chemistry remove revision

There are two ways to find the rate of a reaction: 

• measure the rate at which a reactant is used up
• measure the rate at which a product is formed

factors effecting the rate of reaction:

1. temperature

2. concentration

3. Catalyst

4.Size of particles or surface area

Temperature:

• If the temperature is increased:

  • the reactant particles move more quickly
  • they have more energy
  • the particles collide more often, and more of the collisions result in a reaction
  • the rate of reaction increases

Effect of surface area:

The rate of a chemical reaction can be raised by increasing the surface area of a solid reactant. This is done by cutting the substance into small pieces, or grinding it into a powder.

If a solid reactant is broken into small pieces or ground into a powder:

• its surface area increases
• more particles are exposed to the other reactant
• there are more collisions
• the rate of reaction increases

Effect of catalysts:

A catalyst is a substance that can increase the rate of a reaction. The catalyst itself remains unchanged at the end of the reaction it catalyses. Only a very small amount of catalyst is needed to increase the rate of reaction between large amounts of reactants.

catalystreaction catalysed iron     making ammonia from nitrogen and hydrogen platinum      making nitric acid from ammonia vanadium(V) oxide     making sulphuric acid

The faster the rate, the more reactant is used, or product is made, in a given time. The faster the reaction, the steeper the line on a graph showing total product against time will be.

rate of reaction = total amount of reactant used or product made ÷ time taken

• A reaction finishes when one of the reactants is all used up. No more product is made, so the line on a graph of total product against time will become horizontal.

1. Precipitation

• This is when the product of the reaction is a precipitate which forms a cloudy solution.
• Observe a marker through the solution and measure how long it takes for it to disappear.
• The quicker the marker disappears, the quicker the reaction.

2. Change in mass

• measuring the speed of a reaction that produces a gas can be carried out on a mass balance.
• As the gas is released the mass disappearing is easily measured on the balance.
• the quicker the reading on the balance drops, the quicker the reaction.

3. The volume of gas given off

• this involves the use of a gas syringe to measure the volume of gas given off.
• The more gas given off during a given time, the faster the reaction.
• a graph of gas volume against time could be plotted to give a rate of reation graph.

1. Reaction of hydrochloric acid and marble chips:

• measure the volume of gas released with a gas syringe, take readings at regular intevals.
• Make a table of the readings and plot them as a graph, the time is the independant variable(x) and volume is the dependant variable(y).
• repeat experiment with the same volume of acid and the same mass of marble chips, but with smaller bits of marble, then to follow do the experiment with powdered marble chips.
• This will show that the increase in surface area causes more frequent collisions, so he reaction rate is faster in the sammer and powdered marble chips.

2. Reaction of magnesium metal with dilute HCL:

• This reaction is good for measuring the effects of increased concentration.
• The reaction gives off hydogen gas, which we can measure he loss in mass with a balance as the gas is given off.
• In this experiment time is the independant variable(x) and the mass loss is the dependant variable(y).
• The experiment will show, that with a higher concentration of acid the reaction is faster.

1. Energy must always be suplied to break bonds:

Energy must be suplied to break existing bonds ( give out energy)  - bond breaking is an endothermic process.The energy is usually transferred as heat energy, causing the reaction mixture and its surroundings to get colder. e.g:

• Electrolysis
• The reaction between ethanoic acid and sodium carbonate
• The thermal decomposition of calcium carbonate in a blast furnace

Energy is released when new bonds are formed ( take in energy ) - bond formation is an exothermic process.The energy is usually transferred as heat energy, causing the reaction mixture and its surroundings to become hotter. e.g:

• Combustion (burning)
• Many oxidation reactions, for example rusting
• Neutralisation reactions between acids and alkalis

Exothermic reeaction:                                                  Endothermic reaction: