# Rate

• Created by: ava.scott
• Created on: 23-04-15 14:55

## TERMS: Rate

Rate of reaction:

How fast a reaction is e.g. how quickly reactants are turned into products.

Rate constant:

Quantified rate of reaction. Rate[A][B]

Half life:

The time taken for the concentration of a reactant to decrease by half.

Rate determining step:

The slowest step in a multi-step reaction. ALWAYS 1ST STEP IN OCR.

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## TERMS: order

Order:

The order of a reaction in respect to a reactant is the index to which its concentration term in the rate equation is raised.

Overall order of reaction:

All orders in the rate equation added together.

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## Concentration-time graphs

conc/time graphs

Rate= change in concentration/ change in time

• Draw a tangent from where your chosen time meets the line.
• Concentration minus concentration of where your line meets the axes.
• Time minus time where your line meets the axes.
• UNITS= MOLDM-3 SEC-1

Half-life for 1st order

• 1st order reactions- half life is constant!
• Time at initial concentration- time at half of initial concentration.

CAN BE DRAWN USING CONTINUOUS MEASUREMENTS

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## Drawing/sketching concentration time graphs

• ensure all reactants plateau at the same point.

0 ORDER

• straight line down- as rate doesnt change with concentration
• half life decreases

1st ORDER

• goes down relatively steeply.
• half life is constant

2nd Order

• Decreases more steeply and will decrease by double the amount of any 1st order reactants.
• Plateaus at the same TIME value as 1st order.
• half life increases
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## WAYS TO MEASURE RATE: Acid +Base

pH probe during reaction

• 10^(-pH) will give you H+ concentration.
• [HA]= [H+]
• change in concentration of HA/ change in time.

Titration

• Gives you mols of fluid used until equilibira point
• convert mols to concentrations.
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## WAYS TO MEASURE RATE: involving gas

GAS

Measure change in volume

• volume produced= vol/24 = mol
• convert mol to conc

Measure change in mass of reactants (as gas is produced)

• should decrease
• giving you reacted mols
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## WAYS TO MEASURE RATE: when there's visual change

• Measure mass of precipitate
• Use colorimeter to measure colour change
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## Conc-time graphs: Half-life: zero order

Graph

• Gradient is the same as rate
• So straight negative correlation.
• HALF LIFE DECREASES.
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## HALF-LIFE: first order

The time take for the concentration of a reactant to decrease by half.

First order half lives examples:

• Removal of drugs from the body

Graph

• Half life is constant for 1st order
• Curve will steep decrease and plateau.
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## Conc-time graphs: Half life: 2nd order

Graph

• Very steep decrease
• HALF LIFE INCREASES.
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## ORDERS: assumptions + determination

• Only 2 step reactions
• The first step is the rate determining step

Determination

• Order is determined by the molar ratio in the rate-determining step (NOT overall equation!!)
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## Rate-concentration graphs

zero order

• line is straight but does not meet y axis
• concentration doesnt effect rate

1st order

• line is straight positive correlation
• concentration is proportional to rate
• conc doubles; rate doubles

2nd order

• line increases steeply
• concentration is proportional to rate-squared.

REQUIRES SEPARATE EXPERIMENTS WITH DIFFERENT CONCENTRATIONS of reactants to DRAW.

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## Determing order from experiments PHRASE

• Between experiements ___ and ____
• The concentration of A__
• increases by a factor of ___
• and the initial rate of reaction increases by a factor of _____
• Therefore the order of reaction in respect to _A is ______.

DO PAST PAPER QUESTIONS M8.

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## Determining rate from experimental data

DETERMINING RATE FROM DATA

• Find Orders
• Check headings and use 'x10^-y' values

When using data where two reactants change in concentration

• Use one order you've already worked out
• Times factor of concentration change by order worked out.
• Use this value to times the rate of smaller rate.
• Divide larger rate by this value.
• Compare to factor of change in concentration of unknown reactants.

do questions.

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## Temperature and K

Temperature increase will increase the rate constant and therefore the rate of reaction.

• More heat
• Particles have more energy
• more successful collisions