Enthalpy

Question 1

What’s the 🔼H of reaction

Answer 1

-🔼H = exothermic 
\+🔼H = endothermic

Question 2

What’s the difference between exothermic and endothermic?

Answer 2

Exothermic - heat loss to the surroundings

Endothermic - heat taken in from surroundings

Question 3

Define activation energy

Answer 3

The minimum energy required to start a reaction by breaking bonds.

Question 4

What are standard conditions?

Answer 4

100kPa [pressure] and 298K [temperature]

Question 5

Define enthalpy change of reaction

Answer 5

The enthalpy change that accompanies a reaction in the molar quantities shown in a chemical equation

Question 6

Define enthalpy change of formation

Answer 6

The enthalpy change that takes place when one mole of a compounds formed from its elements [under standard conditions and states]

Question 7

Define enthalpy change of combustion

Answer 7

The enthalpy change that takes place when one mole of a substance reacts completely with oxygen [standard states and conditions]

Question 8

Define Hess’ law

Answer 8

If a reaction can take place by more than one route and the initial and final conditions are the same, the total enthalpy change is the same for each route

Question 9

Define average bond enthalpy

Answer 9

The average enthalpy change that takes

place when breaking by homolytic fission one mole of a given type of bond in the molecules of a gaseous species

Question 10

Define exothermic reaction (also in terms of bonds)

Answer 10

Enthalpy of products is smaller than enthalpy of reactants meaning heat loss to the surroundings (delta H is negative)
[overall more bond making which releases energy]

Question 11

Define endothermic reaction (also in terms of bonds)

Answer 11

Enthalpy of products is greater than enthalpy of reactants meaning heat is taken in from the surroundings (delta H is positive)
[overall more bonds breaking and more energy is required]

Question 12

Define enthalpy change of neutralisation

Answer 12

The energy change that accompanies the reaction of an acid by a base to form one mole of H2O(l)

Question 13

What is a heterogeneous catalyst and how does it work?

Answer 13

-different physical state from reactant (solid)
-reactant molecules are adsorbed (weakly bonded) onto the catalyst surface
-reaction takes place and the product molecules leave the catalyst surface by desorption
E.g. Making ammonia/hydrogenation of alkenes

Question 14

What is a homogeneous catalyst and how does it work?

Answer 14

-same physical state as the reactant
-catalyst reacts with the reactants to form an intermediate
-intermediate breaks down to give the product and regenerates the catalyst
Eg. Preparing esters/ozone depletion (radicals are catalyst)

Question 15

Why do reactions use a catalyst? What is the industrial importance?

Answer 15

Increases the rate of chemical reaction by providing an alternative reaction pathway of lower activation energy
This means a lower temperature can be used as less energy is required

Question 16

What is a heterogeneous catalyst and how does it work?

Answer 16

-different physical state from reactant (solid)
-reactant molecules are adsorbed (weakly bonded) onto the catalyst surface
-reaction takes place and the product molecules leave the catalyst surface by desorption
E.g. Making ammonia/hydrogenation of alkenes

Question 17

What is a homogeneous catalyst and how does it work?

Answer 17

-same physical state as the reactant
-catalyst reacts with the reactants to form an intermediate
-intermediate breaks down to give the product and regenerates the catalyst
Eg. Preparing esters/ozone depletion (radicals are catalyst)

Question 18

Why do reactions use a catalyst?

Answer 18

Increases the rate of chemical reaction by providing an alternative reaction pathway of lower activation energy