practicals - 7a + 7b - needs finishing!!

What needs to be done to measure a reaction rate?

• The concentration of one of the reactants / products must be measured over a period of time

• Temperature must be kept constant as rate varies with temperature

What are the two ways to measure the rate of a chemical reaction?

• Using a continuous monitoring method (7b)

• Using an initial rates method (7a)

The method chosen depends on..

the substance whose concentration is being measured + speed of the reaction

Give the method for continuous monitoring

• You would measure either the ‘amount’ of a product formed or the reactant lost continuously throughout the course of one reaction

• Therefore, could measure concentration of a species reacted / formed, volume of gas formed or mass lost during a reaction

• Plot a concentration against time, volume against time or mass against time → deduce the rate + order of reaction

Even though the units of rate are normally mol dm-3s-1, why can we still use volume / mass of gas formed?

These are proportional to concentration → the units will just be different

How to find the rate of reaction from concentration / volume / mass over time?

If a graph of the concentration, volume of gas or the mass of gas produced is plotted against time, rate can be found by calculating the gradient

• Collect data + plot graph

• Draw a line of best fit

• Draw tangent

• Find gradient of tangent

Determine the order from this graph + explain why

• It’s a straight line

• As concentration changes the gradient, the rate doesn’t change → the order with respect to this reactant is zero

For zero order reactions, what is the rate equation?

Rate = k

*therefore, the rate constant, k, can be determined by calculating the gradient

What is the order of this reaction + how can you tell?

• First order reactions will have a curve as the rate change will equal the change in concentration → use the gradients to work out the order of reaction - should calculate at 2 points

• Change in concentration between the 2 points should be equal to the change in rate for both points

• Rate ∝ concentration

What is the order of this reaction + how can you tell?

• Second order reactions also have a curve

• When the gradient of 2 points on the curve are compared, the change in rate should equal the change in concentration squared

• Rate ∝ concentration²