Chemistry Topic 5+15: Energetics

Define Heat

a form of thermal energy

Define Temperature

a measure of the average kinetic energy of particles

Define an exothermic reaction

heat energy is transferred from a system to the surroundings - the surroundings get hotter. The enthalpy change is negative.

Define an endothermic reaction

a system takes in heat energy from the surroundings - the surroundings get cooler. The enthalpy change is positive.

Define standard conditions

100kPa and a specified temperature

Define standard state

the pure substance at 100kPa and a specified temperature

True or false: total energy is not conserved in a chemical reaction?

false, total energy is conserved in a chemical reaction

How do you find enthalpy change using experimental methods?

measuring the temperature change of a substance (usually water) and using the equation q = mc(delta)T

Give the units of each component in q = mc(delta)T

q = J, m = g, T = degrees celsius SHC = J g-1 degrees celsius -1

If there is a temperature change of 1 K what is that change in degrees celsius?

1 degrees celsius

Define Standard Enthalpy Change of Combustion

the enthalpy change when one mole of a substance is completely burnt in oxygen under standard conditions

Describe the experiment used to find the enthalpy change of combustion of methanol

1. Add a set amount of water to a calorimeter and attach to a clamp stand.

2. weigh the spirit burner with the cap on and note it down

3. Measure the intial temperature of the water and note it down.

4. light the spirit burner and place under the calorimeter

5. place the cap back onto the spirit burner once the water temperature has reached 40 degrees celsius

6. weigh the spirit burner with the cap on and note down the difference in mass

7. use the q = mc(delta) T equation to calculate the enthalpy change of combustion

8. energy given out per mole = - q / mol

9. divide by 1000 to get it in kJ

Give four systematic flaws in this experiment:

1. heat loss to the surroundings
2. incomplete combustion of the methanol
3. evaporation of the methanol
4. evaporation of water

Define Enthalpy Change of Solution

the amount of heat energy given out/taken in when 1 mole of solute dissolves in excess solvent

Describe the experiment used to find the enthalpy change of solution

1. measure out known amounts of reagents and record their initial temperatures
2. mix the reagents together in a polystyrene cup and record the maximum/minimum temperature.
3. use the q = mc(delta)T equation to calculate enthalpy change of solution, you know the volume of water and the mass of the other reagent.

Explain how you would use the q = mc(delta)T equation to find enthalpy of solution

1. use q = mc(delta) T to find energy of solution
2. use n = m/Mr to find moles of reagent (not water)
3. divide the energy by moles to give enthalpy change of solution
4. make sure answer is in kJ mol

What assumptions are made during this experiment to find enthalpy of solution?

1. the density and the specific heat capacity of the solution are the same as the water

What systematic error could occur during this experiment?

heat loss to the surroundings as the polystyrene cup is not a good enough insulator

What is the relationship between enthalpy change of a reaction and the pathway between the initial and final states?

the enthalpy change of a reaction is independent of the pathway between the initial and final states.

What is Hess’ Law used for?

Hess’ Law can be used to work out unknown enthalpy changes from known enthalpy changes.

Define Standard Enthalpy of Formation

the enthalpy change when one mole of the substance is formed from its constituent elements in their standard states under standand conditions.

Give the equation for the enthalpy change of formation of methane

C(s) + 2H2(g) → CH4(g)

Give the enthalpy of formation equation

enthalpy of formation (products) - enthalpy of formation (reactants)

What is the enthalpy change of formation of an element in its standard state?

0

Why is Carbon in its solid state as graphite used for enthalpy of formation equations?

graphite is the most stable form of carbon

Define Average Bond Enthalpy

the average energy required to break one mole of covalent bonds in a gaseous molecule under standard conditions or the energy released when one mole of covalent bonds in a gaseous molecule is made under standard conditions.

Bond making is…

exothermic so releases energy

Bond breaking is…

endothermic so requires energy

Why are average bond enthalpy values flawed?

average bond enthalpies are less reliable as they take the mean value of a compound, introducing inaccuracies into calculations.

What equation do you use to calculate enthalpy change of a reaction using bond enthalpies?

sum of bonds broken - sum of bonds made

When can average bond enthalpy values be used?

they can be used to calculate standard enthalpy change of reactions in a gaseous state only as they do not take intermolecular forces into account.

What do potential energy profiles show?

how the potential energy of the species involved in a chemical reaction changes during that reaction

What is on the y and x axis in an energy profile diagram

x = reaction coordinate y= potential enthalpy

Describe the position of the reactants and products in an exothermic enthalpy profile

reactants are above the products

Describe the position of the reactants and products in an endothermic enthalpy profile

products above the reactants

Why does O2 absorb higher frequency radiation than O3?

because the double bond between 02 is stronger than that in O3 (double and single bond). Bond order of O2 = 2 whereas bond order in O3 =1.5

Write the first step of a born-haber cycle of NaCl(s)

1. atomisation of Na, ½Cl2(g) + Na(g) (endothermic)

Write the second step of a born haber cycle of NaCl(s)

2. atomisation energy of Cl2, Cl(g) + Na(g) (endothermic)

Write the third step of a born haber cycle of NaCl(s)

3. first ionisation energy of Na, Na+ + e + Cl (endothermic)

Write the fourth step of a born haber cycle of NaCl(s)

4. first electron affinity of Cl, Na+(g) + Cl-(g) (exothermic)

Write the fifth step of a born haber cycle of NaCl(s)

5. lattice enthalpy of NaCl (s) (exothermic)

Define enthalpy change of atomisation

enthalpy change when one mole of gaseous atoms is formed from an element - endothermic

Define first ionisation energy

enthalpy change when one electron is removed from each atom in one mole of gaseous atoms - endothermic

Define second ionisation energy

enthalpy change when one electron is removed from each atom in one mole of gaseous atoms - endothermic

Define first electron affinity

enthalpy change when 1 electron is added to one mole of gaseous atoms - exothermic

Is second electron affinity endothermic or exothermic?

endothermic due to increased repulsion from valence electrons

Define lattice enthalpy

the enthalpy change when 1 mole of ionic compound is broken apart into its constituent gaseous ions - endothermic

What value do you use if you are not given a value for enthalpy change of atomisation?

you can divide its bond energy in half and use that value as for atomisation energy.

Write the first step in an enthalpy of solution diagram for CaCl2(s)

1. enthalpy of lattice Ca2+(g) + 2Cl-(g)

Write the second step in an enthalpy of solution diagram for CaCl2(s)

2. enthalpy of hydration Ca2+(aq) + 2Cl-(aq)

Write the combined first and second step in an enthalpy of solution diagram

enthalpy of solution → Ca2+(aq) + 2Cl-(aq)

Define hydration enthalpy

the enthalpy change when 1 mole of gaseous ions are surrounded by water molecules to form an infinitely dilute solution - exothermic

Define enthalpy change of solution

enthalpy change when one mole of solute is dissolved in excess solvent to form a solution of infinite dilution - endothermic or exothermic

Why does lattice enthalpy occur?

lattice enthalpy is the result of electrostatic attractions between oppositely charged ions in the giant lattice

What two things does the strength of lattice enthalpy depend on?

1. charge - the higher the charge on the ions the more strongly they will attract and therefore the greater the lattice enthalpy
2. size - the smaller the ion the stronger the attraction between the nuclei and electrons of other ion, so greater the lattice enthalpy

What type of charge makes the enthalpy change of hydration more exothermic?

the higher the charge

What size ion makes the enthalpy change of hydration more exothermic?

the smaller the ionic radius as this results in more attraction between the ions and water molecules

What is more important, charge or size of ion when affecting the strength of lattice enthalpy/enthalpy of hydration?

charge

Define Entropy

a measure of how the available energy is distributed among the particles

Explain the relationship between disorder and entropy

the higher the disorder the more ways there are of distributing the energy and the higher the entropy

What is the unit for standard entropy?

JK-1mol-1

List the entropy of the states of matter in increasing order

solid < liquid < gas

Why do gases have the highest entropy?

the particles in a gas are moving around more randomly and the energy can be distributed in more ways so the entropy is higher

What does an increase in the number of moles of gas convey about entropy?

entropy increases

What does a decrease in the number of moles of gas convey about entropy?

entropy decreases

Give the equation to find the standard entropy change of a reaction

entropy of products - entropy of reactants

The more positive the entropy…

the greater the disorder

Define a spontaneous reaction

a reaction that occurs without any outside influence

What value must the Gibbs Free energy value have for a reaction to be spontaneous?

below 0 so negative

Give the Gibbs Free Energy equation

G(gibbs) = H(enthalpy change)- T(temp change) x S(entropy)

If entropy is positive how will increasing temperature affect spontaneity?

gibbs becomes more negative so therefore reaction becomes more spontaneous

gibbs is most negative when…

a reaction is at equilibrium