

What is the concentration of a standard solution?

Answer 12

1 moldm-3

Question 13

Is bond breaking endothermic or exothermic?

Answer 13

Endothermic as energy needs to be absorbed to break the bond. If more energy is needed to break than make = endothermic reaction.

Question 14

Is bond breaking endothermic or exothermic?

Answer 14

Exothermic. When new bonds are formed energy is released. If more energy is released when bonds form than absorbed when bonds break the reaction is exothermic.

Question 15

What practical method is used to measure enthalpy change of combustion?

Answer 15

Calorimetry

Question 16

What are the steps of calorimetry?

Answer 16

A fuel is burned to raise the temp of the water by a specific amount.
Weigh fuel before and after to work out the mass of fuel burned.
Lid on top of beaker to stop heat loss
Draft shields around the flame to stop it flickering so energy doesn’t transfer to surroundings.
use q=mc∆T
q= energy lost or gained, J
m of water= g (cm3=g=ml)
c=SHC of water or solution, 4.18 J-1°C
T= °C

Question 17

What is the equation for energy change?

Answer 17

q=mc∆T
q= energy lost or gained, J
m of water= g (cm3=g=ml)
c=SHC of water or solution, 4.18 J-1°C
T= °C

Question 18

How do you calculate enthalpy change?

Answer 18

q/moles
Divide the energy released by the fuel’s combustion (converted to KJ) by the number of moles of fuel.
The energy change is positive or negative depending on if the reaction is exothermic or endothermic.

Question 19

How to calculate bond enthalpy change?

Answer 19

Break-make.
Reactants total bond enthalpy - products total bond enthalpy.

Question 20

What is Hess’s Law?

Answer 20

The TOTAL ENTHALPY CHANGE of a reaction is INDEPENDENT of the route taken.
It helps you work out enthalpy changes you can’t find by doing an experiment.

Question 21

What are the two main cycles in Hess cyles?

Answer 21

Formation and combustion.

Question 22

What does a hess cycle look like when given FORMATION data?

Answer 22

Reactants —————–>Products

               Elements in
        their standard states ∆cH/∆rH on top ∆fH on left arrow pointing UP ∆fH on right arrow pointing UP

Question 23

What does a hess cycle look like when given COMBUSTION data?

Answer 23

Reactants —————–>Products

               COMBUSTION
                PRODUCTS ∆fH/∆rH on top ∆cH on left arrow pointing DOWN ∆cH on right arrow pointing DOWN

Question 24

When the enthalpy change of solution exothermic?

Answer 24

When the sum of enthalpies of hydration are greater than lattice enthalpy

Question 25

When is enthalpy chane of solution endothermic?

Answer 25

When lattice enthalpy is greater than hydration enthalpy

Question 26

What affects enthalpy of hydration and lattic enthalpy?

Answer 26

The smaller the ionic radius, the greater attraction between ions and water molecules = more exothermic lattice and hydration enthalpy
The greater the ionic charge, the greater attraction between ions and water molecules