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iGCSE Chemistry > Physical Chemistry > Flashcards

Physical Chemistry Flashcards

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1
Q

Symbol for enthalpy change and unit

A

ΔH (kj/mol)

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2
Q

Define enthalpy change

A

change in energy

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3
Q

When is enthalpy change positive or negative?

A

+ = Endothermic
- = Exothermic

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4
Q

How can you measure enthalpy change?

A

Through Calorimetry.
- Allows to meansure the amount of energy transferred

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5
Q

Explain the Calorimetry practical for dissolving, displacement and neutralisation reactions.

A

-Take temp of reactants (all the same)
-Mix them
-Measure temp at the end
-Equipment: Polysterene cup (it insulates), thermometer, lid.

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6
Q

Explain the calorimetry experiment for combustion

A

-Energy transferred when fuel is burnt.
-50g of water and record its temp
-Weigh the spirit burner and lid.
-Spirit burner underneath the can and light the wick. Heat the water, stir constantly until temp is about 50º.
-Put out flame + measure temp
-Weigh spirit burner and lid again
-Equipment: Thermometer, Lid, Copper Can, 50cm³ water, draught excluder, spirit burner

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7
Q

Formula for heat energy change

A

q(j)=m(g)xc(heat capacity)xΔT(ºC)

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8
Q

What the specific heat capacity of water

A

4.18/4.2 J/g/ºC

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9
Q

Steps to calculate molar enthalpy change

A

1.calculate q
2.number of moles produced this energy change (mass/mr)
3.q/mol
4.add signs

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10
Q

What is the rate of reaction?

A

How fast the reactants are changed into products.

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11
Q

How do you find the speed of reaction?

A

By recording the amount of product formed or the amount of reactant used up over time.

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12
Q

Characteristics of a graph of Rate of Reaction

A

-Steeper line = faster RoR
-Less steep = reactants are used up
-Flat = Reaction has ended

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13
Q

Describe collision theory

A

-The more collisions there are the faster the reaction is.
-Particles have to collide with enough energy for the collision to be successful.
-Minimum amount of energy = activation energy

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14
Q

What does the RoR depend on?

A

-Temperature
-Concentration
-Pressure
-Surface area
-Catalyst

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15
Q

How does temperature affect the RoR?

A

-Particles move faster
-More collisions
-More energy in collisions (reactions happen if the particles collide with energy
=More successful collisions
=↑RoR

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16
Q

How does the concentration affect the RoR?

A

-More particles of reactant in the same volume.
-Collisions are more likely
=↑RoR

17
Q

How does Surface area affect the RoR?

A

-Larger Surface area to volume ratio
-PArticles around it will have more area to work on
=↑RoR

18
Q

What does a catalyst do?

A

-Increases the RoR without being chemically changed
-Provides an alternative reaction pathway that requires a lower activation energy

19
Q

Formula for the speed of a reaction

A

RoR= Amount of reactant used or amount of product formed/Time

20
Q

What is a reversible reaction?

A

A reaction where the products of the reaction can react with each other and convert back to the original reactants.

21
Q

What is a closed system?

A

Doesn’t allow for the transfer of matter in and out of the system.

22
Q

What is equilibrium?

A

The concentrations of reactant and products will reach a certain balance and stay there.

23
Q

How can the equilibrium change?

A

By changing the temperature and pressure. The position of equilibrium changes. It may give you more product and less reactants. Positioon shifts to right.

24
Q

How would a reverible reaction look like in a graph?

A

It would look like a mirror.

25
Q

What happens when a reversible formula is exothermic?

A

It would be endothermic in the other direction

26
Q

3 methods to calculate the rate of reaction

A

1) Beaker over X. Only when both are aqueous or liquid. Calculate how long it takes for the x to be covered by the precipitate
2) Place reaction on a scale with cotton. Calculate the change in mass against time
3) Reaction connected to a syringe. Calculate the volume of gas produced against time.

27
Q

What happens in a chemical reaction in terms of bond energies?

A

Energy is supplied to break bonds and energy is released when bonds are made
exothermic-energy released from forming bonds is greater than needed to break the bonds
endothermic-energy needed to break the bonds is greater than energy released making them

28
Q

Equation to find enthalpy change in terms of bind energies?

A

Energy of reaction = sum of bond energies of bonds broken - sum of bond energies of bonds made

29
Q

Units for RoR

A

-g/s
-mol/s

30
Q

3 common ways to measure the RoR

A

Loss in mass of reactants, volume of gas produced, time for a solution to become opaque

31
Q

How to find the RoR at some time from a graph f amount of reactant vs time

A

-Pick a point corresponding to time
-Find tangent to the curve at this point
-Tangent = gradient

32
Q

Describe the effect of increasing pressure of a gas on the RoR

A

-Increasing the pressure of reacting gases is the same as increasing the conc.
-t increases the number of gas molecules in the same volume and so increases the frequency of collisions and therefore increases the RoR.

33
Q

Describe Le Chatelier’s Principle

A

If the system is at equilibrium and a change is made to any of the conditions, then the system responds to counteract the change and restore the equilibrium.

34
Q

Describe the effect of changing the concentration of reactant and product on the position of the equilibrium

A

-If conc of one of the reactants or products is changed, it is not longer in equilibrium
-If the concentration of a reactant is increased, more products will be formed until equilibrium
-If the conc of product is decreased, more reactants will react until equilibrium

35
Q

Describe the effect of a change in temp on the position of the equilibrium

A

-If the temp>
-Endothermic = products>
-Exothermic= products<
-If temp<
-Endothermic= products<
-Exothermic= products>

36
Q

Describe the effect of changing pressure on the position of equilibrium

A

FOR GASES
-Equilibrium shifts towards the side with less molecules (moles)

37
Q

Describe the effect of a catalyst on the position of the equilibrium

A

-No effect
-It just speeds up both forward and backward reactions equally
-equilibrium is achieved faster

iGCSE Chemistry (5 decks)

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