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Physical chemistry 2 > Rate Equations > Flashcards

Rate Equations Flashcards

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1
Q

What does a reaction rate tell you?

A

How fast reactants are converted to products

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2
Q

What is reaction rate

A

It is the change in amount of reactant or product per unit time (usually seconds)

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3
Q

If reactants were in a solution, what would be the reaction rate calculated?

A

• The rate will be change in concentration per second. The units will be mol dm-3 s-1

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4
Q

How can you work out the reaction rate from a graph

A

Using a gradient

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5
Q

What is the formula for calculating gradients

A

Gradient= change in y/ change in x

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6
Q

If you draw a graph of the amount of reactant or product against time what would be the reaction rate?

A

The gradient of the graph

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7
Q

Describe the steps of determining the gradient

A

• Draw a line of best fit through the data points
• Pick two points on the line that are easy to read
• Then draw a vertical line down from one point on a horizontal line across from the other to make a triangle

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8
Q

How can you determine the gradient in a curved graph

A

• By working out the gradient of the tangent

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9
Q

What is a tangent

A

A tangent is a line that just touches a curve and has the same gradient as the curve does at that point.

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10
Q

Describe the process of determining the gradient of a curved graph

A

• Find the point on the curve that you need to look at. For example, if you want to find the rate of reaction at 3 minutes, find 3 on the x-axis and go up to the curve from there

• Place a ruler at the point so that it’s just touching the curve. Position the ruler so that you can see the whole curve

• Adjust the ruler until the space between the ruler and the curve is equal on both sides of the point

• Draw a line along the ruler to make the tangent. Extend the line right across the graph, it’ll help to make your gradient calculation easier as you’ll have more points to choose from

• Calculate the gradient of the tangent to find the rate

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11
Q

What is the function of The Rate Equation

A

• Links reaction rate to reactant concentrations
• It also shows how the rate is affected by the concentration of reactants

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12
Q

Give the formula for rate of equations and what the values represent

A

• Rate= K[A]m[B]n
• Rate units: mol dm-3 s-1
• Square brackets: concentration, mol dm3
• M and N are the orders of reaction
• K is the rate constant

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13
Q

What does the orders of reaction tell you?

A

How the reactant concentrations affect the rate

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14
Q

Define orders of reactions

A

• An order of reaction is defined as the power to which the concentration of its reactant is raised in the rate equation

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15
Q

• If [A] changes and the rate remains the same, what is the order or reaction with respect to A and how does this affect the rate of reaction

A

• The order of reaction with respect to A is 0. So if [A] doubles, the rate will stay the same. If [A] triples, the rate will stay the same

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16
Q

If the rate is proportional to [A], then what is the order of reaction with respect to A and how does this affect the rate of reaction

A

• The order of reaction with respect to A is 1
• So if [A] doubles, the rate will double. If [A] triples, the rate will triple

17
Q

If the rate is proportional to [A]2, then what is the order of reaction with respect to A and how does this affect the rate of reaction

A

• The order of reaction with respect to A is 2
• So if [A] doubles, the rate will be 2^2=4 times faster. If [A] triples, the rate will be 3^2=9 times faster

18
Q

What does the constant (k) relate to?

A

Relates the reactant concentrations to Rate at a Particular Temperature

19
Q

What age opens when the value of K becomes bigger

A

The reaction becomes faster

20
Q

What happens when you increase the temperature of reaction

A

• The rate of reaction increases
• Increasing the number of collisions between reactant molecules and also the energy of each collisions
• The concentrations of the reactants and orders of reaction stay the same
• The value of K must increase for the rate equation to balance

21
Q

What method can be used to work out rate equations?

A

Initial Rates Methods

22
Q

When is the initial rate of reaction and where can it be found?

A
  • Right at the start of the reaction
  • This can be found from a concentration-time graph
  • By calculating the gradient of the tangent at time=0
23
Q

Give the formula for initial rate

A

initial rate= y/x

24
Q

Describe how to use the initial rates method

A
  • Repeat an experiment several times using different initial concentrations of the reactants. You should usually only change one of the concentrations at a time keeping the rest constant.
  • Calculate the initial rate for each experiment using the equation
  • See how the initial concentrations affect the initial rates and figure out the order for each reactant. Once you know the orders, you can work out the rate equation (rate=k [A]
25
Q

How can K be calculated using the rate equations?

A
  • By putting the concentrations and the initial rates from one of your experiments into the rate equation
26
Q

How can the initial reaction rate be measured for a:
- Sudden color change reaction
- Precipitate produced reaction
- Other reactions where of course products are formed

A
  • There is a sudden color change when products reach a certain concentration. The rate of reaction can be worked out from measuring the time it takes for the color change to happen- the shorter the time, the faster the rate
  • For reactions that produce a precipitate that clouds the solution, you can measure the time it takes for the mark underneath the reaction vessel to disappear from view
  • For other reactions, you can work out the initial reaction rate by measuring the time taken for a small amount of products to be formed
27
Q

What must be constant during the initial rates method?

A

Temperature

28
Q

How can the rate of reaction be measured?

A

Through continuous monitoring

29
Q

Describe continuous monitoring

A

When a reaction is followed all the way through to the end by recording the amount of product (or reactant) you have at regular time intervals

30
Q

Describe how you can follow the reaction rate of a gas volume produced?

A
  • If a gas is given off, you can collect it in a gas syringe and record how much you’ve got at regular time intervals
  • e.g. reaction between an acid and a carbonate that gives off carbon dioxide gas
  • Use the ideal gas equation, to work out the moles of gas, the molar ratio and concentration of the reactant
31
Q

Describe how you can follow the reaction rate of mass lost?

A
  • If a gas is given off, the system will loose its mass
  • Measure this at regular intervals with a balance
  • Use mole calculations to know how much gas you’ve lost and how many moles are left
32
Q

Describe how you can follow the reaction rate of color change?

A
  • Colorimeter
  • Measures absorbance (the amount of light absorbed by the solution)
  • The concentrated the solution is, the higher the absorbance
  • Plot a calibration curve, a graph of known concentrations of the colored solution plotted against the absorbance
  • Take a sample at regular intervals and record the absorbance
  • Use the curve to convert the absorbance at each point into a concentration
33
Q

Describe how you can follow the reaction rate of a change in pH

A
  • When a reaction uses H+ ions
  • pH of the solution will change
  • Measure the pH of the solution at regular intervals and calculate the concentration of H+
34
Q

What data can you use to construct a rate-concentration graph and what does this tell you?

A
  • You can use a concentration-time graph to construct a rate-concentration graph
  • This can tell you the reaction order
35
Q

How do you draw a rate-concentration graph

A
  • rate on the y-axes and concentration on the x-axes.
  • The shape of the line will tell you the order of the reaction with respect to that reactant
36
Q

Why does the rate of reaction decrease as the reaction proceeds?

A

Concentration of reactants decreases so there are fewer collisions

37
Q

The rate equation for a reaction is
rate = k[E]

Explain qualitatively why doubling the temperature has a much greater effect on the
rate of the reaction than doubling the concentration of E

A
  • Reaction occurs when molecules have E>Ea
  • Doubling T by 10 °C causes many more molecules to have this E
  • Whereas doubling [E] only doubles the number with this E
38
Q

What is meant by the term order of reaction with respect to A?

A

Power (or index or shown as x in [ ]x) of concentration term
(in rate equation)

39
Q

State the effect, if any, on the value of the rate constant k when the temperature is lowered but all other conditions are kept constant. Explain your
answer

A
  • Lowered
  • Fewer particles/ collisions have energy > Ea
  • Fewer have sufficient activation energy to react

Physical chemistry 2 (7 decks)

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