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Flashcards in Energy Changes Deck (67)
1

How do you find the mass of a solution?

Volume x density (assume density is 1 unless told otherwise)

2

What is the specific heat capacity

The amount of energy that is needed to raise the temperature of a solution by 1 degree

3

What is the specific heat capacity of water?

4.2 J/g/•C

4

When do you not add the masses together and when do you?

Don't add the mass of a solid reactant to the mass of the solution, but if two solutions then add them together

5

What is the equation for molar enthalpy change?

Molar enthalpy change (kJmol-1) = - heat energy transferred / no. of moles

Delta H (triangle H) = -Q / n

6

Equation to find energy change

Energy change (J) = mass of solution (g) x specific heat capacity (J/g/•C) x temp change

Q= m x c x Delta T

7

How does temperature, energy, and molar enthalpy change in an exothermic reaction

Temperature increases
Heat energy transferred is positive
Molar enthalpy change is negative

8

How does temperature, energy, and molar enthalpy change in an endothermic reaction

Temperature decreases, heat energy is negative, molar enthalpy is positive

9

What are exothermic reactions and what is the energy of their products like in relation to the reactants

Exothermic reactions release energy into their surroundings
Products have a lower energy than the reactants

10

What are endothermic reactions and what are their products like in relation to their reactants

Endothermic reactions absorb energy from their surroundings
Products have a higher energy than the reactants

11

Example of an exothermic reaction

N2 + 3H2 = 2NH3 (Haber process)

12

Example of an endothermic reaction

H2 + I2 = 2HI

13

KNOW HOW TO DRAW ENERGY CHANGE DIAGRAMS

🐥🐥🐥

14

What is the bond energy

Bond energy is the energy required to break one mole of covalent bonds.

15

Equation for molar enthalpy change from bond energies

Molar enthalpy change = Sum of bonds broken - sum of bonds made

16

What happens when bonds are broken

It requires energy and is an endothermic process

17

What happens when bonds are made

Energy is produced, it's an exothermic process

18

What is the relative atomic mass (Ar)

The weighted average sum of all the isotopes of an element

19

Equation for relative atomic mass

Ar = sum (isotope mass x abundance) / 100

20

Equation for Mr

Sum of Ar

21

Equation for moles

Mass / Mr

22

What is Avogadro's Law

Equal volumes of gases at the same temperature and pressure contain equal numbers of molecules. This means that one mole of any gas will occupy the same volume at the same temperature and pressure.

23

What is the "molar volume" of one mole of any gas?

24dm3 (24000cm3)

24

Equation to work out moles of a gas

Mols of a gas = volume (cm3) / 24000 (cm3)

25

What is a redox reaction?

A reaction in which one species is oxidised and another is reduced

26

What is oxidation

The loss of electrons, loss of hydrogen, addition of oxygen

27

What is reduction

The gain of elections, addition of hydrogen, loss of oxygen

28

What is the full equation for magnesium + copper oxide

Mg + CuO = MgO + Cu

29

What is the ionic equation for magnesium + copper oxide

Mg + Cu 2+ = Mg 2+ + Cu

30

In magnesium + copper oxide what is the reducing agent?

Magnesium is oxidised so is therefore the reducing agent

31

In magnesium + copper oxide what is the oxidising agent

Copper is reduced and therefore is the oxidising agent

32

What are the half equations in magnesium + copper oxide

Mg = Mg 2+ + 2e-

Cu2+ + 2e- = Cu

33

In a half equation what does e- mean?

"One mole of electrons"

34

What is the faraday constant and what is its value

The faraday constant is the charge carried by 1 mole of electrons. It has a value of about 96500C/mol

35

What is the equation for the moles of electrons?

Mol of e- = charge (C) / faraday constant

36

What is the equation for charge

Charge (C) = current (A) x time (s)

37

What does an electrolyte contain

Positive and negative ions

38

What happens at the anode

Negative ions (anions) move to the positive electrode (anode) and lose electrons. This is oxidation.

39

What happens at the cathode

Positive ions (cations) move to the negative electrode and gain electrons. This is reduction.

40

What happens at the anode and cathode with molten salts (PbBr2)
(The equations)

PbBr2 (l) = Pb (l) + Br2 (g)

Anode: 2Br- = Br2 + 2e- (oxidation)

Cathode: Pb 2+ + 2e- = Pb (reduction)

41

What happens at the anode and cathode with concentrated aqueous solutions (NaClaq)
(The equations)

NaCl(aq) ions: Na+, Cl-, H+, OH-

Anode: 2Cl- = Cl2 + 2e-

Cathode: 2H+ + 2e- = H2

42

What happens at the anode and cathode with dilute aqueous solutions (NaClaq)
(The equations) AND WHY

Anode: 4OH- = 2H2O + O2 + 4e-

Cathode: 2H+ + 2e- = H2

Because there is so much more water than sodium chloride

43

What are the rules for electrolysis at the anode

If you have a reasonably concentrated solution of halides you get the halogen
With other common negative ions, OH- is oxidised

44

What are the rules of electrolysis at the cathode

If the metal is high in the reactivity series, you get hydrogen
If the metal is below hydrogen, you get the metal

45

How would you work out this question:
4g of H2 produced, what mass of chlorine is produced?
(Just write what steps you would do)

1) Write out half equations
2) work out mols of hydrogen
3) work out ratio of moles
4) find mass of chlorine

46

How would you work out this question:
What mass of elements would be formed from Pb2+ if 1930C of charge was used?

1) write out half equation
2) work out mols of e-
3) work out ratio of moles of Pb (product) : e-
4) find mass of lead

47

How would you work out this question:
1930C of charge liberates 0.01 mol of cobalt from cobalt nitrate solution, find the charge on the cobalt ion

1) work out the mols of e- for cobalt
2) find ratio of moles of e-: Co
3) write out half equation
4) charge is how many moles of electrons

48

How would you work out this question:
For how long would a current of 1A need to flow to liberate 0.001 mol of silver from silver ions (Ag+) at the cathode?

1) write out half equation
2) work out charge using faraday constant equation
3) work out time using charge and current equation

49

What direction is the arrow in an energy level diagram of an exothermic reaction and why?

Down, because negative enthalpy change

50

What direction is the arrow in an energy level diagram of an endothermic reaction and why?

Up, because positive enthalpy change

51

How do you measure enthalpy change?

Using calorimetry experiments

52

Describe the calorimetry experiments for dissolving, displacement and neutralisation reactions

To measure the amount of energy transferred in these reactions:
1) Take the temperature of the reagents (making sure they're the same)
2) Mix them in a polystyrene cup
3) Measure the temperature at the end of the reaction

53

How to reduce heat loss in calorimetry experiments for dissolving, displacement and neutralisation reactions

1) polystyrene is an insulator
2) put the polystyrene cup into a beaker of cotton wool to give more insulation
3) put a lid on the cup to reduce heat loss by evaporation

54

Describe the calorimetry experiment for combustion (to measure the amount of energy produced when a fuel is burnt)

1) To reduce heat loss use a screen to act as a draught excluder
2) Put 50g of water in a copper can and record it's temperature
3) Weigh the spirit burner and lid
4) Put the spirit burner underneath the can and light the wick. Heat the water, stirring constantly, until the temperature reaches about 50 degrees
5) Put out the flame using the burner lid, and measure the final temperature of the water
6) Weigh the spirit burner and lid again
7) Use the measurements to calculate the molar enthalpy change

55

Experimental points for the calorimetry experiment for combustion

1) Use a medal can (eg copper) because it is a good heat conductor
2) Reduce heat loss by reducing draughts - by using a screen as a draught excluder and by not doing the experiment near an open window

56

Apart from burning, name 3 simple exothermic reactions:

• the reaction of metals with acids
• neutralisation reactions
• adding water to calcium oxide

57

In an exothermic energy diagram where are the reactants/products?

The reactants start with a higher energy than the products then draw a sudden arrow down at 90 degrees until it reaches the lower energy of the products. The arrow can be labelled 'heat evolved'

58

what is on the x and y coordinates of an energy diagram

progress of the reaction on x
energy on y

59

In an exothermic reaction is Delta H negative or positive?

negative because the reactants are losing energy as heat, which is transferred to the surroundings

60

What is Delta H measured in?

kJ mol-1

61

What is thermal decomposition?

Breaking something up by heating it

62

What do carbonates split up to give when they are heated?

Most carbonates split up to give a metal oxide and a carbon dioxide

63

Give the products of the thermal decomposition of copper carbonate

Copper (II) carbonate is a green powder which decomposes on heating to give black copper (II) oxide.

64

Give the products of the thermal decomposition of zinc carbonate

zinc carbonate is a white powder which decomposes to give zinc oxide, yellow when hot, white when cooled

65

Give the products of the thermal decomposition of calcium carbonate

takes a LOT of heat but this reaction is commercially important as it turns calcium carbonate (limestone) into calcium oxide (quick lime)

66

In an endothermic energy diagram where are the reactants/products?

reactants are lower than the products with an arrow at 90 degrees upwards to the reactants labelled heat absorbed leading to the products.

67

Why do reactions give out or absorb heat?

During chemical reaction bonds in the reactants have to be broken and new ones formed. Breaking bonds needs energy and energy is released when new bonds are made.