# 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.

## 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.

## Concentration-time graphs

conc/time graphs

**Rate= change in concentration/ change in** **tim**e

- 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**

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

CAN BE DRAWN USING CONTINUOUS MEASUREMENTS

## Drawing/sketching concentration time graphs

- ensure all reactants plateau at the same point.
- Gradient=rate

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

## 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.

## 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

## WAYS TO MEASURE RATE: when there's visual change

- Measure mass of precipitate
- Use colorimeter to measure colour change

## Conc-time graphs: Half-life: zero order

Graph

- Gradient is the same as rate
- So straight negative correlation.
- HALF LIFE DECREASES.

## HALF-LIFE: first order

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

First order half lives examples:

- Radioactive chemicals
- Removal of drugs from the body

Graph

- Half life is constant for 1st order
- Curve will steep decrease and plateau.

## Conc-time graphs: Half life: 2nd order

Graph

- Very steep decrease
- HALF LIFE INCREASES.

## 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!!)

## 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.

## 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.

## 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.

## 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
- leading to more reaction

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