2.1 - Thermochemistry

Question 1

In a reaction bonds are broken in the _____ and made in the _______. For bonds to be broken, energy is _____ __ from the surroundings and when bonds are formed energy is _____ ____

Answer 1

reactants
products
taken in
given out

Question 2

When energy is taken in from the surroundings, the enthalpy change is _____.
When energy is released, the enthalpy change is _______.

Answer 2

positive
negative

Question 3

How do you calculate overall enthalpy change?

Answer 3

change in H = Hproducts - Hreactants

Question 4

What is an exothermic and endothermic reaction? What is the change in H?

Answer 4

An exothermic reaction is one that releases energy to the surroundings, there is a temp rise and change H is negative

An endothermic reaction is one that takes in energy form the surroundings, there is a temp drop and change H is positive

See NC1

Question 5

What is enthalpy?

Answer 5

All the heat energy stored in a chemical system

Question 6

Enthalpy changes for reactions depend on conditions, so if they’re to be compared, then enthalpy change is measured when fixed or standard conditions are used. What are the standard condition?

Answer 6

All substances in their standard states
A temp of 298K
A pressure of 1 atmosphere (101,000 Pa)
A conc of 1 mol dm-3 for chemical in solution

Question 7

What is the principle of conservation of energy?

Answer 7

States that energy cannot be created or destroyed only changed from one form to another

Question 8

What is the definition of the enthalpy of formation?

Answer 8

The enthalpy change when one mol of a substance (the product) is produced from its elements under standard conditions

See NC 2

Question 9

If an element like O2(g) is being formed from the element O2(g) then standard enthalpy change of formation must be =

Answer 9

0 as there is no chemical change. All elements in their standard state have a standard enthalpy change of formation of 0 KJ mol-1

Question 10

What is the definition of the Standard enthalpy change of combustion?

Answer 10

Enthalpy change when 1 mole of a substance is completely combusted in oxygen under standard conditions

See NC 2

Question 11

What is the definition of the enthalpy change of a reaction?

Answer 11

Enthalpy change in any reaction between the number of moles of reactants shown in the equation for the reaction

Question 12

How do you calculate the standard enthalpy change of a reaction?

Answer 12

From the standard enthalpies of formation of all the reactants and products involved.

H change of reaction = Total H change of formation (products) - Total H change of formation (reactants)

See NC 2

Question 13

Energy in a reaction system must be conserved as it cannot be created or destroyed. Therefore the overall enthalpy change for a reaction is the same, regardless of the route taken. State Hess’s law.

Answer 13

The total enthalpy change for a reaction is independent of the route taken from the reactants to the products

See NC 3

Question 14

Hess’s law can be used to calculate an enthalpy change using given enthalpies of formation. When setting up the triangular diagram, the arrows point ____ from the central product C as both A and B are formed from the _____ at C

What is the enthalpy change of formation? equation?

Answer 14

out
elements
Hproducts - Hreactants

See NC 4

Question 15

Hess’s law can also be used to calculate an enthalpy change using given enthalpies of combustion. When setting up the triangular diagram, the arrows point ______ the central product (which is always ___ and ____) as both A and B burn to form products at C

What is the enthalpy change of combustion equation?

Answer 15

Towards
H20
CO2

Hreactants -Hproducts

See NC 5

Question 16

What is the definition of a bond enthalpy?

Answer 16

The amount of energy needed to break a covalent bond during a chemical reaction

Question 17

What is the definition of an average bond enthalpy?

Answer 17

The average value of the enthalpy required to break 1 mole of bonds

Question 18

What are the 4 steps involved in calculations using bond enthalpies

Answer 18

1) Draw out molecule so you can see all bonds
2) Calc energy needed to break all the bonds in the reactants (endo)
3) Calc energy released when bonds are formed in products (exo)
4) change in H = total (bonds broken) - total (bonds formed)

See NC 6

Question 19

What is calorimetry? What type of container is it carried out in and why?

Answer 19

Method for finding enthalpy change by measuring temp change over time. A reaction is carried out in a sealed, insulated container and the temp noted at regular intervals over a period of time .This type of container helps to reduce the energy lost to the surroundings

See NC7

Question 20

The measured change in temp is proportional to the energy change. What is the equation for the amount of heat transferred (q) and what do you assume?

Answer 20

q = mc(change in)T

m = mass of solution in calorimeter
c = specific heat capacity of solution
change in T = change in temp

Assuming that:
all of the heat is exchanged with just the solution
the solution has same c as water
the density of solution is 1 g cm-3 (so the mass is the same as the final vol in the colorimeter

Question 21

What is the definition of specific heat capacity?

Answer 21

The energy required to raise 1g of a substance by 1k without a change of state

Question 22

How do you calculate enthalpy change per mole?

Answer 22

Enthalpy change = -q / n

where n is the amount of moles reacted