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Chemistry preliminary 2020 > Module 4: Drivers of Reactions > Flashcards

Module 4: Drivers of Reactions Flashcards

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1
Q

Exothermic definition + example

A

Exothermic= releasing heat to the surroundings

E.g. Respiration releases energy in the form of ATP molecules so it is exothermic

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2
Q

Endothermic definition + example

A

Endothermic=absorbing heat from the surroundings

E.g. Photosynthesis requires an input of light energy so it endothermic

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3
Q

Chemical Arithmetic:

A
  • Energy must be added to break bonds in reactants to form products
  • The difference in bond energy indicated a net absorbance or release of energy
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4
Q

Chemical arithmetic and exothermic

A

-When total energy of products is lower than the total energy of the reactants, energy has been transferred from the system into the surrounding (exothermic)

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5
Q

Chemical arithmetic and endothermic

A

-When the total energy of the products is higher than the total energy of the reactants, energy has been transferred to the system to the system from the surrounding (endothermic)

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6
Q

What is enthalpy?

A
  • The amount of thermal heat energy released or absorbed during a chemical reaction
  • Assumes constant pressure
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7
Q

WHat is molar heat of a solution?

A
  • Measure of the energy change when 1 mole of a substance dissolves in water
  • If enthalpy is negative exothermic
  • If enthalpy is positive endothermic
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8
Q

Why are styrofoam cups useful in reactions?

A

Styrofoam holds more heat energy in and exchanges less energy with the surroundings

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9
Q

What is dissolution?

-relate to exo and endo

A
  • To cause a solute to pass into a solution
  • The dissolutions in water of many salts can be exo or endo
  • Depends on how strong the bonds are
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10
Q

What happens when a substance absorbs heat?

A

When a substance absorbs heat, its temperature changes in proportion to the amount of heat it has absorbed.

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11
Q

What does the quantity of the heat absorbed depend on?

A

The quantity of heat absorbed is symbolised by q and depends on the mass of the substance, the specific heat capacity of the substance and the change in its temperature.

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12
Q

What is specific heat capacity?

A

The specific heat capacity of a substance is the amount of heat required to increase the temperature of the substance by 1oC.

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13
Q

How do you calculate specific heat capacity?

A

q = mcΔT

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14
Q

what does q symbolis

q = mcΔT

A

q = change in heat energy, measured in Joules (J)

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15
Q

what does m symbolise

q = mcΔT

A

m = the mass of the substance (water) in grams (g)

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16
Q

what does c symbolise

q = mcΔT

A

c = specific heat capacity (of water) in J/K/g or J/°C/g

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17
Q

what does t symbolise

q = mcΔT

A

ΔT = change in the temperature of the medium in K or °C.

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18
Q

Molar enthalpy change limitations

A

Enthalpy is a measure of the total energy possessed by a substance/s. We can only measure changes in enthalpy not the total energy of a substance.

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19
Q

how do we measure molar enthalpy change

A

ΔH= enthalpy of products-enthalpy of reactants

The molar enthalpy change for a substance is the change in enthalpy per mole of substance reacting.

20
Q

if temp is positive …

-molar enthalpy change

A

exo

21
Q

if temp is negative …

-molar enthalpy change

A

endo

22
Q

WHat si activation energy?

A

Activation Energy is the minimum energy required by the reactants to be able to produce the products. In other words, it is the energy needed to begin a chemical reaction.

23
Q

What are catalysts?

A

A catalyst is a substance which increases the rate of a reaction without being changed by the reaction. Catalysts lower the activation energy.

24
Q

How do catalysts lower activation energy?

A

The catalyst helps break down bonds in one of the substances to help it re-bond to the other substance more readily.

25
Q

How do catalysts increase rate of reaction?

A

At a fixed temperature, the catalyst ensures that a higher proportion of particles possess energies greater than the lowered activation energy. This increases the rate of the reaction.

26
Q

What classifies a substance to be a catalyst?

A
  • cause a reaction to proceed at a faster rate than a measured control.
  • not be used up or changed by the end of the reaction (it may react but must reform at the end).
27
Q

Bond energies in chemical reactions

A

In any chemical reaction bonds within the reactants must be broken and new bonds formed to create the products. Energy is required to break chemical bonds and energy is released when bonds are formed. Hence a chemical reaction is a transfer of energy.

28
Q

define bond energies

A

Bond energy is the energy required to break specific bonds

29
Q

How to find net gain or loss of energy:

A

ΔH= (energy to break bonds)- (energy released when bonds form)

30
Q

What does Hess’ law state?

A

If a reaction is carried out in a series of steps, ΔH for the overall reaction will equal the sum of the enthalpy changes for the individual steps.

31
Q

Important info about enthalpy:

A

Enthalpy change is constant regardless of the reaction steps taken to convert reactants into products.
• Used to support the law of conservation of energy.
• If reaction is reversed, sign of DH changes.
• If reaction is doubled then DH doubles.
• Need to add the reaction steps together to get total bond energy.

32
Q

Hess’ law and photosynthesis

A

We can use Hess’ Law to calculate the enthalpy change for photosynthesis by using the enthalpies of formation of water, carbon dioxide and glucose

33
Q

Hess’ law and respiration

A

The equation is reversed, the sign is reversed and this means respiration is an exothermic process. This is exactly what we would expect as respiration is the process by which energy is made available to the cells in the form of an energy-rich molecule called ATP (adenosine triphosphate).

34
Q

Law of entropy states…

A

Unless energy is expended to counter it, things become random and break down over time.

35
Q

How does entropy increase?

A

Entropy increases as the distribution of particles becomes more random. We can model this by thinking about the changes in state from a solid to a liquid to a gas. If we were to plot entropy vs temperature, we might have a graph which looks similar to the one below.

36
Q

define entropy:

A

Entropy can be defined as an increase in disorder.

37
Q

Indicators of increased entropy in a chemical reaction:

A
  • Gas has more entropy than liquids, and liquids have more entropy than solids
  • Increase in temperature from reactants to products (exothermic) shows more entropy
  • Increase in number of moles from reactants to products = more disorder
  • Increasing concentration
  • Increasing pressure/decreasing volume
38
Q

What is standar molar entropy?

A

The standard molar entropy of a substance is the entropy of one mole of the substance in its standard state at the specified temperature

39
Q

How do entropy and enthalpy work together?

A

The reason for this relationship is that if energy is added to or released from the system, it has to be partitioned into new states. Thus, an enthalpy change can also have an effect on entropy.

40
Q

WHat is gibbs free enrgy equation?

A

Δ G = Δ H − T Δ S

41
Q

Δ G = Δ H − T Δ S

-what if g is positive?

A

the reaction goes in the reverse direction.

42
Q

Δ G = Δ H − T Δ S

-What if s is positive?

A

ΔG decreases as temperature increases so increased change of spontaneity.

43
Q

Δ G = Δ H − T Δ S

-What if s is negative?

A

then ΔG increases as temperature increases and so the chances of spontaneity decreases as temperature increases.

44
Q

What is gibbs free energy?

A

Gibbs free energy is a measure of the net results of the energy and entropy drives for a reaction.

45
Q

What do the constants in gibbs free energy equation stand for?

A

H is enthalpy
T is temperature in Kelvin
S is entropy

46
Q

Δ G = Δ H − T Δ S

-what if g is negative?

A

the reaction goes in the forward direction

47
Q

IS exo or endo mostly spontaneous?

A

EXOTHERMIC WILL BE SPONTANEOUS

Chemistry preliminary 2020 (5 decks)

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