chemistry topic 6- rates of reaction

rate of a chemical reaction

how fast the reactants are changed into products

3 ways to measure rate of reaction

• precipitation/ colour change

• measuring amount of reactant used

• measuring amount of product formed

rate of reaction formula(s)

amount of reactant used or amount of product formed (cm³) / time(s)

understanding graphs for rate of a reaction

• steeper line- the faster the rate of reaction

• over time line becomes less steep- reactants are used up

• quickest reactions- steepest lines- flat in the least time

• flat line shows reaction has finished

collision theory

reactions can only take place when particles collide with enough energy. reaction rates explained by collision theory:

1. collision frequency- more collisions, faster reaction. doubling frequency doubles rate

2. energy transferred during a collision- particles need to collide with enough energy for the collision to be successful (activation energy)

factors affecting rates of reaction

1. temperature

2. surface area

3. concentration

4. catalysts

affect of temp on rate

• increasing temperature increases rate of reaction

• more energy so particles move faster

• collide more frequently

• more energy so more of the collisions will have enough energy to make the reaction happen

affect of conc on rate

if a solution is more concentrated there are more particles in the same volume of water or solvent.. particles are more crowded

(similarly when pressure of gas is increased, same number of particles occupy smaller space)

collisions between reactant particle more frequent

double conc= double rate

affect of surface area on rate

catalyst

catalyst is a substance that speeds up a reaction without being used in the reaction itself.

affect of catalyst on rate

using a catalyst will increase the rate of reaction

it decreases the activation energy needed for the reaction

it provides an alternative reaction pathway with a lower activation energy

measuring colour change

1. you can record visual change in a reaction if initial solution is transparent and product is a precipitate which clouds the solution

2. observe a mark through the solution and how long it takes to disappear. faster it disappears quicker the reaction

3. time how long it takes to change colour

4. results are subjective though and you can't plot a graph

observing colour change (disappearing cross) practical method

1. use measuring cylinder to put 40cm cubed of 8 g/ dm cubed sodium thiosulfate into conical flask and place it on the cross

2. measure 10 cm cubed HCl

3. pour acid into flask. gently swirl and start stop clock

4. look down through top of flask, stop when you can no longer see cross

5. collect results and repeat 3 times

6. repeat with different concentrations of sodium thiosulfate

7. plot graph- mean time taken for cross to disappear in s and sodium thiosulfate conc g/dm cubed

disappearing cross method variables

independent- sodium thiosulfate concentration

dependent- time taken for cross to disappear

controls- volume of solution (HCl and sodium thiosulfate), size and thickness of cross, person judging

measuring the volume of gas produced practical method

1. measure 50 cm cubed of 1M HCl using a measuring cylinder and put into the conical flask

2. set up equipment fixing a gas syringe to a clamp stand and push it to 0

3. add 3cm Mg strip to conical flask and put the bung on and start the stop clock

4. record the volume of H2 gas given off every 10 seconds for 120 seconds or until no more is given off

5. repeat this using 1.5 and 2.0 mol HCl

measuring volume of gas practical variables

independent- concentration acid (HCl)

dependent- volume of gas produced

controls- volume of acid, length of Mg wire

calculating rates using tangents

1. draw a dot/ cross at specific time in the question

2. draw a line that sits on the curve and goes through the dot/ cross

3. using the squares, draw an appropriate triangle (length doesn't matter)

4. calculate values for x and y

5. gradient= change in y/ change in x

reversible reactions

• a physical change is reversible. no new substances are made (e.g. ice melting, puddles drying)

• occurs in forward and backwards directions.

hydrated copper sulfate+ heat

• blue before heating

• white after heating

• endothermic

it boiled then the water began to evaporate and the copper sulfate formed white crystals

anhydrous copper sulfate + water

• white before water

• blue after water

• exothermic

when water was added the white powder turned blue and the liquid was warm

equilibrium

equilibrium is the state in which a forward and backwards reaction happens at the same rate and the concentration of the reactants and the products are constant in a closed system

Le Chatelier's principle

equilibrium will shift if the conditions are changed in the system to counteract them

affect of changing concentration on equilibrium

increasing conc sends equilibrium to opposite side

decreasing conc sends equilibrium towards same side

affect of changing pressure on equilibrium

only affects if in gaseos state

increasing pressure sends to side with less molecules

decreasing pressure sends to side with more molecules

affect of changing temperature on equilibrium

increasing temp to endothermic direction

decreasing temp to exothermic direction

affect of catalyst on equilibrium

a catalyst doesn't affect equilibrium

has an equal effect on both forwards and backwards reaction