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Chemistry > 1.6: Chemical Energy > Flashcards

1.6: Chemical Energy Flashcards

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1
Q

What is the energy stored within chemical bonds called?

A

Enthalpy

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2
Q

What does bond breaking require?

A

requires energy, which is an endothermic process

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3
Q

What does bond making do?

A

release energy, which is an exothermic process.

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4
Q

What occurs in an exothermic reaction?

A

when more energy is released when bonds form than was needed to break bonds.
△H is negative.

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5
Q

What occurs in an endothermic reaction?

A

When more energy is needed to break bonds than is released when bonds form.
△H is positive.

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6
Q

What is bond enthalpy?

A

The energy required to break one mole of bonds in a gaseous molecule.

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7
Q

How do you calculate enthalpy change △H?

A

Enthalpy of bonds broken - enthalpy of bonds formed.

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8
Q

What is activation energy?

A

The energy needed to start a reaction by breaking bonds.

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9
Q

How do you measure the enthalpy change of combustion?

A

A simple apparatus is set up, where a known mass of fuel is burned and is used to heat up a known mass of water (or other substance), the fuel (usually in a spirit burner) is then reweighed and the temperature rise is measured.

The temperature rise is then used together with the specific heat capacity of the water (4.18 JK-1g-1) and the mass of water (1 cm3 = 1 g of water) to calculate the amount of energy released:

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10
Q

What is the equation for calculating an energy change?

A

q = mc△T
q = mass x specific heat capacity x temp change

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11
Q

How do you calculate enthalpy change? (step by step)

A
  1. q = mc△T
  2. calcuate the number of moles, n, of fuel burnt
  3. calculate the enthalpy change of reaction
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12
Q

For calculations to be accurate several assumptions are made, what are these?

A
  • all the heat energy released by the burning fuel is absorbed by the water.
  • the fuel undergoes complete combustion.
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13
Q

What is the reality of the assumptions made about heat energy?

A
  • Heat energy is lost to the surroundings (air; apparatus etc)
  • incomplete combustion occurs.
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14
Q

What improves the accuracy of the heat energy calculation/ method?

A
  • The combustion chamber is surrounded by water to maximise the absorption of the heat energy.
  • A good supply of air / oxygen promotes complete combustion of the fuel.
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15
Q

What is enthalpy of neutralisation?

A

The standard enthalpy change of neutralisation is the enthalpy change when solutions of an acid and an alkali react together under standard conditions to produce 1 mole of water.

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16
Q

How do you calculate the number of moles of acid or alkali?

A

using the stoichiometry / mole ratio in the symbol equation to determine the moles of water.

17
Q

What is the law of conservation of energy?

A

Energy can be neither created nor destroyed, only transferred from one form to another.

18
Q

What is Hess’ law?

A

The enthalpy change of a reaction is independent of the route taken, provided that the initial and final conditions are the same

19
Q

What applications can be made using Hess’ law?

A

as long as we start with the same reactants in the same physical states at the same temperature, and finish with the same products in the same physical states at the same temperature, then overall the amount of energy taken in or given out will be the same.

Allows us to carry out and measure q for a unmeasurable directly reaction and ones that you can’t carry out under normal lab conditions.

20
Q

What is enthalpy change of formation?

A

Def: Enthalpy Change of Formation: the enthalpy change when 1 mole of a compound is formed from its elements in their standard states, under standard conditions. (298 K or 25oC and 1atm pressure

21
Q

What is the enthalpy change of combustion?

A

The enthalpy change when 1 mole of a substance is reacted with excess oxygen, under standard conditions, all reactants and products in their standard states.

22
Q

What equation shows the formation of methane?

A

C(s) + 2H₂(g) –> CH₄(g)

23
Q

What equation shows the formation of carbon dioxide?

A

C(s) + O₂(g). –> CO₂ (g)

24
Q

What equation shows the formation of water?

A

H₂(g) + 1/2 O₂(g) –> H₂O(l)

25
Q

What equation shows the combustion of methane?

A

CH₄(g) + 2O₂(g) –> CO₂ + 2H₂O(l)

26
Q

What equation shows the composition of carbon?

A

C(s) + O₂(g) –> CO₂(g)

27
Q

What equation shows the composition of hydrogen?

A

H₂(g) + ½ O₂(g) –> H2O(l)

28
Q

What is the enthalpy cycles and Hess’s law?

A

Hess’s law tells us that if we use an alternative route – via E + F,
i.e. A + B → E + F → C + D
then the overall enthalpy change will be the same as long as the initial and final conditions are the same.

29
Q

What are the steps to calculate enthalpy change, using the enthalpy change formation

A

Step 1: Write the equation for the enthalpy change you want to determine across the top.

Step 2: Construct a cycle If ΔHf data, write the elements at the bottom. Arrows from elements to compounds. Pointing UP.

Step 3: Add the data values. Check No of moles

Step 4: Show the alternative route.

Step 5: Add the values. Check arrow direction & amend signs.

30
Q

What are the steps to calculate enthalpy of formation/combustion using Hess’s law?

A

Step 1: Write the equation for the enthalpy change you want to determine across the top.

Step 2: Produce a Hess’s cycle. If ΔHc data, write the combustion products at the bottom and draw the arrows down to the combustion products.

Step 3: Add the enthalpy change data. Check No of moles

Step 4: Show the alternative route.

Step 5: Add the values. Check arrow direction & amend signs.

31
Q

What are born-haber cycles?

A

cycles that allow us to calculate the value of any of the enthalpy changes. on the cycle, provided that we know all the others. Usually, they involve calculating the lattice enthalpy or possibly the enthalpy of electron affinity.

32
Q

what is lactice enthalpy?

A

The enthalpy change when one mole of ionic solid is formed from its gaseous ions, under standard conditions.

33
Q

What is enthalpy of atomisation?

A

The enthalpy change when one mole of gaseous atoms is formed from the element in its standard state under standard conditions.

34
Q

What is 1st Ionisation energy?

A

The energy required to remove one mole of electrons from one mole of gaseous atoms, under standard conditions.

35
Q

What is 1st Electron affinity?

A

The enthalpy change when one mole of electrons is added to one mole of gaseous atoms, under standard conditions.

36
Q
A

Chemistry (8 decks)

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