Percentage Yield

Percentage Yield

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When chemists study a chemical reaction, they often wonder if all the reactants turn into the product. But sometimes not all the reactants change, and that's where percentage yield comes in. It helps us figure out how much product should be produced versus how much is actually produced. In this article, we'll explain what percentage yield is, what factors affect it, and how to calculate it. We'll also talk about limiting reactants and how to find them in a chemical reaction. Lastly, we'll discuss percentage errors and how to reduce them. To understand how percentage yield works, let's look at the reaction between ethene and water to make ethanol as an example. We can determine the amount of product we'll get by using the molecular mass of the samples involved.

Percentage Yield
Percentage Yield

What is percentage yield?

The equation above shows that 1 mole of ethene reacts with water to produce 1 mole of ethanol. Based on this, we might think that reacting 28g of ethene with water would give us 46g of ethanol. However, this is just a theoretical amount. In reality, the actual amount of product we get is lower due to the inefficiency of the reaction.

If we conducted an experiment with exactly 1 mole of ethene and excess water, we would get less than 1 mole of ethanol. To determine how effective a reaction is, we can compare the actual amount of product we get to the theoretical amount from the balanced equation. This is known as percentage yield. Percentage yield is a measure of how well a chemical reaction works. It tells us the percentage of our reactants that successfully turn into a product.

Factors that affect percentage yield

The reaction process can be inefficient for a number of reasons, including:

  • Some of the reactants don't convert into a product.
  • Some of the reactants escape into the air (if they're a gas).
  • Unwanted products are formed in side-reactions.
  • The reaction reaches equilibrium.
  • Impurities interfere with the reaction.

Calculating percentage yield

To find the percentage yield of this reaction, we need to first determine the theoretical yield and the actual yield.

From the balanced equation, we see that 1 mole of methane makes 1 mole of carbon dioxide. We can use the molar mass of methane (16 g/mol) to calculate the number of moles of methane we have:

34 g CH4 / 16 g/mol = 2.125 mol CH4

So, according to the balanced equation, we should get:

2.125 mol CO2

To convert this to grams, we use the molar mass of carbon dioxide (44 g/mol):

2.125 mol CO2 x 44 g/mol = 93.5 g CO2

This is the theoretical yield, or the maximum amount of product we can get from the reaction.

Now, let's say we actually carried out the reaction and measured the amount of carbon dioxide we got. We found that we only got 73 g of CO2. This is the actual yield.

To calculate the percentage yield, we use the formula:

% yield = (actual yield / theoretical yield) x 100

% yield = (73 g / 93.5 g) x 100

% yield = 78.1%

Therefore, the percentage yield of this reaction is 78.1%. This means that we got 78.1% of the maximum amount of product we could have gotten, due to the inefficiency of the reaction process.

What are limiting reactants?

Sometimes we do not have enough of a reactant to form the amount of product we need.

Imagine you make nine cupcakes for a party but eleven guests show up. You should have made more cupcakes! Now the cupcakes are a limiting factor.

Limiting reactant
Limiting reactant


In the same way, if you do not have enough of a certain reactant for a chemical reaction, the reaction will stop when the reactant is all used up. We call the reactant a limiting reactant. A limiting reactant is a reactant that is all used up in a chemical reaction. Once the limiting reactant is all used up, the reaction stops. One or more of the reactants may be in excess. They are not all used up in a chemical reaction. We call them excess reactants.

How to find the limiting reactant

Another way to determine the limiting reactant is to calculate the amount of product that each reactant would produce if it were the limiting reactant, and then compare those amounts to see which is the smallest.

Using the same example:

If we have 1 mole of ethene, we can see that we would need 1 mole of chlorine to react with it completely, based on the balanced equation. This would give us 1 mole of dichloroethane.

If we have 1 mole of chlorine, we can see that we would need 1 mole of ethene to react with it completely, based on the balanced equation. This would also give us 1 mole of dichloroethane.

However, if we have 1.5 moles of chlorine, we can see that we would still only need 1 mole of ethene to react with it completely, based on the balanced equation. This would give us 1 mole of dichloroethane, and we would be left with 0.5 moles of chlorine that did not react.

Therefore, we can see that ethene is the limiting reactant in this case, as it is completely used up in the reaction while there is still some chlorine left over.

Percentage errors

When we carry out an experiment, we use different apparatus to measure things. For example, a balance or a measuring cylinder. Now, when using these to measure they are not entirely accurate and instead have something called a percentage error, and when we carry out experiments we need to be able to calculate percentage error. So how do we do this?

1. First we need to find the margin of error of the apparatus and we then need to see how many times we used the apparatus for a single measurement.

2. Then we need to see how much of a substance we measured.

3. Lastly, we use the figures and plug them into the following equation: maximum error/measured value x 100

1. A burette has a margin of error of 0.05cm3 and when we use this apparatus to record a measurement we use it twice. So we do 0.05 x 2 = 0.10, this is the margin error 2. Let us say we have measured 5.00 cm3 of a solution. This is the amount of substance we measured.3. Now, we can put the figures into the equation:0.10/5 x 100 = 2%So this has a 2% error.

How to minimise percentage error?

Yes, those are great key takeaways! To add to reducing percentage error, another way is to repeat the measurements multiple times and take an average. This can help to reduce random errors and increase the accuracy of the measurements. It's also important to use proper measurement techniques, such as ensuring the apparatus is properly calibrated and using the appropriate units of measurement. Additionally, minimizing external factors such as temperature and pressure fluctuations can help to reduce systematic errors.

Percentage Yield

How to work out percentage yield?

We work out percentage yield using the formula below:actual yield/ theoretical yield x 100

What does percentage yield mean?

Percentage yield measures the effectiveness of a chemical reaction. It tells us how much of our reactants (in percent) successfully turned into a product.

Why is it important to have a high percentage yield?

A high percentage yield lets us know how effective our reaction was. We usually only care about one of the products in a chemical reaction. Percentage yield lets us know how much of our reactants turned into a desired product.

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