6.1.1 - Calculating Rates of Reactions

How is rate of reaction calculated?

• rate of reaction = amount (e.g. grams, cm³) of reactant used/product formed ÷ time

• rate of reaction (mol/s) = moles of reactant used or product formed ÷ time

What are the various units for rate of reaction?

• Can include g/s, cm/s, or mol/s

• Generally, mass ÷ time, volume ÷ time, moles ÷ time

Name three common ways of measuring rate of reaction.

• Loss in mass of reactants.

• Volume of gas produced.

• Time for a solution to become opaque.

Describe measuring the rate by monitoring mass loss.

• Place the reaction flask on a balance.

• In these reactions (e.g. metal carbonate + acid), a gas is given off, so record the decrease in mass in time intervals (note hydrogen is too light).

• Plot a graph of mass v.s. time.

Describe measuring the rate by monitoring the volume of a gas.

• Connect a gas syringe to a reaction flask.

• Measure the volume of a gas formed in time intervals.

• Plot a graph of volume v.s. time.

Describe measuring the rate by monitoring the disappearance of a cross.

• Take a piece of paper and mark a cross (❌) on it.

• Put the reaction flask on this cross.

• Mix the reagents, and measure how long it takes for a cloudy mixture to conceal the cross.

How do you find a rate of reaction at some time, t, from a graph of amount of reactant v.s. time?

• Pick a point corresponding to the time, t, and find the tangent to the curve at this point.

• The tangent is the gradient of this graph - it tells you how fast the reaction proceeds at this point. The steeper the tangent line, the faster the rate.

• Gradient of tangent can be expressed in change in y values over change in x values.