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In today's T-Level Chemistry lesson, we are going to study A twelve point two, Calculating Rate of Reaction.

By the end of this lesson, you should be able to calculate average rate from tables and graphs, using amount of reactant used or product formed divided by time, and give the correct compound unit.

Reaction rate is how quickly reactants are used or products are formed.

Average rate over a chosen interval is the change in amount divided by the time taken.

Amount may be mass in grams, gas volume in cubic centimetres or cubic decimetres, or amount in moles. Use the units given. Do not convert unless the question requires it or the units are inconsistent.

If a reactant amount decreases, the amount used is initial minus final. That change must be positive.

Rate units are compound units. Grams per second. Cubic centimetres per second. Moles per second. If time is in minutes, the rate is per minute unless you convert.

One minute equals sixty seconds.

Worked example. Seventy two cubic centimetres of gas form in twenty four seconds. Seventy two divided by twenty four is three point zero cubic centimetres per second.

The same method gives three point zero cubic centimetres per second for forty five cubic centimetres in fifteen seconds, and for eighty four cubic centimetres in twenty eight seconds.

Now a reactant example. A solid falls from eight point four zero grams to six point nine zero grams in fifty seconds.

The change is one point five zero grams. One point five zero divided by fifty is zero point zero three zero grams per second.

Check another. Twelve point five grams to ten point one grams in eighty seconds. The change is two point four grams. Two point four divided by eighty is also zero point zero three zero grams per second.

Time conversion. Two point five minutes is one hundred and fifty seconds.

One hundred and twenty cubic centimetres in two point zero minutes is sixty cubic centimetres per minute. Two point zero minutes is one hundred and twenty seconds, so the rate is also one point zero cubic centimetre per second.

Now a table. Gas volume is zero at zero seconds, eighteen at twenty seconds, forty two at fifty seconds, and fifty eight at eighty seconds.

From twenty to fifty seconds the change is twenty four cubic centimetres in thirty seconds. Twenty four divided by thirty is zero point eight zero cubic centimetres per second.

From fifty to eighty seconds the change is sixteen cubic centimetres in thirty seconds, which is zero point five three cubic centimetres per second to two significant figures. The first interval is faster.

On a product–time graph, average rate is the gradient of the chord between two points: vertical change divided by horizontal change.

From eighteen cubic centimetres at twenty seconds to forty two cubic centimetres at fifty seconds, the chord is again twenty four over thirty, zero point eight zero cubic centimetres per second.

An examination graph uses fifteen cubic centimetres at twenty seconds and thirty nine cubic centimetres at sixty seconds. The change is twenty four cubic centimetres in forty seconds, so the rate is zero point six zero cubic centimetres per second.

Correct three common errors.

Eighty cubic centimetres in twenty seconds is four cubic centimetres per second, not four cubic centimetres. The unit must include time.

If mass falls from nine point zero grams to seven point five grams in thirty seconds, do not divide seven point five by thirty. The change is one point five grams, so the rate is zero point zero five zero grams per second.

Ninety cubic centimetres in one point five minutes is sixty cubic centimetres per minute, or one point zero cubic centimetre per second, not sixty cubic centimetres per second.

Compare rates only when the units are compatible. Convert first if needed.

Examination-style question.

Part a. Ninety six cubic centimetres form in thirty two seconds. Calculate the rate. Two marks.

Ninety six divided by thirty two is three point zero cubic centimetres per second.

Part b. Mass falls from six point eight zero grams to five point three zero grams in seventy five seconds. Three marks.

The change is one point five zero grams. One point five zero divided by seventy five is zero point zero two zero grams per second.

Part c. A graph is read as fifteen cubic centimetres at twenty seconds and thirty nine cubic centimetres at sixty seconds. Three marks.

Twenty four cubic centimetres in forty seconds is zero point six zero cubic centimetres per second.

Part d. Why is the unit essential? One mark.

The unit identifies the amount measured and the time basis.

Before we finish, complete the method. Find the start and end values. Calculate the change. Find the time interval. Divide change by time. Write the compound unit and round sensibly.

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